Extract all list items matching a template

[JensRantil]

I have a list of items, Input and a template #MyType. How do I extract a list of all the items that match #MyType?

Example input:

Input: [
  {type:"want", x: 5},
  42,
  {type:"want"},
]

#MyType: {
  type: "want"
  x: number
}

The expected output should be

Output: [
  {type:"want", x: 5},
]

[deepbrook]

You can use a comparison of the unified result and bottom (_|_) like this:

output: [for o in Input if (o & #MyType) != _|_ {o}]

If you combine this with the switch pattern, you can create a type switch

but note that your schema currently does not require the field x - it's currently simply added with the non-concrete value of number if not present, and validated against number if it is.

If you need the result of that field to be concrete, you can either:

• declare a default value if it isn't present: x: number | *0 - populates/adds the field x with the default value (0) if it isn't present or does not have a concrete value,, and validates it against number otherwise
• mark it as required: x!: number - requires the field to be present, throwing an error otherwise
• or mark it optional: x?: number - ignores/skips the x field during unification if it isn't declared/present.

The latter two options would produce the output you want.

[JensRantil]

Thanks for your answer! I tried your loop, combined with making x required, but Output still includes the third item in the list: https://cuelang.org/play/?id=dsStzPTLv7E#w=function&i=cue&f=export&o=cue Am I missing something?

(https://cuelang.org/play/?id=gv6ZMRibr6y#w=function&i=cue&f=export&o=cue requires Output to also be a list of #MyType - and fails validation - as it should)

[deepbrook]

Ok, that is weird. @mvdan can you shed a light on this? I would've expected the if to kick out the last element if the required field is absent.🤔

[infogulch]

The key is testing (close(o) & #MyType) != _|_: https://cuelang.org/play/?id=BmdBQCRUbsv#w=function&i=cue&f=export&o=cue

Here I add Output: [...#MyType] constraint because the if expression evaluation does not propagate the constraint to Output. But because close(o) is omitted cue fails while evaluating it for output:

Output: [...#MyType]
Output: [for o in Input if (o & #MyType) != _|_ {o}]

->

Output.1.x: field is required but not present:
    -:9:3

The confusion here is that by default cue doesn't require open structs to pass all validations until they are actually output by the top level because the user might end up unifying the result with something else that fixes it later. You can force it to evaluate immediately with close(o) & #MyType.

Output: [for o in Input if (close(o) & #MyType) != _|_ {o}]

I'm not exactly sure if this is the best/only way but it works.

[JensRantil]

@infogulch This was very useful. Thanks!

@deepbrook @mvdan Sort of a follow-up since this was quite confusing, does this discussion warrant creating an issue in any way?