search

Perl / perl5

šŸŖ The Perl programming language
https://dev.perl.org/perl5/
Other
1.98k stars 558 forks source link

smartmatch PATCH 2 of 2 for 5.14.1: perlsyn.pod #11365

Closed p5pRT closed 12 years ago

p5pRT commented 13 years ago

Migrated from rt.perl.org#90926 (status was 'resolved')

Searchable as RT90926$

p5pRT commented 13 years ago

From tchrist@perl.com

As previously explained\, this patch against perlsyn is part and parcel of the previous one against perlop.

--tom

p5pRT commented 13 years ago

From tchrist@perl.com

perlsyn-given.patch ```diff --- perlsyn.pod 2011-05-10 08:54:22.000000000 -0600 +++ /tmp/perlsyn.pod 2011-05-17 18:05:49.000000000 -0600 @@ -6,12 +6,13 @@ =head1 DESCRIPTION A Perl program consists of a sequence of declarations and statements -which run from the top to the bottom. Loops, subroutines and other +which run from the top to the bottom. Loops, subroutines, and other control structures allow you to jump around within the code. -Perl is a B language, you can format and indent it however -you like. Whitespace mostly serves to separate tokens, unlike -languages like Python where it is an important part of the syntax. +Perl is a B language: you can format and indent it however +you like. Whitespace serves mostly to separate tokens, unlike +languages like Python where it is an important part of the syntax, +or Fortran where it is immaterial. Many of Perl's syntactic elements are B. Rather than requiring you to put parentheses around every function call and @@ -31,36 +32,34 @@ X X X X The only things you need to declare in Perl are report formats and -subroutines (and sometimes not even subroutines). A variable holds -the undefined value (C) until it has been assigned a defined -value, which is anything other than C. When used as a number, -C is treated as C<0>; when used as a string, it is treated as -the empty string, C<"">; and when used as a reference that isn't being -assigned to, it is treated as an error. If you enable warnings, -you'll be notified of an uninitialized value whenever you treat -C as a string or a number. Well, usually. Boolean contexts, -such as: +subroutines (and sometimes not even subroutines). A scalar variable holds +the undefined value (C) until it has been assigned a defined value, +which is anything other than C. When used as a number, C is +treated as C<0>; when used as a string, it is treated as the empty string, +C<"">; and when used as a reference that isn't being assigned to, it is +treated as an error. If you enable warnings, you'll be notified of an +uninitialized value whenever you treat C as a string or a number. +Well, usually. Boolean contexts, such as: - my $a; if ($a) {} are exempt from warnings (because they care about truth rather than definedness). Operators such as C<++>, C<-->, C<+=>, -C<-=>, and C<.=>, that operate on undefined left values such as: +C<-=>, and C<.=>, that operate on undefined variables such as: - my $a; + undef $a; $a++; are also always exempt from such warnings. -A declaration can be put anywhere a statement can, but has no effect on -the execution of the primary sequence of statements--declarations all -take effect at compile time. Typically all the declarations are put at -the beginning or the end of the script. However, if you're using -lexically-scoped private variables created with C, you'll -have to make sure -your format or subroutine definition is within the same block scope -as the my if you expect to be able to access those private variables. +A declaration can be put anywhere a statement can, but has no effect on the +execution of the primary sequence of statements: declarations all take +effect at compile time. All declarations are typically put at the +beginning or the end of the script. However, if you're using +lexically-scoped private variables created with C, C, or +C, you'll have to make sure your format or subroutine definition is +within the same block scope as the my if you expect to be able to access +those private variables. Declaring a subroutine allows a subroutine name to be used as if it were a list operator from that point forward in the program. You can declare a @@ -70,11 +69,21 @@ sub myname; $me = myname $0 or die "can't get myname"; -Note that myname() functions as a list operator, not as a unary operator; -so be careful to use C instead of C<||> in this case. However, if -you were to declare the subroutine as C, then -C would function as a unary operator, so either C or -C<||> would work. +A bare declaration like that declares the function to be a list operator, +not a unary operator, so you have to be careful to use parentheses (or +C instead of C<||>.) The C<||> operator binds too tightly to use after +list operators; it becomes part of the last element. You can always use +parentheses around the list operators arguments to turn the list operator +back into something that behaves more like a function call. Alternatively, +you can use the prototype C<($)> to turn the subroutine into a unary +operator: + + sub myname ($); + $me = myname $0 || die "can't get myname"; + +That now parses as you'd expect, but you still ought to get in the habit of +using parentheses in that situation. For more on prototypes, see +L Subroutines declarations can also be loaded up with the C statement or both loaded and imported into your namespace with a C statement. @@ -97,23 +106,24 @@ X X X X<;> The only kind of simple statement is an expression evaluated for its -side effects. Every simple statement must be terminated with a +side-effects. Every simple statement must be terminated with a semicolon, unless it is the final statement in a block, in which case -the semicolon is optional. (A semicolon is still encouraged if the +the semicolon is optional. But put the semicolon in anyway if the block takes up more than one line, because you may eventually add -another line.) Note that there are some operators like C and -C that look like compound statements, but aren't (they're just -TERMs in an expression), and thus need an explicit termination if used +another line. Note that there are operators like C, C, and +C that I like compound statements, but aren't--they're just +TERMs in an expression--and thus need an explicit termination when used as the last item in a statement. =head2 Truth and Falsehood X X X X X X X X<0> -The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and -C are all false in a boolean context. All other values are true. -Negation of a true value by C or C returns a special false value. -When evaluated as a string it is treated as C<''>, but as a number, it -is treated as 0. +The number 0, the strings C<"0"> and C<"">, the empty list C<()>, +and C are all false in a boolean context. All other values +are true. Negation of a true value by C or C returns a +special false value. When evaluated as a string it is treated as +C<"">, but as a number, it is treated as 0. Most Perl operators +that return true or false behave this way. =head2 Statement Modifiers X X X X X @@ -127,40 +137,24 @@ unless EXPR while EXPR until EXPR - when EXPR for LIST foreach LIST + when EXPR The C following the modifier is referred to as the "condition". Its truth or falsehood determines how the modifier will behave. C executes the statement once I and only if the condition is true. C is the opposite, it executes the statement I -the condition is true (i.e., if the condition is false). +the condition is true (that is, if the condition is false). print "Basset hounds got long ears" if length $ear >= 10; go_outside() and play() unless $is_raining; -C executes the statement I C<$_> smart matches C, and -then either Cs out if it's enclosed in a C scope or skips -to the C element when it lies directly inside a C loop. -See also L. - - given ($something) { - $abc = 1 when /^abc/; - $just_a = 1 when /^a/; - $other = 1; - } - - for (@names) { - admin($_) when [ qw/Alice Bob/ ]; - regular($_) when [ qw/Chris David Ellen/ ]; - } - -The C modifier is an iterator: it executes the statement once +The C modifier is an iterator: it executes the statement once for each item in the LIST (with C<$_> aliased to each item in turn). - print "Hello $_!\n" foreach qw(world Dolly nurse); + print "Hello $_!\n" for qw(world Dolly nurse); C repeats the statement I the condition is true. C does the opposite, it repeats the statement I the @@ -172,14 +166,15 @@ The C and C modifiers have the usual "C loop" semantics (conditional evaluated first), except when applied to a -C-BLOCK (or to the deprecated C-SUBROUTINE statement), in -which case the block executes once before the conditional is -evaluated. This is so that you can write loops like: +C- BLOCK (or to the Perl4 C-SUBROUTINE statement), in which +case the block executes once before the conditional is evaluated. + +This is so that you can write loops like: do { $line = ; ... - } until $line eq ".\n"; + } until !defined($line) || $line eq ".\n" See L. Note also that the loop control statements described later will I work in this construct, because modifiers don't take @@ -196,24 +191,36 @@ For C, you have to be more elaborate: X - LOOP: { + LOOP: { do { last if $x = $y**2; # do something here } while $x++ <= $z; } -B The behaviour of a C statement modified with a statement -modifier conditional or loop construct (e.g. C) is -B. The value of the C variable may be C, any -previously assigned value, or possibly anything else. Don't rely on -it. Future versions of perl might do something different from the -version of perl you try it out on. Here be dragons. -X +B The behaviour of a C, C, or C modified with a +statement modifier conditional or loop construct (for example, C) is B. The value of the C variable may be C, +any previously assigned value, or possibly anything else. Don't rely on +it. Future versions of Perl might do something different from the version +of Perl you try it out on. Here be dragons. X + +The C modifier is an experimental feature that first appeared in Perl +5.14. To use it, you should include a C declaration. +(Technically, it requires only the C feature, but that aspect of it +was not available before 5.14.) Operative only from within a C +loop or a C block, it executes the statement only if the smartmatch +C<< $_ ~~ I >> is true. If the statement executes, it is followed by +a C from inside a C and C from inside a C. + +Under the current implementation, the C loop can be +anywhere within the C modifier's dynamic scope, but must be +within the C block's lexical scope. This restricted may +be relaxed in a future release. See L<"Switch statements"> below. =head2 Compound Statements X X X X X -X<{> X<}> X X X X X X X +X<{> X<}> X X X X X X X X In Perl, a sequence of statements that defines a scope is called a block. Sometimes a block is delimited by the file containing it (in the case @@ -227,29 +234,45 @@ if (EXPR) BLOCK if (EXPR) BLOCK else BLOCK + if (EXPR) BLOCK elsif (EXPR) BLOCK ... if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK + unless (EXPR) BLOCK unless (EXPR) BLOCK else BLOCK + unless (EXPR) BLOCK elsif (EXPR) BLOCK ... unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK + + given (EXPR) BLOCK + LABEL while (EXPR) BLOCK LABEL while (EXPR) BLOCK continue BLOCK + LABEL until (EXPR) BLOCK LABEL until (EXPR) BLOCK continue BLOCK + LABEL for (EXPR; EXPR; EXPR) BLOCK + + LABEL foreach (LIST) BLOCK + LABEL foreach (LIST) BLOCK continue BLOCK LABEL foreach VAR (LIST) BLOCK LABEL foreach VAR (LIST) BLOCK continue BLOCK + + LABEL BLOCK LABEL BLOCK continue BLOCK -Note that, unlike C and Pascal, these are defined in terms of BLOCKs, +The experimental C statement is I; see +L below for how to do so, and the attendant caveats. + +Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs, not statements. This means that the curly brackets are I--no dangling statements allowed. If you want to write conditionals without -curly brackets there are several other ways to do it. The following +curly brackets, there are several other ways to do it. The following all do the same thing: - if (!open(FOO)) { die "Can't open $FOO: $!"; } + if (!open(FOO)) { die "Can't open $FOO: $!" } die "Can't open $FOO: $!" unless