infix operators

[Ingo60]

It looks like infix declarations are not honoured, nor imports of packages that have infix operators defined:

frege> import frege.control.Monoid
frege> :t `<>`
12: syntax error, the next definition cannot
start with token "`<>`"
frege> :t (<>)
12: syntax error on end of file
12: unexpected '}' while trying to parse an
annotated item
frege> Sum 0 <> Sum 1
12: syntax error on end of file
12: unexpected '}' while trying to parse
left hand side of a function or pattern
binding
frege>  

[Ingo60]

The following works, however:

frege> :t (+)
Num α => α -> α -> α
frege> :t (<)
Ord α => α -> α -> Bool
frege>  

[Ingo60]

As well as this:

frege> :t (`<>`)
Semigroup α => α -> α -> α
frege> (`<>`) (Sum 0) (Sum 1)
res4 = Sum 1
frege> Sum 0 `<>` Sum 1
res5 = Sum 1

This is, of course, not exactly a nice workaround, but it shows at least that the symbol <> is well known, just the scanner doesn't know.

[mmhelloworld]

Considering the following: 1) Sum 0 <> Sum 1 is not working but foo = Sum 0 <> Sum 1 is working 2) To parse <>, the imports must be in scope

I thought that instead of trying to directly parse an expression from the grammar, we could use the following template to check whether the script is an expression:

module SomeName where
<previous scripts>
someName = (<current script>)

On this template after applying the scripts, we will just invoke the lexer and parser and if both succeed, it is an expression otherwise it is either an invalid script or a set of declarations. It turned out to be working well:

frege> :t <>
8: syntax error, the next definition cannot
start with token "<"
frege> import frege.control.Monoid

frege> :t <>
Semigroup α => α -> α -> α
frege> :t +
Num α => α -> α -> α
frege> Sum 1 <> Sum 0
res5 = Sum 1

This fix is available in App Engine as well as frege-scripting-1.0.jar in the downloads page.

With this approach, the grammar changes which I made for parsing just expressions are obsolete.

Do you see any issues with this approach? If this is good, I will push the changes later today.

[Ingo60]

Sounds amazingly simple, indeed. Is it necessary to write the current script in (...)? If not, this would allow inputs like 2+2 where 2+2=5 which would be quite cool.

[mmhelloworld]

Is it necessary to write the current script in (...)?

It is not necessary. I initially thought that the scripts like + can be compiled if we wrap it in parenthesis by default but it doesn't make sense since it is not actually an expression since foo = + is not valid. I have fixed this as well. Now inputs like 2+2 where 2+2=5 will be working.

[Ingo60]

You're right, a lexical operator on it's own does not make a syntactically valid expression. Hence, I was in error when I tried:

:t `<>`

since only

:t (<>)

needs to work if the syntax is strictly :t expression