spec/comprehension: if clause is missing else

[cueckoo]

Originally opened by @xinau in cuelang/cue#111

atm. the only way to implement a else is to add the same if clause and negating the expression

foo: ["dragons"]

if ((foo & string) != _|_) {
  bar: foo
} 
if ((foo & string) == _|_) {
  bar: 20
} 

while this works, it feels strange from a user perspective coming from other languages.
I also checked the spec and it seems to be missing https://github.com/cuelang/cue/blob/master/doc/ref/spec.md#comprehensions

[cueckoo]

Original reply by @mpvl in cuelang/cue#111 (comment)

It is not missing, it was deliberately omitted. I’m not entirely against adding it, but only after getting more data and experience with comprehensions and seeing how they develop. Conditional expression tend to lead to awful configurations, and although comprehensions are grammatically not expressions, they are getting close and just might develop into them.

[cueckoo]

Original reply by @mpvl in cuelang/cue#111 (comment)

BTW, note that the above could be approximated as:

{
  foo: ["dragons"]

  {
    foo: string
    bar: foo
  } | {
    foo: [...] 
    bar: 20
  }
}

Disjunctions will often be a good approach for more complicated structural alternatives.

A more likely language extension, which would also be useful for JSON schema compatibility would be to allow !string or not(string) to mean null | bool | number | bytes | {} | [...].

[cueckoo]

Original reply by @mpvl in cuelang/cue#111 (comment)

You could even fully simulate else with default values:

{
  foo: ["dragons"]

 *{
    foo: string
    bar: foo
  } | *{
    foo: int
    bar: 10
  } | {
    bar: 20
  }
}

where only the else block is not marked as a default. Note you can have multiple defaults, which makes sense to use if there is a discriminator field, like in this case.

[cueckoo]

Original reply by @xinau in cuelang/cue#111 (comment)

@mpvl Thanks for the input.

[cueckoo]

Original reply by @seh in cuelang/cue#111 (comment)

You could even fully simulate else with default values:

I discovered today that this example works up until the "foo" field has a value that's a string or an int. If it matches either of the first two disjunction alternatives, we have to ensure that it doesn't match the third as well.

We need something like the following in the third alternative:

foo: null | bool | bytes | float | [...] | {...}
bar: 20