open(FOO); - open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! - open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; + open(FOO) || die "Can't open $FOO: $!"; + open(FOO) ? () : die "Can't open $FOO: $!"; # a bit exotic, that last one The C statement is straightforward. Because BLOCKs are always @@ -261,18 +284,15 @@ language construct, as everyone reading your code will have to think at least twice before they can understand what's going on. -The C statement executes the block as long as the expression is -L. -The C statement executes the block as long as the expression is -false. -The LABEL is optional, and if present, consists of an identifier followed -by a colon. The LABEL identifies the loop for the loop control -statements C, C, and C. -If the LABEL is omitted, the loop control statement -refers to the innermost enclosing loop. This may include dynamically -looking back your call-stack at run time to find the LABEL. Such -desperate behavior triggers a warning if you use the C -pragma or the B<-w> flag. +The C statement executes the block so long as the expression is +L. The C statement executes the block +so long as the expression is false. The LABEL is optional, and if present, +consists of an identifier followed by a colon. The LABEL identifies the +loop for the loop control statements C, C, and C. If the +LABEL is omitted, the loop control statement refers to the innermost +enclosing loop. This may include dynamically looking back your call-stack +at run time to find the LABEL. Such desperate behaviour triggers a warning +if you use the C pragma or the C> flag. If there is a C BLOCK, it is always executed just before the conditional is about to be evaluated again. Thus it can be used to @@ -280,7 +300,7 @@ the C statement. Extension modules can also hook into the Perl parser to define new -kinds of compound statement. These are introduced by a keyword which +kinds of compound statements. These are introduced by a keyword which the extension recognizes, and the syntax following the keyword is defined entirely by the extension. If you are an implementor, see L for the mechanism. If you are using such @@ -310,9 +330,9 @@ This command is normally used by programs that want to lie to themselves about what was just input. -For example, when processing a file like F. -If your input lines might end in backslashes to indicate continuation, you -want to skip ahead and get the next record. +For example, when processing a file like F. If your input +lines might end in backslashes to indicate continuation, you want to skip +ahead and get the next record. while (<>) { chomp; @@ -323,7 +343,7 @@ # now process $_ } -which is Perl short-hand for the more explicitly written version: +which is Perl shorthand for the more explicitly written version: LINE: while (defined($line = )) { chomp($line); @@ -354,7 +374,7 @@ test is reversed, but the conditional is still tested before the first iteration. -The loop control statements don't work in an C or C, since +Loop control statements don't work in an C or C, since they aren't loops. You can double the braces to make them such, though. if (/pattern/) {{ @@ -431,9 +451,7 @@ X X The C keyword is actually a synonym for the C keyword, so -you can use C for readability or C for brevity. (Or because -the Bourne shell is more familiar to you than I, so writing C -comes more naturally.) If VAR is omitted, C<$_> is set to each value. +you can use either. If VAR is omitted, C<$_> is set to each value. X<$_> If any element of LIST is an lvalue, you can modify it by modifying @@ -459,8 +477,9 @@ $elem *= 2; } - for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { - print $count, "\n"; sleep(1); + for $count (reverse(1..10), "BOOM") { + print $count, "\n"; + sleep(1); } for (1..15) { print "Merry Christmas\n"; } @@ -489,7 +508,7 @@ next OUTER if $wid > $jet; $wid += $jet; } - } + } See how much easier this is? It's cleaner, safer, and faster. It's cleaner because it's less noisy. It's safer because if code gets added @@ -518,34 +537,336 @@ $nothing = 1; } -Such constructs are quite frequently used, because older versions -of Perl had no official C statement. +You'll also find that C loop used to create a topicalizer +and a switch: + + SWITCH: + for ($var) { + if (/^abc/) { $abc = 1; last SWITCH; } + if (/^def/) { $def = 1; last SWITCH; } + if (/^xyz/) { $xyz = 1; last SWITCH; } + $nothing = 1; + } + +Such constructs are quite frequently used, both because older versions of +Perl had no official C statement, and also because the new version +described immediately below remains experimental and can sometimes be confusing. -=head2 Switch statements +=head2 Switch Statements X X X X X -Starting from Perl 5.10, you can say +Starting from Perl 5.10.1 (well, 5.10.0, but it didn't work +right), you can say use feature "switch"; -which enables a switch feature that is closely based on the -Perl 6 proposal. +to enable an experimental switch feature. This is loosely based on an +old version of a Perl 6 proposal, but it no longer resembles the Perl 6 +construct. You also get the switch feature whenever you declare that your +code prefers to run under a version of Perl that is 5.10 or later. For +example: + + use v5.14; + +Under the "switch" feature, Perl gains the experimental keywords C, +C, C, C, and C. The keywords C and +C are analogous to C and C in other languages, so the +code in the previous section could be rewritten as + + use v5.10.1; + for ($var) { + when (/^abc/) { $abc = 1 } + when (/^def/) { $def = 1 } + when (/^xyz/) { $xyz = 1 } + default { $nothing = 1 } + } + +The C is the non-experimental way to set a topicalizer. +If you wish to use the highly experimental C, that could be +written like this: + + use v5.10.1; + given ($var) { + when (/^abc/) { $abc = 1 } + when (/^def/) { $def = 1 } + when (/^xyz/) { $xyz = 1 } + default { $nothing = 1 } + } + +As fo 5.14, that can also be written this way: + + use v5.14; + for ($var) { + $abc = 1 when /^abc/; + $def = 1 when /^def/; + $xyz = 1 when /^xyz/; + default { $nothing = 1 } + } + +Or if you don't care to play it safe, like this: + + use v5.14; + given ($var) { + $abc = 1 when /^abc/; + $def = 1 when /^def/; + $xyz = 1 when /^xyz/; + default { $nothing = 1 } + } + +The arguments to C and C are in scalar context, +and C assigns the C<$_> variable its topic value. + +Exactly what the I argument to C does is hard to describe +precisely, but in general, it tries to guess what you want done. Sometimes +it is interpreted as C<< $_ ~~ I >>, and sometimes it does not. It +also behaves differently when lexically enclosed by a C block than +it does when dynamically enclosed by a C loop. The rules are far +too difficult to understand to be described here. See L later on. + +Due to an unfortunate bug in how C was implemented between Perl 5.10 +and 5.14, under those implementations the version of C<$_> governed by +C is merely a lexically scoped copy of the original, not a +dynamically scoped alias to the original, as it would be if it were a +C or under both the original and the current Perl 6 language +specification. This bug is expected to be addressed in a future release of +Perl. For forwards compatibility, if you really want a lexical C<$_>, +specify that explicitly: + + given(my $_ = EXPR) { ... } + +In the meanwhile, stick to C for your topicalizer and +you will be less unhappy. + +=head2 Goto +X + +Although not for the faint of heart, Perl does support a C +statement. There are three forms: C-LABEL, C-EXPR, and +C-&NAME. A loop's LABEL is not actually a valid target for +a C; it's just the name of the loop. + +The C-LABEL form finds the statement labeled with LABEL and resumes +execution there. It may not be used to go into any construct that +requires initialization, such as a subroutine or a C loop. It +also can't be used to go into a construct that is optimized away. It +can be used to go almost anywhere else within the dynamic scope, +including out of subroutines, but it's usually better to use some other +construct such as C or C. The author of Perl has never felt the +need to use this form of C (in Perl, that is--C is another matter). + +The C-EXPR form expects a label name, whose scope will be resolved +dynamically. This allows for computed Cs per FORTRAN, but isn't +necessarily recommended if you're optimizing for maintainability: + + goto(("FOO", "BAR", "GLARCH")[$i]); + +The C-&NAME form is highly magical, and substitutes a call to the +named subroutine for the currently running subroutine. This is used by +C subroutines that wish to load another subroutine and then +pretend that the other subroutine had been called in the first place +(except that any modifications to C<@_> in the current subroutine are +propagated to the other subroutine.) After the C, not even C +will be able to tell that this routine was called first. + +In almost all cases like this, it's usually a far, far better idea to use the +structured control flow mechanisms of C, C, or C instead of +resorting to a C. For certain applications, the catch and throw pair of +C and die() for exception processing can also be a prudent approach. + +=head2 The Ellipsis Statement + +X<...> +X<... statement> +X +X +X +X +X + +Beginning in Perl 5.12, Perl accepts an ellipsis, "C<...>", as a +placeholder for code that you haven't implemented yet. This form of +ellipsis, the unimplemented statement, should not be confused with the +binary flip-flop C<...> operator. One is a statement and the other an +operator. (Perl doesn't usually confuse them because usually Perl can tell +whether it wants an operator or a statement, but see below for exceptions.) + +When Perl 5.12 or later encounters an ellipses statement, it parses this +without error, but if and when you should actually try to execute it, Perl +throws an exception with the text C: + + use v5.12 + sub unimplemented { ... } + eval { unimplemented() }; + if ($@ eq "Unimplemented") { + say "I found an ellipsis!"; + } + +You can only use the elliptical statement to stand in for a +complete statement. These examples of how the ellipsis works: + + use v5.12; + { ... } + sub foo { ... } + ...; + eval { ... }; + sub somemeth { + my $self = shift; + ...; + } + $x = do { + my $n; + ...; + say "Hurrah!"; + $n; + }; + +The elliptical statement cannot stand in for an expression that +is part of a larger statement, since the C<...> is also the three-dot +version of the flip-flop operator (see L). + +These examples of attempts to use an ellipsis are syntax errors: + + use v5.12; + + print ...; + open(my $fh, ">", "/dev/passwd") or ...; + if ($condition && ... ) { say "Howdy" }; + +There are some cases where Perl can't immediately tell the difference +between an expression and a statement. For instance, the syntax for a block +and an anonymous hash reference constructor look the same unless there's +something in the braces to give Perl a hint. The ellipsis is a syntax +error if Perl doesn't guess that the C<{ ... }> is a block. In that case, +it doesn't think the C<...> is an ellipsis because it's expecting an +expression instead of a statement: + + @transformed = map { ... } @input; # syntax error + +You can use a C<;> inside your block to denote that the C<{ ... }> is a +block and not a hash reference constructor. Now the ellipsis works: + + @transformed = map {; ... } @input; # ; disambiguates + + @transformed = map { ...; } @input; # ; disambiguates + +Note: Some folks colloquially refer to this bit of punctuation as a +"yada-yada", but its true name is actually an ellipsis. Perl does not yet +accept the Unicode version, U+2026 HORIZONTAL ELLIPSIS, as an alias for +C<...