Closed mpvl closed 3 years ago
This is so good to see.
One problem to consider with the "Switch" section: You write, more or less, if a {} else if b {} ...
, but quite frequently b
is !a
or not a
, which requires restating a
. Could let
help here to define the result of a
once, and express it being both true for the consequent branch and its negation for the alternate branch?
Also, while head
is evocative, it does so little that it barely justifies its inclusion. I thought of coalesce
as a good name for picking the first suitable item in a sequence that can accommodate "null" or disqualified values. Against that, though, in your "Switch" example, I suppose the list should never wind up with more than one value, as opposed to it being prefixed by any number of "null" values.
@seh: yes, let
could be used here that way, though outside the list. We could perhaps consider allowing let
in lists.
Also, one could mimic this behavior with: head([if a {}, {}]), where the second element is the "default", and thus
!a`.
Regarding head
: I agree its utility is a bit meager. We did consider a select
builtin which I think is close to what you're proposing, where it would pick the first of any valid entry. The main problem with this pattern seems that it will be too easy to ignore potential errors, so it may be a less safe approach. Having said that, it reads quite nice and we have seen configurations where this would have merit. So it is something to consider. It just seemed safer to see how far one would get with this seemingly safer approach.
I'm not sure I understand the point with the null values, but maybe this answers your question.
Do you think adding head
is not warranted and using a [...][0]
pattern is sufficient?
I was not sure that CUE has the same notion of "null" values that SQL, HCL, Jsonnet, and other languages have, so the semantics of a hypothetical coalesce
function might not apply.
I don't think head
is warranted without tail
(or rest
), and perhaps nth
. My Lisp is showing. I haven't yet reached for any functions like that, though. I'd rather spend those tokens on set manipulation functions for lists.
Would it be possible to write a CUE "function" that encapsulates your [if a {consequent}, {alternate}][0]
technique? It would require at least two inputs; the alternate could be optional. It's not much compression, but might cut down on the "syntactic noise" with those brackets. Yes, I confess that I'm still looking for else
.
@seh: you can do else with the switch approach and I’m not in favor of a dedicated If-else construct, as it encourages bad patterns.
But I see your points otherwise. I guess you could indeed express this as cue macros neatly if we had the call shorthand. head
would then be defined as:
head: { #0[0], #0: […] }
One problem is that the first element cannot have a conflicting definition of #0.
But maybe this is enough for now to just point out the pattern and suggest that people comment the construct:
aSwitch: [ // select first match
if a { … },
if b { … },
c // default
][0]
anIfElse: [ // if then else
if a { … },
c // else
][0]
This would not require any additions to the language and we can get some experience to see what works. The query addition may also provide useful patterns that obviates the need for this.
@seh in CUE, bottom (incomplete errors,
to be more specific) is a bit like null
in those languages. null
can mean various things, often not compatible with the notion of null
here. So it seemed impossible to assign any specific meaning to it.
This issue has been migrated to https://github.com/cue-lang/cue/issues/943.
For more details about CUE's migration to a new home, please see https://github.com/cue-lang/cue/issues/1078.
Definitions
Before we introduce some of the proposed builtins, we formally introduce some as-of-yet undocumented language features.
Functions
We propose cue supports named argument functions and calls to “structs” as a shorthand for the common macro pattern (e.g.
(s & { _, a: x}).out
).A function argument is now defined as:
Any named argument must be followed by other named arguments.
The expression
s(a: x, b: y)
, wheres
is a struct, is now a shorthand fors & {_, a: x, b: y)
.Validator
A validator is a special builtin that is evaluated by unifying it with other values whereby the result is one of a few outcomes:
_
if the validation is successful and making the value with which it was unified more specific does not change this result (or it is a final evaluation).A validator must be run at the last stage of evaluating a node, after a fixed point is reached evaluating all all non-validator values, in which case any error is considered a fatal error. A validator may be run at earlier stages of the evaluation of a node, in which case an incomplete error signifies that the decision on validity must be postponed.
An example of a language-level validator is
<10
.struct.MinFields
andstruct.MaxFields
are examples of validators of builtin packages.Validators can be thought of as a Go function that has an error return signature.