>, but someday it may. + +=head2 PODs: Embedded Documentation +X X + +Perl has a mechanism for intermixing documentation with source code. +While it's expecting the beginning of a new statement, if the compiler +encounters a line that begins with an equal sign and a word, like this + + =head1 Here There Be Pods! + +Then that text and all remaining text up through and including a line +beginning with C<=cut> will be ignored. The format of the intervening +text is described in L. -The keywords C and C are analogous -to C and C in other languages, so the code -above could be written as +This allows you to intermix your source code +and your documentation text freely, as in - given($_) { - when (/^abc/) { $abc = 1; } - when (/^def/) { $def = 1; } - when (/^xyz/) { $xyz = 1; } - default { $nothing = 1; } + =item snazzle($) + + The snazzle() function will behave in the most spectacular + form that you can possibly imagine, not even excepting + cybernetic pyrotechnics. + + =cut back to the compiler, nuff of this pod stuff! + + sub snazzle($) { + my $thingie = shift; + ......... } -This construct is very flexible and powerful. For example: +Note that pod translators should look at only paragraphs beginning +with a pod directive (it makes parsing easier), whereas the compiler +actually knows to look for pod escapes even in the middle of a +paragraph. This means that the following secret stuff will be +ignored by both the compiler and the translators. + + $a=3; + =secret stuff + warn "Neither POD nor CODE!?" + =cut back + print "got $a\n"; + +You probably shouldn't rely upon the C being podded out forever. +Not all pod translators are well-behaved in this regard, and perhaps +the compiler will become pickier. + +One may also use pod directives to quickly comment out a section +of code. + +=head2 Plain Old Comments (Not!) +X X X<#> X X + +Perl can process line directives, much like the C preprocessor. Using +this, one can control Perl's idea of filenames and line numbers in +error or warning messages (especially for strings that are processed +with C). The syntax for this mechanism is almost the same as for +most C preprocessors: it matches the regular expression + + # example: '# line 42 "new_filename.plx"' + /^\# \s* + line \s+ (\d+) \s* + (?:\s("?)([^"]+)\g2)? \s* + $/x + +with C<$1> being the line number for the next line, and C<$3> being +the optional filename (specified with or without quotes). Note that +no whitespace may precede the C<< # >>, unlike modern C preprocessors. + +There is a fairly obvious gotcha included with the line directive: +Debuggers and profilers will only show the last source line to appear +at a particular line number in a given file. Care should be taken not +to cause line number collisions in code you'd like to debug later. + +Here are some examples that you should be able to type into your command +shell: + + % perl + # line 200 "bzzzt" + # the '#' on the previous line must be the first char on line + die 'foo'; + __END__ + foo at bzzzt line 201. + + % perl + # line 200 "bzzzt" + eval qq[\n#line 2001 ""\ndie 'foo']; print $@; + __END__ + foo at - line 2001. + + % perl + eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; + __END__ + foo at foo bar line 200. + + % perl + # line 345 "goop" + eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; + print $@; + __END__ + foo at goop line 345. + +=head2 Experimental Details on given and when + +As previously mentioned, the "switch" feature is considered highly +experimental; it is subject to change with little notice. In particular, +both C and C have tricky behaviours that are expected to +change to become less tricky in the future. Do not rely upon their +current (mis)implementations. + +Here is a longer example of C: use feature ":5.10"; - given($foo) { + given ($foo) { when (undef) { say '$foo is undefined'; } @@ -567,134 +888,158 @@ } } -C will assign the value of EXPR to C<$_> -within the lexical scope of the block, so it's similar to +As currently implemented, C assign the value of I to +merely a lexically scoped I> (!) of C<$_>, not a dynamically +scoped alias the way C does. That makes it similar to do { my $_ = EXPR; ... } except that the block is automatically broken out of by a -successful C or an explicit C. - -Most of the power comes from implicit smart matching: - - when($foo) - -is exactly equivalent to - - when($_ ~~ $foo) - -Most of the time, C is treated as an implicit smart match of -C<$_>, i.e. C<$_ ~~ EXPR>. (See L for more -information on smart matching.) But when EXPR is one of the below -exceptional cases, it is used directly as a boolean: +successful C or an explicit C. Because it is only a +copy, and because it is only lexically scoped, not dynamically +scoped, you cannot do the things with it that you are used to in +a C loop. In particular, you probably cannot use +arbitrary function calls. Best stick to C for that. + +Most of the power comes from the implicit smartmatching that can +sometimes apply. Most of the time, C is treated as an +implicit smartmatch of C<$_>, that is, C<$_ ~~ EXPR>. (See +L for more information on smartmatching.) +But when I is one of the nine exceptional cases (or things like them) +listed below, it is used directly as a boolean. + +B Understand that this is not an exclusive list, because the +mysteries of how the C operator actually behaves have been lost to +the loremasters. We must therefore couch are descriptions with fuzzier +language than is our wont. This makes the behaviour of the C +operator is unreliable, because it cannot be fully described. At best, +you should diligently study the lists below and confine your cases to only +those for which you can find clear, understandable rules and examples. =over 4 -=item * +=item 1. -a subroutine or method call +A user-defined subroutine call or a method invocation. -=item * +=item 2. -a regular expression match, i.e. C or C<$foo =~ /REGEX/>, -or a negated regular expression match (C or C<$foo !~ /REGEX/>). +A regular expression match in the form of C, C<$foo =~ /REGEX/>, or +perhaps also C<$foo =~ EXPR>. Also, a negated regular expression match in +the form C, C<$foo !~ /REGEX/>, or perhaps also C<$foo !~ EXPR>. -=item * +=item 3. -a comparison such as C<$_ E 10> or C<$x eq "abc"> -(or of course C<$_ ~~ $c>) +A comparison such as C<$_ E 10> or C<$x eq "abc"> +(or of course C<$_ ~~ $c>). There may be other comparisons +that work, but we're no longer sure what those are. -=item * +B You will often have to use C<$c ~~ $_> because +the default case uses C<$_ ~~ $c> , which is frequently +just the opposite of what you want. -C, C, or C +=item 4. -=item * +At least the three builtin functions C, +C, and C. We might add more later +when we think of them. -a negated expression C or C, or a logical -exclusive-or C<(...) xor (...)>. +=item 5. -=item * +A negated expression C or C, or a logical +exclusive-or C<(...) xor (...)>. We don't think it works with +the bitwise operators. Unfortunately. -a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>, -that return numerical values, not boolean ones. +=item 6. -=item * +A filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>, +that return numerical values, not boolean ones. We don't know why the +C<-z> operator is excluded. + +=item 7. -the C<..> and C<...> flip-flop operators. +The C<..> and C<...> flip-flop operators. Note that the C<...> flip-flop +operator is completely different from the C<...> elliptical statement +just described. =back -In those cases the value of EXPR is used directly as a boolean. +In those 7 cases sketched out above, the value of EXPR is used directly as +a boolean, and no smartmatching is done. You may think of C as a +smartsmartmatch, if you can figure that one out. -Furthermore, Perl inspects the operands of the binary boolean operators to -decide whether to use smart matching for each one by applying the above test to +Furthermore, Perl inspects the operands of logical operators to +decide whether to use smartmatching for each one by applying the above test to the operands: =over 4 -=item * +=item 1. -If EXPR is C<... && ...> or C<... and ...>, the test -is applied recursively to both operands. If I -operands pass the test, then the expression is treated -as boolean; otherwise, smart matching is used. +If EXPR is C<... && ...> or C<... and ...>, the test is applied recursively +to both operands. If I operands pass the test, then the expression is +treated as boolean; otherwise, smartmatching is used. -=item * +=item 2. If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test is applied recursively to the first operand (which may be a higher-precedence AND operator, for example). If the first operand -is to use smart matching, then both operands will do so; if it is -not, then the second argument will not be either. +is to use smartmatching, then both operands will do so; if it is +not, then the second argument will not be either. This is quite +different from the C case just described. Be careful. =back -These rules look complicated, but usually they will do what -you want. For example: +These rules are very complicated, but the goal is for them to +I do what you want even if you don't really understand +what they are doing. For example: when (/^\d+$/ && $_ < 75) { ... } -will be treated as a boolean match because the rules say both a regex match and -an explicit test on $_ will be treated as boolean. +will be treated as a boolean match because the rules say both +a regex match and an explicit test on C<$_> will be treated +as boolean. Also: when ([qw(foo bar)] && /baz/) { ... } -will use smart matching because only I of the operands is a boolean; the -other uses smart matching, and that wins. +will use smartmatching because only I of the operands is a boolean; the +other uses smartmatching, and that wins. Further: when ([qw(foo bar)] || /^baz/) { ... } -will use smart matching (only the first operand is considered), whereas +will use smartmatching (only the first operand is considered), whereas when (/^baz/ || [qw(foo bar)]) { ... } -will test only the regex, which causes both operands to be treated as boolean. -Watch out for this one, then, because an arrayref is always a true value, which -makes it effectively redundant. - -Tautologous boolean operators are still going to be optimized away. Don't be -tempted to write - - when ('foo' or 'bar') { ... } - -This will optimize down to C<'foo'>, so C<'bar'> will never be considered (even -though the rules say to use a smart match on C<'foo'>). For an alternation like -this, an array ref will work, because this will instigate smart matching: +will test only the regex, which causes both operands to be +treated as boolean. Watch out for this one, then, because an +arrayref is always a true value, which makes it effectively +redundant. Not a good idea. + +Tautologous boolean operators are still going to be optimized +away. Don't be tempted to write + + when ("foo" or "bar") { ... } + +This will optimize down to C<"foo">, so C<"bar"> will never be considered (even +though the rules say to use a smartmatch on C<"foo">). For an alternation like +this, an array ref will work, because this will instigate smartmatching: when ([qw(foo bar)] { ... } This is somewhat equivalent to the C-style switch statement's fallthrough -functionality (not to be confused with I fallthrough functionality - see -below), wherein the same block is used for several C statements. - -Another useful shortcut is that, if you use a literal array -or hash as the argument to C, it is turned into a -reference. So C is the same as C, -for example. +functionality (not to be confused with I fallthrough +functionality--see below), wherein the same block is used for several +C statements. + +Another useful shortcut is that, if you use a literal array or hash as the +argument to C, it is turned into a reference. So C is +the same as C, for example. C behaves exactly like C, which is to say that it always matches. @@ -712,29 +1057,29 @@ given($foo) { when (/x/) { say '$foo contains an x'; continue } - when (/y/) { say '$foo contains a y' } - default { say '$foo does not contain a y' } + when (/y/) { say '$foo contains a y' } + default { say '$foo does not contain a y' } } =head3 Return value -When a C statement is also a valid expression (e.g. -when it's the last statement of a block), it evaluates to : +When a C statement is also a valid expression (for example, +when it's the last statement of a block), it evaluates to: =over 4 =item * -an empty list as soon as an explicit C is encountered. +An empty list as soon as an explicit C is encountered. =item * -the value of the last evaluated expression of the successful -C/C clause, if there's one. +The value of the last evaluated expression of the successful +C/C clause, if there happens to be one. =item * -the value of the last evaluated expression of the C block if no +The value of the last evaluated expression of the C block if no condition is true. =back @@ -745,15 +1090,17 @@ Note that, unlike C and C, failed C statements always evaluate to an empty list. - my $price = do { given ($item) { - when ([ 'pear', 'apple' ]) { 1 } - break when 'vote'; # My vote cannot be bought - 1e10 when /Mona Lisa/; - 'unknown'; - } }; + my $price = do { + given ($item) { + when ([ "pear", "apple" ]) { 1 } + break when "vote"; # My vote cannot be bought + 1e10 when /Mona Lisa/; + "unknown"; + } + }; -Currently, C blocks can't always be used as proper expressions. This -may be addressed in a future version of perl. +Currently, C blocks can't always be used as proper +expressions. This may be addressed in a future version of Perl. =head3 Switching in a loop @@ -761,256 +1108,84 @@ For example, here's one way to count how many times a particular string occurs in an array: + use v5.10.1; my $count = 0; for (@array) { when ("foo") { ++$count } } print "\@array contains $count copies of 'foo'\n"; -At the end of all C blocks, there is an implicit C. -You can override that with an explicit C if you're only -interested in the first match. +Or in a more recent version: + + use v5.14; + my $count = 0; + for (@array) { + ++$count when "foo"; + } + print "\@array contains $count copies of 'foo'\n"; -This doesn't work if you explicitly specify a loop variable, -as in C. You have to use the default -variable C<$_>. (You can use C.) - -=head3 Smart matching in detail - -The behaviour of a smart match depends on what type of thing its arguments -are. The behaviour is determined by the following table: the first row -that applies determines the match behaviour (which is thus mostly -determined by the type of the right operand). Note that the smart match -implicitly dereferences any non-blessed hash or array ref, so the "Hash" -and "Array" entries apply in those cases. (For blessed references, the -"Object" entries apply.) - -Note that the "Matching Code" column is not always an exact rendition. For -example, the smart match operator short-circuits whenever possible, but -C does not. - - $a $b Type of Match Implied Matching Code - ====== ===== ===================== ============= - Any undef undefined !defined $a - - Any Object invokes ~~ overloading on $object, or dies - - Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a - Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a - Any CodeRef scalar sub truth $b->($a) - - Hash Hash hash keys identical (every key is found in both hashes) - Array Hash hash keys intersection grep { exists $b->{$_} } @$a - Regex Hash hash key grep grep /$a/, keys %$b - undef Hash always false (undef can't be a key) - Any Hash hash entry existence exists $b->{$a} - - Hash Array hash keys intersection grep { exists $a->{$_} } @$b - Array Array arrays are comparable[2] - Regex Array array grep grep /$a/, @$b - undef Array array contains undef grep !defined, @$b - Any Array match against an array element[3] - grep $a ~~ $_, @$b - - Hash Regex hash key grep grep /$b/, keys %$a - Array Regex array grep grep /$b/, @$a - Any Regex pattern match $a =~ /$b/ - - Object Any invokes ~~ overloading on $object, or falls back: - Any Num numeric equality $a == $b - Num numish[4] numeric equality $a == $b - undef Any undefined !defined($b) - Any Any string equality $a eq $b - - 1 - empty hashes or arrays will match. - 2 - that is, each element smart-matches the element of same index in the - other array. [3] - 3 - If a circular reference is found, we fall back to referential equality. - 4 - either a real number, or a string that looks like a number - -=head3 Custom matching via overloading - -You can change the way that an object is matched by overloading -the C<~~> operator. This may alter the usual smart match semantics. - -It should be noted that C<~~> will refuse to work on objects that -don't overload it (in order to avoid relying on the object's -underlying structure). - -Note also that smart match's matching rules take precedence over -overloading, so if C<$obj> has smart match overloading, then - - $obj ~~ X - -will not automatically invoke the overload method with X as an argument; -instead the table above is consulted as normal, and based in the type of X, -overloading may or may not be invoked. +At the end of all C blocks, there is an implicit C. +You can override that with an explicit C if you're +interested in only the first match alone. -See L. +This doesn't work if you explicitly specify a loop variable, as +in C. You have to use the default variable C<$_>. =head3 Differences from Perl 6 -The Perl 5 smart match and C/C constructs are not -absolutely identical to their Perl 6 analogues. The most visible -difference is that, in Perl 5, parentheses are required around -the argument to C and C (except when this last -one is used as a statement modifier). Parentheses in Perl 6 -are always optional in a control construct such as C, -C, or C; they can't be made optional in Perl -5 without a great deal of potential confusion, because Perl 5 -would parse the expression - - given $foo { - ... - } +The Perl 5 smartmatch and C/C constructs are not compatible +with their Perl 6 analogues. The most visible difference and least +important difference is that, in Perl 5, parentheses are required around +the argument to C and C (except when this last one is used +as a statement modifier). Parentheses in Perl 6 are always optional in a +control construct such as C, C, or C; they can't be +made optional in Perl 5 without a great deal of potential confusion, +because Perl 5 would parse the expression + + given $foo { + ... + } as though the argument to C were an element of the hash C<%foo>, interpreting the braces as hash-element syntax. -The table of smart matches is not identical to that proposed by the -Perl 6 specification, mainly due to the differences between Perl 6's -and Perl 5's data models. - -In Perl 6, C will always do an implicit smart match -with its argument, whilst it is convenient in Perl 5 to -suppress this implicit smart match in certain situations, -as documented above. (The difference is largely because Perl 5 -does not, even internally, have a boolean type.) - -=head2 Goto -X - -Although not for the faint of heart, Perl does support a C -statement. There are three forms: C-LABEL, C-EXPR, and -C-&NAME. A loop's LABEL is not actually a valid target for -a C; it's just the name of the loop. - -The C-LABEL form finds the statement labeled with LABEL and resumes -execution there. It may not be used to go into any construct that -requires initialization, such as a subroutine or a C loop. It -also can't be used to go into a construct that is optimized away. It -can be used to go almost anywhere else within the dynamic scope, -including out of subroutines, but it's usually better to use some other -construct such as C or C. The author of Perl has never felt the -need to use this form of C (in Perl, that is--C is another matter). - -The C-EXPR form expects a label name, whose scope will be resolved -dynamically. This allows for computed Cs per FORTRAN, but isn't -necessarily recommended if you're optimizing for maintainability: - - goto(("FOO", "BAR", "GLARCH")[$i]); - -The C-&NAME form is highly magical, and substitutes a call to the -named subroutine for the currently running subroutine. This is used by -C subroutines that wish to load another subroutine and then -pretend that the other subroutine had been called in the first place -(except that any modifications to C<@_> in the current subroutine are -propagated to the other subroutine.) After the C, not even C -will be able to tell that this routine was called first. - -In almost all cases like this, it's usually a far, far better idea to use the -structured control flow mechanisms of C, C, or C instead of -resorting to a C. For certain applications, the catch and throw pair of -C and die() for exception processing can also be a prudent approach. - -=head2 PODs: Embedded Documentation -X X - -Perl has a mechanism for intermixing documentation with source code. -While it's expecting the beginning of a new statement, if the compiler -encounters a line that begins with an equal sign and a word, like this - - =head1 Here There Be Pods! - -Then that text and all remaining text up through and including a line -beginning with C<=cut> will be ignored. The format of the intervening -text is described in L. - -This allows you to intermix your source code -and your documentation text freely, as in - - =item snazzle($) - - The snazzle() function will behave in the most spectacular - form that you can possibly imagine, not even excepting - cybernetic pyrotechnics. +However, their are many, many other differences. For example, +this works in Perl 5: - =cut back to the compiler, nuff of this pod stuff! + use v5.12; + my @primary = ("red", "blue", "green"); - sub snazzle($) { - my $thingie = shift; - ......... + if (@primary ~~ "red") { + say "primary smartmatches red"; } -Note that pod translators should look at only paragraphs beginning -with a pod directive (it makes parsing easier), whereas the compiler -actually knows to look for pod escapes even in the middle of a -paragraph. This means that the following secret stuff will be -ignored by both the compiler and the translators. - - $a=3; - =secret stuff - warn "Neither POD nor CODE!?" - =cut back - print "got $a\n"; - -You probably shouldn't rely upon the C being podded out forever. -Not all pod translators are well-behaved in this regard, and perhaps -the compiler will become pickier. - -One may also use pod directives to quickly comment out a section -of code. - -=head2 Plain Old Comments (Not!) -X X X<#> X X - -Perl can process line directives, much like the C preprocessor. Using -this, one can control Perl's idea of filenames and line numbers in -error or warning messages (especially for strings that are processed -with C). The syntax for this mechanism is almost the same as for -most C preprocessors: it matches the regular expression + if ("red" ~~ @primary) { + say "red smartmatches primary"; + } - # example: '# line 42 "new_filename.plx"' - /^\# \s* - line \s+ (\d+) \s* - (?:\s("?)([^"]+)\g2)? \s* - $/x + say "that's all, folks!"; -with C<$1> being the line number for the next line, and C<$3> being -the optional filename (specified with or without quotes). Note that -no whitespace may precede the C<< # >>, unlike modern C preprocessors. +But it doesn't work at all in Perl 6. Instead, you should +use the (parallelizable) C operator instead: -There is a fairly obvious gotcha included with the line directive: -Debuggers and profilers will only show the last source line to appear -at a particular line number in a given file. Care should be taken not -to cause line number collisions in code you'd like to debug later. + if any(@primary) eq "red" { + say "primary smartmatches red"; + } -Here are some examples that you should be able to type into your command -shell: + if "red" eq any(@primary) { + say "red smartmatches primary"; + } - % perl - # line 200 "bzzzt" - # the `#' on the previous line must be the first char on line - die 'foo'; - __END__ - foo at bzzzt line 201. +The table of smartmatches in L is not +identical to that proposed by the Perl 6 specification, mainly due to +differences between Perl 6's and Perl 5's data models, but also because +the Perl 6 spec has changed since Perl 5 rushed into early adoption. - % perl - # line 200 "bzzzt" - eval qq[\n#line 2001 ""\ndie 'foo']; print $@; - __END__ - foo at - line 2001. - - % perl - eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; - __END__ - foo at foo bar line 200. - - % perl - # line 345 "goop" - eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; - print $@; - __END__ - foo at goop line 345. +In Perl 6, C will always do an implicit smartmatch with its +argument, while in Perl 5 it is convenient albeit potentially confusing) to +suppress this implicit smartmatch in various rather loosely-defined +situations, as roughly outlined above. (The difference is largely because +Perl 5 does not have, even internally, a boolean type.) =cut + ```
p5pRT commented 13 years ago