Inferred validators
Optional: Builtin functions that have the signature
foo(x1, x2, …, xn) bool
may be implicitly interpreted as validators of the signaturefoo(x2, …, xn) error
.The CUE function notation
We define the following signature format for cue functions:
Either all or none of the arguments should be named.
The following rules apply for calling functions with this signature:
These rules could be relaxed later.
Proposed builtins
builtins to replace
_|_
(bottom)Although
_|_
is part of the standard CUE idiom, it has several issues:We intend to deprecate the bottom symbol (keeping it around for backwards compatibility) and replace it with builtins that clearer conveys the intent of its usage.
Comparison is not supported by the spec (arguably), but it is a crucial piece of functionality for many CUE configurations. The meaning of it is unclear, however. In many cases, it is used to check whether a reference exists. In some cases, however, the intended meaning is to check that a value is valid. In reality, CUE implements a semantic that is somewhere in between the two cases: it checks the validity of a value, but not recursively.
Note that if any of these builtins return false, they may still be satisfied at a later point in time. Evaluation should take this into account, as usual.
_|_
replacement:error(msg: string | *null) :: _|_
The use of
error(msg)
replaces the common use of_|_
with the added ability to associate a user message with an error. When used within a disjunction, the error will get eliminated as usual, but upon failure of the disjunction, the user-supplied error is used as an alternative error message.Comparison to bottom
Uses of comparison against bottom will need to be replaced with one of the following builtins.
isconcrete(expr) :: bool
isconcrete
reports whetherexpr
resolves to a concrete value, returning true if it does and false otherwise. It is a fatal error if an expression can never evaluate to true.Example:
Purpose: replaces
if a.foo != _|_ {
, where it is checked whethera.foo
exists with the purpose of determining whether it is a concrete value.exists(expr) :: bool
(optional)exists
reports whetherexpr
resolves to any value.Example:
Purpose: replaces
if a.foo != _|_ {
, where it is checked whethera.foo
exists regardless of concreteness.validator builtins
must(expr: _, msg: string | *null) :: _
must(expr)
passes ifexpr
evaluates totrue
and fails otherwise.Must can be used to turn arbitrary expressions into constraints. For instance,
a: <10
can be written asa: must(a < 10)
. See Issue #575 for detailsnot(expr) :: _
not(expr)
passes if unified with a valuex
for whichexpr&x
fails and false otherwise. See #571 for details.Examples:
numexist(count, ...expr) :: _
numexist(count, ...expr)
passes if the number of expressions for whichexists(x)
evaluates totrue
unifies withcount
.The main purpose of
numexist
is to indicate mutual exclusivity of fields.numconcrete(count, ...expr) :: _
(optional)numconcrete(count, ...expr)
passes if the number of expressions for whichisconcrete(x)
evaluates totrue
unifies withcount
.numvalid(count, ...expr) :: _
(optional)numvalid(count, ...expr)
passes if the number of expressions for whichisvalid(x)
evaluates totrue
unifies withcount
.Builtins related to concrete values
Purpose: combine schema of different instances of the same package that would otherwise fail because there are conflicting definitions.
manifest(x) :: _
manifest
evaluatesx
stripping it of any optional fields and definitions and disambiguating disjunctions after their removal.Use cases:
Defining ranges
Looking around at other languages, defining range numbers clearly is a hard problem, as it is often not clear from just looking at the syntax, or even wording, whether or not ranges are inclusive.
CUE’s unary comparators provide a possible solution to this issue.
range(from: int, to: int, by: int | *1) :: [...int]
Builtin
range
returns a stream of values, starting fromfrom
(must be concrete) , addingby
(defaults to1
) as long as unification withto
succeeds. It is an error to define a range that never terminates.Examples:
Switching
CUE’s
if
is not paired with anelse
. This is partly becauseif
really is a comprehension. But another reason is that the use ofelse
quickly leads to nested conditions. Aswitch
statement is generally more conducive to readability in this case.A switch statement can be simulated in CUE using lists:
is equivalent to the hypothetical
The issue is that the hidden
[0]
at the end of theswitch
is impairing readability.head
A
head
builtin could make the above more readable. It would do nothing more than select the first element in a list, but doing so by more clearly signaling the intention at the start of the list.Package
std
We’re considering making all core builtins available under the package
std
, so that they can be referenced unambiguously and more clearly than using the__
prefix.