From tchrist@perl.com

Here's a replacement patch again perlsyn\, with the new lore folded into it. This is against the current perlsyn in blead.

It is to be applied at the same time as the perlop patch\, because they will not make sense unless applied in tandem.

--tom

p5pRT commented 13 years ago

From tchrist@perl.com

perlsyn-given.patch ```diff --- perlsyn.pod 2011-05-10 08:54:22.000000000 -0600 +++ /tmp/perlsyn.pod 2011-05-17 22:08:45.000000000 -0600 @@ -6,12 +6,13 @@ =head1 DESCRIPTION A Perl program consists of a sequence of declarations and statements -which run from the top to the bottom. Loops, subroutines and other +which run from the top to the bottom. Loops, subroutines, and other control structures allow you to jump around within the code. -Perl is a B language, you can format and indent it however -you like. Whitespace mostly serves to separate tokens, unlike -languages like Python where it is an important part of the syntax. +Perl is a B language: you can format and indent it however +you like. Whitespace serves mostly to separate tokens, unlike +languages like Python where it is an important part of the syntax, +or Fortran where it is immaterial. Many of Perl's syntactic elements are B. Rather than requiring you to put parentheses around every function call and @@ -31,36 +32,34 @@ X X X X The only things you need to declare in Perl are report formats and -subroutines (and sometimes not even subroutines). A variable holds -the undefined value (C) until it has been assigned a defined -value, which is anything other than C. When used as a number, -C is treated as C<0>; when used as a string, it is treated as -the empty string, C<"">; and when used as a reference that isn't being -assigned to, it is treated as an error. If you enable warnings, -you'll be notified of an uninitialized value whenever you treat -C as a string or a number. Well, usually. Boolean contexts, -such as: +subroutines (and sometimes not even subroutines). A scalar variable holds +the undefined value (C) until it has been assigned a defined value, +which is anything other than C. When used as a number, C is +treated as C<0>; when used as a string, it is treated as the empty string, +C<"">; and when used as a reference that isn't being assigned to, it is +treated as an error. If you enable warnings, you'll be notified of an +uninitialized value whenever you treat C as a string or a number. +Well, usually. Boolean contexts, such as: - my $a; if ($a) {} are exempt from warnings (because they care about truth rather than definedness). Operators such as C<++>, C<-->, C<+=>, -C<-=>, and C<.=>, that operate on undefined left values such as: +C<-=>, and C<.=>, that operate on undefined variables such as: - my $a; + undef $a; $a++; are also always exempt from such warnings. -A declaration can be put anywhere a statement can, but has no effect on -the execution of the primary sequence of statements--declarations all -take effect at compile time. Typically all the declarations are put at -the beginning or the end of the script. However, if you're using -lexically-scoped private variables created with C, you'll -have to make sure -your format or subroutine definition is within the same block scope -as the my if you expect to be able to access those private variables. +A declaration can be put anywhere a statement can, but has no effect on the +execution of the primary sequence of statements: declarations all take +effect at compile time. All declarations are typically put at the +beginning or the end of the script. However, if you're using +lexically-scoped private variables created with C, C, or +C, you'll have to make sure your format or subroutine definition is +within the same block scope as the my if you expect to be able to access +those private variables. Declaring a subroutine allows a subroutine name to be used as if it were a list operator from that point forward in the program. You can declare a @@ -70,11 +69,21 @@ sub myname; $me = myname $0 or die "can't get myname"; -Note that myname() functions as a list operator, not as a unary operator; -so be careful to use C instead of C<||> in this case. However, if -you were to declare the subroutine as C, then -C would function as a unary operator, so either C or -C<||> would work. +A bare declaration like that declares the function to be a list operator, +not a unary operator, so you have to be careful to use parentheses (or +C instead of C<||>.) The C<||> operator binds too tightly to use after +list operators; it becomes part of the last element. You can always use +parentheses around the list operators arguments to turn the list operator +back into something that behaves more like a function call. Alternatively, +you can use the prototype C<($)> to turn the subroutine into a unary +operator: + + sub myname ($); + $me = myname $0 || die "can't get myname"; + +That now parses as you'd expect, but you still ought to get in the habit of +using parentheses in that situation. For more on prototypes, see +L Subroutines declarations can also be loaded up with the C statement or both loaded and imported into your namespace with a C statement. @@ -97,23 +106,24 @@ X X X X<;> The only kind of simple statement is an expression evaluated for its -side effects. Every simple statement must be terminated with a +side-effects. Every simple statement must be terminated with a semicolon, unless it is the final statement in a block, in which case -the semicolon is optional. (A semicolon is still encouraged if the +the semicolon is optional. But put the semicolon in anyway if the block takes up more than one line, because you may eventually add -another line.) Note that there are some operators like C and -C that look like compound statements, but aren't (they're just -TERMs in an expression), and thus need an explicit termination if used +another line. Note that there are operators like C, C, and +C that I like compound statements, but aren't--they're just +TERMs in an expression--and thus need an explicit termination when used as the last item in a statement. =head2 Truth and Falsehood X X X X X X X X<0> -The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and -C are all false in a boolean context. All other values are true. -Negation of a true value by C or C returns a special false value. -When evaluated as a string it is treated as C<''>, but as a number, it -is treated as 0. +The number 0, the strings C<"0"> and C<"">, the empty list C<()>, +and C are all false in a boolean context. All other values +are true. Negation of a true value by C or C returns a +special false value. When evaluated as a string it is treated as +C<"">, but as a number, it is treated as 0. Most Perl operators +that return true or false behave this way. =head2 Statement Modifiers X X X X X @@ -127,40 +137,24 @@ unless EXPR while EXPR until EXPR - when EXPR for LIST foreach LIST + when EXPR The C following the modifier is referred to as the "condition". Its truth or falsehood determines how the modifier will behave. C executes the statement once I and only if the condition is true. C is the opposite, it executes the statement I -the condition is true (i.e., if the condition is false). +the condition is true (that is, if the condition is false). print "Basset hounds got long ears" if length $ear >= 10; go_outside() and play() unless $is_raining; -C executes the statement I C<$_> smart matches C, and -then either Cs out if it's enclosed in a C scope or skips -to the C element when it lies directly inside a C loop. -See also L. - - given ($something) { - $abc = 1 when /^abc/; - $just_a = 1 when /^a/; - $other = 1; - } - - for (@names) { - admin($_) when [ qw/Alice Bob/ ]; - regular($_) when [ qw/Chris David Ellen/ ]; - } - -The C modifier is an iterator: it executes the statement once +The C modifier is an iterator: it executes the statement once for each item in the LIST (with C<$_> aliased to each item in turn). - print "Hello $_!\n" foreach qw(world Dolly nurse); + print "Hello $_!\n" for qw(world Dolly nurse); C repeats the statement I the condition is true. C does the opposite, it repeats the statement I the @@ -172,14 +166,15 @@ The C and C modifiers have the usual "C loop" semantics (conditional evaluated first), except when applied to a -C-BLOCK (or to the deprecated C-SUBROUTINE statement), in -which case the block executes once before the conditional is -evaluated. This is so that you can write loops like: +C- BLOCK (or to the Perl4 C-SUBROUTINE statement), in which +case the block executes once before the conditional is evaluated. + +This is so that you can write loops like: do { $line = ; ... - } until $line eq ".\n"; + } until !defined($line) || $line eq ".\n" See L. Note also that the loop control statements described later will I work in this construct, because modifiers don't take @@ -196,24 +191,36 @@ For C, you have to be more elaborate: X - LOOP: { + LOOP: { do { last if $x = $y**2; # do something here } while $x++ <= $z; } -B The behaviour of a C statement modified with a statement -modifier conditional or loop construct (e.g. C) is -B. The value of the C variable may be C, any -previously assigned value, or possibly anything else. Don't rely on -it. Future versions of perl might do something different from the -version of perl you try it out on. Here be dragons. -X +B The behaviour of a C, C, or C modified with a +statement modifier conditional or loop construct (for example, C) is B. The value of the C variable may be C, +any previously assigned value, or possibly anything else. Don't rely on +it. Future versions of Perl might do something different from the version +of Perl you try it out on. Here be dragons. X + +The C modifier is an experimental feature that first appeared in Perl +5.14. To use it, you should include a C declaration. +(Technically, it requires only the C feature, but that aspect of it +was not available before 5.14.) Operative only from within a C +loop or a C block, it executes the statement only if the smartmatch +C<< $_ ~~ I >> is true. If the statement executes, it is followed by +a C from inside a C and C from inside a C. + +Under the current implementation, the C loop can be +anywhere within the C modifier's dynamic scope, but must be +within the C block's lexical scope. This restricted may +be relaxed in a future release. See L<"Switch statements"> below. =head2 Compound Statements X X X X X -X<{> X<}> X X X X X X X +X<{> X<}> X X X X X X X X In Perl, a sequence of statements that defines a scope is called a block. Sometimes a block is delimited by the file containing it (in the case @@ -227,29 +234,45 @@ if (EXPR) BLOCK if (EXPR) BLOCK else BLOCK + if (EXPR) BLOCK elsif (EXPR) BLOCK ... if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK + unless (EXPR) BLOCK unless (EXPR) BLOCK else BLOCK + unless (EXPR) BLOCK elsif (EXPR) BLOCK ... unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK + + given (EXPR) BLOCK + LABEL while (EXPR) BLOCK LABEL while (EXPR) BLOCK continue BLOCK + LABEL until (EXPR) BLOCK LABEL until (EXPR) BLOCK continue BLOCK + LABEL for (EXPR; EXPR; EXPR) BLOCK + + LABEL foreach (LIST) BLOCK + LABEL foreach (LIST) BLOCK continue BLOCK LABEL foreach VAR (LIST) BLOCK LABEL foreach VAR (LIST) BLOCK continue BLOCK + + LABEL BLOCK LABEL BLOCK continue BLOCK -Note that, unlike C and Pascal, these are defined in terms of BLOCKs, +The experimental C statement is I; see +L below for how to do so, and the attendant caveats. + +Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs, not statements. This means that the curly brackets are I--no dangling statements allowed. If you want to write conditionals without -curly brackets there are several other ways to do it. The following +curly brackets, there are several other ways to do it. The following all do the same thing: - if (!open(FOO)) { die "Can't open $FOO: $!"; } + if (!open(FOO)) { die "Can't open $FOO: $!" } die "Can't open $FOO: $!" unless open(FOO); - open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! - open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; + open(FOO) || die "Can't open $FOO: $!"; + open(FOO) ? () : die "Can't open $FOO: $!"; # a bit exotic, that last one The C statement is straightforward. Because BLOCKs are always @@ -261,18 +284,15 @@ language construct, as everyone reading your code will have to think at least twice before they can understand what's going on. -The C statement executes the block as long as the expression is -L. -The C statement executes the block as long as the expression is -false. -The LABEL is optional, and if present, consists of an identifier followed -by a colon. The LABEL identifies the loop for the loop control -statements C, C, and C. -If the LABEL is omitted, the loop control statement -refers to the innermost enclosing loop. This may include dynamically -looking back your call-stack at run time to find the LABEL. Such -desperate behavior triggers a warning if you use the C -pragma or the B<-w> flag. +The C statement executes the block so long as the expression is +L. The C statement executes the block +so long as the expression is false. The LABEL is optional, and if present, +consists of an identifier followed by a colon. The LABEL identifies the +loop for the loop control statements C, C, and C. If the +LABEL is omitted, the loop control statement refers to the innermost +enclosing loop. This may include dynamically looking back your call-stack +at run time to find the LABEL. Such desperate behaviour triggers a warning +if you use the C pragma or the C> flag. If there is a C BLOCK, it is always executed just before the conditional is about to be evaluated again. Thus it can be used to @@ -280,7 +300,7 @@ the C statement. Extension modules can also hook into the Perl parser to define new -kinds of compound statement. These are introduced by a keyword which +kinds of compound statements. These are introduced by a keyword which the extension recognizes, and the syntax following the keyword is defined entirely by the extension. If you are an implementor, see L for the mechanism. If you are using such @@ -310,9 +330,9 @@ This command is normally used by programs that want to lie to themselves about what was just input. -For example, when processing a file like F. -If your input lines might end in backslashes to indicate continuation, you -want to skip ahead and get the next record. +For example, when processing a file like F. If your input +lines might end in backslashes to indicate continuation, you want to skip +ahead and get the next record. while (<>) { chomp; @@ -323,7 +343,7 @@ # now process $_ } -which is Perl short-hand for the more explicitly written version: +which is Perl shorthand for the more explicitly written version: LINE: while (defined($line = )) { chomp($line); @@ -354,7 +374,7 @@ test is reversed, but the conditional is still tested before the first iteration. -The loop control statements don't work in an C or C, since +Loop control statements don't work in an C or C, since they aren't loops. You can double the braces to make them such, though. if (/pattern/) {{ @@ -431,9 +451,7 @@ X X The C keyword is actually a synonym for the C keyword, so -you can use C for readability or C for brevity. (Or because -the Bourne shell is more familiar to you than I, so writing C -comes more naturally.) If VAR is omitted, C<$_> is set to each value. +you can use either. If VAR is omitted, C<$_> is set to each value. X<$_> If any element of LIST is an lvalue, you can modify it by modifying @@ -459,8 +477,9 @@ $elem *= 2; } - for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { - print $count, "\n"; sleep(1); + for $count (reverse(1..10), "BOOM") { + print $count, "\n"; + sleep(1); } for (1..15) { print "Merry Christmas\n"; } @@ -489,7 +508,7 @@ next OUTER if $wid > $jet; $wid += $jet; } - } + } See how much easier this is? It's cleaner, safer, and faster. It's cleaner because it's less noisy. It's safer because if code gets added @@ -518,34 +537,336 @@ $nothing = 1; } -Such constructs are quite frequently used, because older versions -of Perl had no official C statement. +You'll also find that C loop used to create a topicalizer +and a switch: + + SWITCH: + for ($var) { + if (/^abc/) { $abc = 1; last SWITCH; } + if (/^def/) { $def = 1; last SWITCH; } + if (/^xyz/) { $xyz = 1; last SWITCH; } + $nothing = 1; + } + +Such constructs are quite frequently used, both because older versions of +Perl had no official C statement, and also because the new version +described immediately below remains experimental and can sometimes be confusing. -=head2 Switch statements +=head2 Switch Statements X X X X X -Starting from Perl 5.10, you can say +Starting from Perl 5.10.1 (well, 5.10.0, but it didn't work +right), you can say use feature "switch"; -which enables a switch feature that is closely based on the -Perl 6 proposal. +to enable an experimental switch feature. This is loosely based on an +old version of a Perl 6 proposal, but it no longer resembles the Perl 6 +construct. You also get the switch feature whenever you declare that your +code prefers to run under a version of Perl that is 5.10 or later. For +example: + + use v5.14; + +Under the "switch" feature, Perl gains the experimental keywords C, +C, C, C, and C. The keywords C and +C are analogous to C and C in other languages, so the +code in the previous section could be rewritten as + + use v5.10.1; + for ($var) { + when (/^abc/) { $abc = 1 } + when (/^def/) { $def = 1 } + when (/^xyz/) { $xyz = 1 } + default { $nothing = 1 } + } + +The C is the non-experimental way to set a topicalizer. +If you wish to use the highly experimental C, that could be +written like this: + + use v5.10.1; + given ($var) { + when (/^abc/) { $abc = 1 } + when (/^def/) { $def = 1 } + when (/^xyz/) { $xyz = 1 } + default { $nothing = 1 } + } + +As fo 5.14, that can also be written this way: + + use v5.14; + for ($var) { + $abc = 1 when /^abc/; + $def = 1 when /^def/; + $xyz = 1 when /^xyz/; + default { $nothing = 1 } + } + +Or if you don't care to play it safe, like this: + + use v5.14; + given ($var) { + $abc = 1 when /^abc/; + $def = 1 when /^def/; + $xyz = 1 when /^xyz/; + default { $nothing = 1 } + } + +The arguments to C and C are in scalar context, +and C assigns the C<$_> variable its topic value. + +Exactly what the I argument to C does is hard to describe +precisely, but in general, it tries to guess what you want done. Sometimes +it is interpreted as C<< $_ ~~ I >>, and sometimes it does not. It +also behaves differently when lexically enclosed by a C block than +it does when dynamically enclosed by a C loop. The rules are far +too difficult to understand to be described here. See L later on. + +Due to an unfortunate bug in how C was implemented between Perl 5.10 +and 5.14, under those implementations the version of C<$_> governed by +C is merely a lexically scoped copy of the original, not a +dynamically scoped alias to the original, as it would be if it were a +C or under both the original and the current Perl 6 language +specification. This bug is expected to be addressed in a future release of +Perl. For forwards compatibility, if you really want a lexical C<$_>, +specify that explicitly: + + given(my $_ = EXPR) { ... } + +In the meanwhile, stick to C for your topicalizer and +you will be less unhappy. + +=head2 Goto +X + +Although not for the faint of heart, Perl does support a C +statement. There are three forms: C-LABEL, C-EXPR, and +C-&NAME. A loop's LABEL is not actually a valid target for +a C; it's just the name of the loop. + +The C-LABEL form finds the statement labeled with LABEL and resumes +execution there. It may not be used to go into any construct that +requires initialization, such as a subroutine or a C loop. It +also can't be used to go into a construct that is optimized away. It +can be used to go almost anywhere else within the dynamic scope, +including out of subroutines, but it's usually better to use some other +construct such as C or C. The author of Perl has never felt the +need to use this form of C (in Perl, that is--C is another matter). + +The C-EXPR form expects a label name, whose scope will be resolved +dynamically. This allows for computed Cs per FORTRAN, but isn't +necessarily recommended if you're optimizing for maintainability: + + goto(("FOO", "BAR", "GLARCH")[$i]); + +The C-&NAME form is highly magical, and substitutes a call to the +named subroutine for the currently running subroutine. This is used by +C subroutines that wish to load another subroutine and then +pretend that the other subroutine had been called in the first place +(except that any modifications to C<@_> in the current subroutine are +propagated to the other subroutine.) After the C, not even C +will be able to tell that this routine was called first. + +In almost all cases like this, it's usually a far, far better idea to use the +structured control flow mechanisms of C, C, or C instead of +resorting to a C. For certain applications, the catch and throw pair of +C and die() for exception processing can also be a prudent approach. + +=head2 The Ellipsis Statement + +X<...> +X<... statement> +X +X +X +X +X + +Beginning in Perl 5.12, Perl accepts an ellipsis, "C<...>", as a +placeholder for code that you haven't implemented yet. This form of +ellipsis, the unimplemented statement, should not be confused with the +binary flip-flop C<...> operator. One is a statement and the other an +operator. (Perl doesn't usually confuse them because usually Perl can tell +whether it wants an operator or a statement, but see below for exceptions.) + +When Perl 5.12 or later encounters an ellipses statement, it parses this +without error, but if and when you should actually try to execute it, Perl +throws an exception with the text C: + + use v5.12 + sub unimplemented { ... } + eval { unimplemented() }; + if ($@ eq "Unimplemented") { + say "I found an ellipsis!"; + } + +You can only use the elliptical statement to stand in for a +complete statement. These examples of how the ellipsis works: + + use v5.12; + { ... } + sub foo { ... } + ...; + eval { ... }; + sub somemeth { + my $self = shift; + ...; + } + $x = do { + my $n; + ...; + say "Hurrah!"; + $n; + }; + +The elliptical statement cannot stand in for an expression that +is part of a larger statement, since the C<...> is also the three-dot +version of the flip-flop operator (see L). + +These examples of attempts to use an ellipsis are syntax errors: + + use v5.12; + + print ...; + open(my $fh, ">", "/dev/passwd") or ...; + if ($condition && ... ) { say "Howdy" }; + +There are some cases where Perl can't immediately tell the difference +between an expression and a statement. For instance, the syntax for a block +and an anonymous hash reference constructor look the same unless there's +something in the braces to give Perl a hint. The ellipsis is a syntax +error if Perl doesn't guess that the C<{ ... }> is a block. In that case, +it doesn't think the C<...> is an ellipsis because it's expecting an +expression instead of a statement: + + @transformed = map { ... } @input; # syntax error + +You can use a C<;> inside your block to denote that the C<{ ... }> is a +block and not a hash reference constructor. Now the ellipsis works: + + @transformed = map {; ... } @input; # ; disambiguates + + @transformed = map { ...; } @input; # ; disambiguates + +Note: Some folks colloquially refer to this bit of punctuation as a +"yada-yada", but its true name is actually an ellipsis. Perl does not yet +accept the Unicode version, U+2026 HORIZONTAL ELLIPSIS, as an alias for +C<...>, but someday it may. + +=head2 PODs: Embedded Documentation +X X + +Perl has a mechanism for intermixing documentation with source code. +While it's expecting the beginning of a new statement, if the compiler +encounters a line that begins with an equal sign and a word, like this + + =head1 Here There Be Pods! + +Then that text and all remaining text up through and including a line +beginning with C<=cut> will be ignored. The format of the intervening +text is described in L. -The keywords C and C are analogous -to C and C in other languages, so the code -above could be written as +This allows you to intermix your source code +and your documentation text freely, as in - given($_) { - when (/^abc/) { $abc = 1; } - when (/^def/) { $def = 1; } - when (/^xyz/) { $xyz = 1; } - default { $nothing = 1; } + =item snazzle($) + + The snazzle() function will behave in the most spectacular + form that you can possibly imagine, not even excepting + cybernetic pyrotechnics. + + =cut back to the compiler, nuff of this pod stuff! + + sub snazzle($) { + my $thingie = shift; + ......... } -This construct is very flexible and powerful. For example: +Note that pod translators should look at only paragraphs beginning +with a pod directive (it makes parsing easier), whereas the compiler +actually knows to look for pod escapes even in the middle of a +paragraph. This means that the following secret stuff will be +ignored by both the compiler and the translators. + + $a=3; + =secret stuff + warn "Neither POD nor CODE!?" + =cut back + print "got $a\n"; + +You probably shouldn't rely upon the C being podded out forever. +Not all pod translators are well-behaved in this regard, and perhaps +the compiler will become pickier. + +One may also use pod directives to quickly comment out a section +of code. + +=head2 Plain Old Comments (Not!) +X X X<#> X X + +Perl can process line directives, much like the C preprocessor. Using +this, one can control Perl's idea of filenames and line numbers in +error or warning messages (especially for strings that are processed +with C). The syntax for this mechanism is almost the same as for +most C preprocessors: it matches the regular expression + + # example: '# line 42 "new_filename.plx"' + /^\# \s* + line \s+ (\d+) \s* + (?:\s("?)([^"]+)\g2)? \s* + $/x + +with C<$1> being the line number for the next line, and C<$3> being +the optional filename (specified with or without quotes). Note that +no whitespace may precede the C<< # >>, unlike modern C preprocessors. + +There is a fairly obvious gotcha included with the line directive: +Debuggers and profilers will only show the last source line to appear +at a particular line number in a given file. Care should be taken not +to cause line number collisions in code you'd like to debug later. + +Here are some examples that you should be able to type into your command +shell: + + % perl + # line 200 "bzzzt" + # the '#' on the previous line must be the first char on line + die 'foo'; + __END__ + foo at bzzzt line 201. + + % perl + # line 200 "bzzzt" + eval qq[\n#line 2001 ""\ndie 'foo']; print $@; + __END__ + foo at - line 2001. + + % perl + eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; + __END__ + foo at foo bar line 200. + + % perl + # line 345 "goop" + eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; + print $@; + __END__ + foo at goop line 345. + +=head2 Experimental Details on given and when + +As previously mentioned, the "switch" feature is considered highly +experimental; it is subject to change with little notice. In particular, +both C and C have tricky behaviours that are expected to +change to become less tricky in the future. Do not rely upon their +current (mis)implementations. + +Here is a longer example of C: use feature ":5.10"; - given($foo) { + given ($foo) { when (undef) { say '$foo is undefined'; } @@ -567,134 +888,154 @@ } } -C will assign the value of EXPR to C<$_> -within the lexical scope of the block, so it's similar to +As currently implemented, C assign the value of I to +merely a lexically scoped I> (!) of C<$_>, not a dynamically +scoped alias the way C does. That makes it similar to do { my $_ = EXPR; ... } except that the block is automatically broken out of by a -successful C or an explicit C. - -Most of the power comes from implicit smart matching: +successful C or an explicit C. Because it is only a +copy, and because it is only lexically scoped, not dynamically +scoped, you cannot do the things with it that you are used to in +a C loop. In particular, you probably cannot use +arbitrary function calls. Best stick to C for that. + +Most of the power comes from the implicit smartmatching that can +sometimes apply. Most of the time, C is treated as an +implicit smartmatch of C<$_>, that is, C<$_ ~~ EXPR>. (See +L for more information on smartmatching.) +But when I is one of the 10 exceptional cases (or things like them) +listed below, it is used directly as a boolean. - when($foo) +=over 4 -is exactly equivalent to +=item 1. - when($_ ~~ $foo) +A user-defined subroutine call or a method invocation. -Most of the time, C is treated as an implicit smart match of -C<$_>, i.e. C<$_ ~~ EXPR>. (See L for more -information on smart matching.) But when EXPR is one of the below -exceptional cases, it is used directly as a boolean: +=item 3. -=over 4 +A regular expression match in the form of C, C<$foo =~ /REGEX/>, +or C<$foo =~ EXPR>. Also, a negated regular expression match in +the form C, C<$foo !~ /REGEX/>, or C<$foo !~ EXPR>. -=item * +=item 3. -a subroutine or method call - -=item * +A smart match that uses an explicit C<~~> operator, such as C. -a regular expression match, i.e. C or C<$foo =~ /REGEX/>, -or a negated regular expression match (C or C<$foo !~ /REGEX/>). +=item 4. -=item * +A boolean comparison operator such as C<$_ E 10> or C<$x eq "abc"> The +relational operators that this applies to are the six numeric comparisons +(C<< < >>, C<< > >>, C<< <= >>, C<< >= >>, C<< == >>, and C<< != >>), and +the six string comparisons (C, C, C, C, C, and C). -a comparison such as C<$_ E 10> or C<$x eq "abc"> -(or of course C<$_ ~~ $c>) +B You will often have to use C<$c ~~ $_> because +the default case uses C<$_ ~~ $c> , which is frequently +the opposite of what you want. -=item * +=item 5. -C, C, or C +At least the three builtin functions C, C, and +C. We might someday add more of these later if we think of them. -=item * +=item 6. -a negated expression C or C, or a logical -exclusive-or C<(...) xor (...)>. +A negated expression, whether C or C, or a logical +exclusive-or, C<(EXPR1) xor (EXPR2)>. The bitwise versions (C<~> and C<^>) +are not included. -=item * +=item 7. -a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>, -that return numerical values, not boolean ones. +A filetest operator, with exactly 4 exceptions: C<-s>, C<-M>, C<-A>, and +C<-C>, as these return numerical values, not boolean ones. The C<-z> +filetest operator is not included in the exception list. -=item * +=item 8. -the C<..> and C<...> flip-flop operators. +The C<..> and C<...> flip-flop operators. Note that the C<...> flip-flop +operator is completely different from the C<...> elliptical statement +just described. =back -In those cases the value of EXPR is used directly as a boolean. +In those 8 cases above, the value of EXPR is used directly as a boolean, so +no smartmatching is done. You may think of C as a smartsmartmatch. -Furthermore, Perl inspects the operands of the binary boolean operators to -decide whether to use smart matching for each one by applying the above test to -the operands: +Furthermore, Perl inspects the operands of logical operators to +decide whether to use smartmatching for each one by applying the +above test to the operands: =over 4 -=item * - -If EXPR is C<... && ...> or C<... and ...>, the test -is applied recursively to both operands. If I -operands pass the test, then the expression is treated -as boolean; otherwise, smart matching is used. +=item 9. -=item * - -If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test -is applied recursively to the first operand (which may be a -higher-precedence AND operator, for example). If the first operand -is to use smart matching, then both operands will do so; if it is -not, then the second argument will not be either. +If EXPR is C or C, the test is applied +I to both EXPR1 and EXPR2. Only if I operands also pass the +test, I, will the expression be treated as boolean. Otherwise, +smartmatching is used. + +=item 10. + +If EXPR is C, C, or C, the +test is applied I to EXPR1 only (which might itself be a +higher-precedence AND operator, for example, and thus subject to the +previous rule), not to EXPR2. If EXPR1 is to use smartmatching, then EXPR2 +also does so, no matter what EXPR2 contains. But if EXPR2 does not get to +use smartmatching, then the second argument will not be either. This is +quite different from the C<&&> case just described, so be careful. =back -These rules look complicated, but usually they will do what -you want. For example: +These rules are complicated, but the goal is for them to do what you want +(even if you don't quite understand why they are doing it). For example: when (/^\d+$/ && $_ < 75) { ... } -will be treated as a boolean match because the rules say both a regex match and -an explicit test on $_ will be treated as boolean. +will be treated as a boolean match because the rules say both +a regex match and an explicit test on C<$_> will be treated +as boolean. Also: when ([qw(foo bar)] && /baz/) { ... } -will use smart matching because only I of the operands is a boolean; the -other uses smart matching, and that wins. +will use smartmatching because only I of the operands is a boolean: +the other uses smartmatching, and that wins. Further: when ([qw(foo bar)] || /^baz/) { ... } -will use smart matching (only the first operand is considered), whereas +will use smartmatching (only the first operand is considered), whereas when (/^baz/ || [qw(foo bar)]) { ... } -will test only the regex, which causes both operands to be treated as boolean. -Watch out for this one, then, because an arrayref is always a true value, which -makes it effectively redundant. - -Tautologous boolean operators are still going to be optimized away. Don't be -tempted to write - - when ('foo' or 'bar') { ... } - -This will optimize down to C<'foo'>, so C<'bar'> will never be considered (even -though the rules say to use a smart match on C<'foo'>). For an alternation like -this, an array ref will work, because this will instigate smart matching: +will test only the regex, which causes both operands to be +treated as boolean. Watch out for this one, then, because an +arrayref is always a true value, which makes it effectively +redundant. Not a good idea. + +Tautologous boolean operators are still going to be optimized +away. Don't be tempted to write + + when ("foo" or "bar") { ... } + +This will optimize down to C<"foo">, so C<"bar"> will never be considered (even +though the rules say to use a smartmatch on C<"foo">). For an alternation like +this, an array ref will work, because this will instigate smartmatching: when ([qw(foo bar)] { ... } This is somewhat equivalent to the C-style switch statement's fallthrough -functionality (not to be confused with I fallthrough functionality - see -below), wherein the same block is used for several C statements. - -Another useful shortcut is that, if you use a literal array -or hash as the argument to C, it is turned into a -reference. So C is the same as C, -for example. +functionality (not to be confused with I fallthrough +functionality--see below), wherein the same block is used for several +C statements. + +Another useful shortcut is that, if you use a literal array or hash as the +argument to C, it is turned into a reference. So C is +the same as C, for example. C behaves exactly like C, which is to say that it always matches. @@ -712,29 +1053,29 @@ given($foo) { when (/x/) { say '$foo contains an x'; continue } - when (/y/) { say '$foo contains a y' } - default { say '$foo does not contain a y' } + when (/y/) { say '$foo contains a y' } + default { say '$foo does not contain a y' } } =head3 Return value -When a C statement is also a valid expression (e.g. -when it's the last statement of a block), it evaluates to : +When a C statement is also a valid expression (for example, +when it's the last statement of a block), it evaluates to: =over 4 =item * -an empty list as soon as an explicit C is encountered. +An empty list as soon as an explicit C is encountered. =item * -the value of the last evaluated expression of the successful -C/C clause, if there's one. +The value of the last evaluated expression of the successful +C/C clause, if there happens to be one. =item * -the value of the last evaluated expression of the C block if no +The value of the last evaluated expression of the C block if no condition is true. =back @@ -745,15 +1086,17 @@ Note that, unlike C and C, failed C statements always evaluate to an empty list. - my $price = do { given ($item) { - when ([ 'pear', 'apple' ]) { 1 } - break when 'vote'; # My vote cannot be bought - 1e10 when /Mona Lisa/; - 'unknown'; - } }; + my $price = do { + given ($item) { + when (["pear", "apple"]) { 1 } + break when "vote"; # My vote cannot be bought + 1e10 when /Mona Lisa/; + "unknown"; + } + }; -Currently, C blocks can't always be used as proper expressions. This -may be addressed in a future version of perl. +Currently, C blocks can't always be used as proper +expressions. This may be addressed in a future version of Perl. =head3 Switching in a loop @@ -761,256 +1104,84 @@ For example, here's one way to count how many times a particular string occurs in an array: + use v5.10.1; my $count = 0; for (@array) { when ("foo") { ++$count } } print "\@array contains $count copies of 'foo'\n"; -At the end of all C blocks, there is an implicit C. -You can override that with an explicit C if you're only -interested in the first match. +Or in a more recent version: + + use v5.14; + my $count = 0; + for (@array) { + ++$count when "foo"; + } + print "\@array contains $count copies of 'foo'\n"; -This doesn't work if you explicitly specify a loop variable, -as in C. You have to use the default -variable C<$_>. (You can use C.) - -=head3 Smart matching in detail - -The behaviour of a smart match depends on what type of thing its arguments -are. The behaviour is determined by the following table: the first row -that applies determines the match behaviour (which is thus mostly -determined by the type of the right operand). Note that the smart match -implicitly dereferences any non-blessed hash or array ref, so the "Hash" -and "Array" entries apply in those cases. (For blessed references, the -"Object" entries apply.) - -Note that the "Matching Code" column is not always an exact rendition. For -example, the smart match operator short-circuits whenever possible, but -C does not. - - $a $b Type of Match Implied Matching Code - ====== ===== ===================== ============= - Any undef undefined !defined $a - - Any Object invokes ~~ overloading on $object, or dies - - Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a - Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a - Any CodeRef scalar sub truth $b->($a) - - Hash Hash hash keys identical (every key is found in both hashes) - Array Hash hash keys intersection grep { exists $b->{$_} } @$a - Regex Hash hash key grep grep /$a/, keys %$b - undef Hash always false (undef can't be a key) - Any Hash hash entry existence exists $b->{$a} - - Hash Array hash keys intersection grep { exists $a->{$_} } @$b - Array Array arrays are comparable[2] - Regex Array array grep grep /$a/, @$b - undef Array array contains undef grep !defined, @$b - Any Array match against an array element[3] - grep $a ~~ $_, @$b - - Hash Regex hash key grep grep /$b/, keys %$a - Array Regex array grep grep /$b/, @$a - Any Regex pattern match $a =~ /$b/ - - Object Any invokes ~~ overloading on $object, or falls back: - Any Num numeric equality $a == $b - Num numish[4] numeric equality $a == $b - undef Any undefined !defined($b) - Any Any string equality $a eq $b - - 1 - empty hashes or arrays will match. - 2 - that is, each element smart-matches the element of same index in the - other array. [3] - 3 - If a circular reference is found, we fall back to referential equality. - 4 - either a real number, or a string that looks like a number - -=head3 Custom matching via overloading - -You can change the way that an object is matched by overloading -the C<~~> operator. This may alter the usual smart match semantics. - -It should be noted that C<~~> will refuse to work on objects that -don't overload it (in order to avoid relying on the object's -underlying structure). - -Note also that smart match's matching rules take precedence over -overloading, so if C<$obj> has smart match overloading, then - - $obj ~~ X - -will not automatically invoke the overload method with X as an argument; -instead the table above is consulted as normal, and based in the type of X, -overloading may or may not be invoked. +At the end of all C blocks, there is an implicit C. +You can override that with an explicit C if you're +interested in only the first match alone. -See L. +This doesn't work if you explicitly specify a loop variable, as +in C. You have to use the default variable C<$_>. =head3 Differences from Perl 6 -The Perl 5 smart match and C/C constructs are not -absolutely identical to their Perl 6 analogues. The most visible -difference is that, in Perl 5, parentheses are required around -the argument to C and C (except when this last -one is used as a statement modifier). Parentheses in Perl 6 -are always optional in a control construct such as C, -C, or C; they can't be made optional in Perl -5 without a great deal of potential confusion, because Perl 5 -would parse the expression - - given $foo { - ... - } +The Perl 5 smartmatch and C/C constructs are not compatible +with their Perl 6 analogues. The most visible difference and least +important difference is that, in Perl 5, parentheses are required around +the argument to C and C (except when this last one is used +as a statement modifier). Parentheses in Perl 6 are always optional in a +control construct such as C, C, or C; they can't be +made optional in Perl 5 without a great deal of potential confusion, +because Perl 5 would parse the expression + + given $foo { + ... + } as though the argument to C were an element of the hash C<%foo>, interpreting the braces as hash-element syntax. -The table of smart matches is not identical to that proposed by the -Perl 6 specification, mainly due to the differences between Perl 6's -and Perl 5's data models. - -In Perl 6, C will always do an implicit smart match -with its argument, whilst it is convenient in Perl 5 to -suppress this implicit smart match in certain situations, -as documented above. (The difference is largely because Perl 5 -does not, even internally, have a boolean type.) - -=head2 Goto -X - -Although not for the faint of heart, Perl does support a C -statement. There are three forms: C-LABEL, C-EXPR, and -C-&NAME. A loop's LABEL is not actually a valid target for -a C; it's just the name of the loop. - -The C-LABEL form finds the statement labeled with LABEL and resumes -execution there. It may not be used to go into any construct that -requires initialization, such as a subroutine or a C loop. It -also can't be used to go into a construct that is optimized away. It -can be used to go almost anywhere else within the dynamic scope, -including out of subroutines, but it's usually better to use some other -construct such as C or C. The author of Perl has never felt the -need to use this form of C (in Perl, that is--C is another matter). - -The C-EXPR form expects a label name, whose scope will be resolved -dynamically. This allows for computed Cs per FORTRAN, but isn't -necessarily recommended if you're optimizing for maintainability: - - goto(("FOO", "BAR", "GLARCH")[$i]); - -The C-&NAME form is highly magical, and substitutes a call to the -named subroutine for the currently running subroutine. This is used by -C subroutines that wish to load another subroutine and then -pretend that the other subroutine had been called in the first place -(except that any modifications to C<@_> in the current subroutine are -propagated to the other subroutine.) After the C, not even C -will be able to tell that this routine was called first. - -In almost all cases like this, it's usually a far, far better idea to use the -structured control flow mechanisms of C, C, or C instead of -resorting to a C. For certain applications, the catch and throw pair of -C and die() for exception processing can also be a prudent approach. - -=head2 PODs: Embedded Documentation -X X - -Perl has a mechanism for intermixing documentation with source code. -While it's expecting the beginning of a new statement, if the compiler -encounters a line that begins with an equal sign and a word, like this - - =head1 Here There Be Pods! - -Then that text and all remaining text up through and including a line -beginning with C<=cut> will be ignored. The format of the intervening -text is described in L. - -This allows you to intermix your source code -and your documentation text freely, as in - - =item snazzle($) - - The snazzle() function will behave in the most spectacular - form that you can possibly imagine, not even excepting - cybernetic pyrotechnics. +However, their are many, many other differences. For example, +this works in Perl 5: - =cut back to the compiler, nuff of this pod stuff! + use v5.12; + my @primary = ("red", "blue", "green"); - sub snazzle($) { - my $thingie = shift; - ......... + if (@primary ~~ "red") { + say "primary smartmatches red"; } -Note that pod translators should look at only paragraphs beginning -with a pod directive (it makes parsing easier), whereas the compiler -actually knows to look for pod escapes even in the middle of a -paragraph. This means that the following secret stuff will be -ignored by both the compiler and the translators. - - $a=3; - =secret stuff - warn "Neither POD nor CODE!?" - =cut back - print "got $a\n"; - -You probably shouldn't rely upon the C being podded out forever. -Not all pod translators are well-behaved in this regard, and perhaps -the compiler will become pickier. - -One may also use pod directives to quickly comment out a section -of code. - -=head2 Plain Old Comments (Not!) -X X X<#> X X - -Perl can process line directives, much like the C preprocessor. Using -this, one can control Perl's idea of filenames and line numbers in -error or warning messages (especially for strings that are processed -with C). The syntax for this mechanism is almost the same as for -most C preprocessors: it matches the regular expression + if ("red" ~~ @primary) { + say "red smartmatches primary"; + } - # example: '# line 42 "new_filename.plx"' - /^\# \s* - line \s+ (\d+) \s* - (?:\s("?)([^"]+)\g2)? \s* - $/x + say "that's all, folks!"; -with C<$1> being the line number for the next line, and C<$3> being -the optional filename (specified with or without quotes). Note that -no whitespace may precede the C<< # >>, unlike modern C preprocessors. +But it doesn't work at all in Perl 6. Instead, you should +use the (parallelizable) C operator instead: -There is a fairly obvious gotcha included with the line directive: -Debuggers and profilers will only show the last source line to appear -at a particular line number in a given file. Care should be taken not -to cause line number collisions in code you'd like to debug later. + if any(@primary) eq "red" { + say "primary smartmatches red"; + } -Here are some examples that you should be able to type into your command -shell: + if "red" eq any(@primary) { + say "red smartmatches primary"; + } - % perl - # line 200 "bzzzt" - # the `#' on the previous line must be the first char on line - die 'foo'; - __END__ - foo at bzzzt line 201. +The table of smartmatches in L is not +identical to that proposed by the Perl 6 specification, mainly due to +differences between Perl 6's and Perl 5's data models, but also because +the Perl 6 spec has changed since Perl 5 rushed into early adoption. - % perl - # line 200 "bzzzt" - eval qq[\n#line 2001 ""\ndie 'foo']; print $@; - __END__ - foo at - line 2001. - - % perl - eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; - __END__ - foo at foo bar line 200. - - % perl - # line 345 "goop" - eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; - print $@; - __END__ - foo at goop line 345. +In Perl 6, C will always do an implicit smartmatch with its +argument, while in Perl 5 it is convenient albeit potentially confusing) to +suppress this implicit smartmatch in various rather loosely-defined +situations, as roughly outlined above. (The difference is largely because +Perl 5 does not have, even internally, a boolean type.) =cut + ```
p5pRT commented 13 years ago

From tchrist@perl.com

Here is a patch against the second patch\, fixing typos reported to me.

  perlsyn-smartmatch-patch-on-patch.patch

--tom

p5pRT commented 13 years ago

From tchrist@perl.com

perlsyn-smartmatch-patch-on-patch.patch ```diff --- perlsyn.pod 2011-05-17 22:08:45.000000000 -0600 +++ new/perlsyn.pod 2011-05-18 07:06:58.000000000 -0600 @@ -593,7 +593,7 @@ default { $nothing = 1 } } -As fo 5.14, that can also be written this way: +As of 5.14, that can also be written this way: use v5.14; for ($var) { @@ -914,7 +914,7 @@ A user-defined subroutine call or a method invocation. -=item 3. +=item 2. A regular expression match in the form of C, C<$foo =~ /REGEX/>, or C<$foo =~ EXPR>. Also, a negated regular expression match in ```
p5pRT commented 13 years ago

From @cpansprout

On Tue May 17 17​:09​:27 2011\, tom christiansen wrote​:

As previously explained\, this patch against perlsyn is part and parcel of the previous one against perlop.

--tom

patching file pod/perlsyn.pod Hunk #17 FAILED at 537. Hunk #18 succeeded at 889 (offset 1 line). Hunk #19 succeeded at 1054 (offset 1 line). Hunk #20 succeeded at 1087 (offset 1 line). patch unexpectedly ends in middle of line patch​: **** unexpected end of file in patch

p5pRT commented 13 years ago

The RT System itself - Status changed from 'new' to 'open'

p5pRT commented 12 years ago

From @cpansprout

On Tue May 17 21​:15​:56 2011\, tom christiansen wrote​:

Here's a replacement patch again perlsyn\, with the new lore folded into it. This is against the current perlsyn in blead.

It is to be applied at the same time as the perlop patch\, because they will not make sense unless applied in tandem.

Applied as c2f1e229171\, with the follow-up corrections as 8e15b1894e4.

This patch contains a section entitled ā€˜The Ellipsis Statementā€™. But another patch of yours was applied to perlop as c543c01b\, with a section entitled ā€˜The Triple-Dot Operatorā€™.

The previous patch against perlop removes the ā€˜Yada Yada Operatorā€™ section\, with the expectation that *this* patch will put it in perlsyn.

Does the name ā€˜The Triple-Dot Operatorā€™ makes sense in perlsyn?

Which version of that section do you prefer?

For now\, Iā€™ve left both in (perlop and perlsyn)\, but we obviously need to fix that before 5.16.

--

Father Chrysostomos

p5pRT commented 12 years ago

From tchrist@perl.com

On Tue May 17 21​:15​:56 2011\, tom christiansen wrote​:

Here's a replacement patch again perlsyn\, with the new lore folded into it. This is against the current perlsyn in blead.

It is to be applied at the same time as the perlop patch\, because they will not make sense unless applied in tandem.

Applied as c2f1e229171\, with the follow-up corrections as 8e15b1894e4.

This patch contains a section entitled ā€˜The Ellipsis Statementā€™. But another patch of yours was applied to perlop as c543c01b\, with a section entitled ā€˜The Triple-Dot Operatorā€™.

The previous patch against perlop removes the ā€˜Yada Yada Operatorā€™ section\, with the expectation that *this* patch will put it in perlsyn.

Does the name ā€˜The Triple-Dot Operatorā€™ makes sense in perlsyn?

Which version of that section do you prefer?

For now\, Iā€™ve left both in (perlop and perlsyn)\, but we obviously need to fix that before 5.16.

Well\, the Camel calls it an ellipsis statement\, or maybe sometimes an elliptical statement. I suspect that the two patches were me trying to zero in on that. I guess I call it triple-dot in my head\, and am trying to bring myself around to calling it a real ellipsis. Larry has a crack about the real Unicode ellipsis character in there somewhere now. I'd go for something elliptical\, since it means something was left out. Ellipsis is fine.

Thanks very much.

--tom

p5pRT commented 12 years ago

From @cpansprout

On Thu Jan 05 20​:35​:42 2012\, tom christiansen wrote​:

On Tue May 17 21​:15​:56 2011\, tom christiansen wrote​:

Here's a replacement patch again perlsyn\, with the new lore folded into it. This is against the current perlsyn in blead.

It is to be applied at the same time as the perlop patch\, because they will not make sense unless applied in tandem.

Applied as c2f1e229171\, with the follow-up corrections as 8e15b1894e4.

This patch contains a section entitled ā€˜The Ellipsis Statementā€™. But another patch of yours was applied to perlop as c543c01b\, with a section entitled ā€˜The Triple-Dot Operatorā€™.

The previous patch against perlop removes the ā€˜Yada Yada Operatorā€™ section\, with the expectation that *this* patch will put it in perlsyn.

Does the name ā€˜The Triple-Dot Operatorā€™ makes sense in perlsyn?

Which version of that section do you prefer?

For now\, Iā€™ve left both in (perlop and perlsyn)\, but we obviously need to fix that before 5.16.

Well\, the Camel calls it an ellipsis statement\, or maybe sometimes an elliptical statement. I suspect that the two patches were me trying to zero in on that. I guess I call it triple-dot in my head\, and am trying to bring myself around to calling it a real ellipsis. Larry has a crack about the real Unicode ellipsis character in there somewhere now. I'd go for something elliptical\, since it means something was left out. Ellipsis is fine.

Thanks very much.

OK\, Iā€™ve removed the triple-dot section from perlop in commit c7102404 and added it as an alias to perlsyn with commit 81104cdf1.

--

Father Chrysostomos

p5pRT commented 12 years ago

@cpansprout - Status changed from 'open' to 'resolved'