Better syntax support for user-defined "functions"

[myitcv]

#484 (reply in thread)

Is your feature request related to a problem? Please describe.

Per #484, user-defined "functions" are somewhat clumsy right now in that they have to be defined via something like:

#udf: {
  #foo: string
  out: "\(#foo)bar"
}

"quux": (#udf & {#foo: "foo"}).out

with named parameter fields and an output field

Describe the solution you'd like

Per @mpvl, something like this could work:

#udf: {
  #0: string
  
  "\(#0)bar"
}

"quux": #udf("foo")

which uses a combination of numbered definitions as parameter fields and an embedded scalar as the result.

Open question: how struct values would work in this case.

Describe alternatives you've considered

The workaround described above.

Additional context

n/a

[madhadron]

Would something closer to Prolog be more general and more useful? A first cut:

{
    n: 42,
    paddedN: s |- pad10 & { out: s, n: n }
}

pad10 { n: <10, out: "0\(n)" }
pad10 { n: >=10, out: "\(n)" }

where |- is a "given that" sign. The expression to the left of |- is evaluated after the expression on the right is unified. Then we can write things like

s |- (s<100 & pad10 & { out: s, in: n }) | s="too big"

This kind of layering of guards and conditionals is something we would do in a sequence of expressions in an imperative language. In Cue right now, it has to be moved somewhere else in the program instead of being kept local to the value.

[myitcv]

Closing this in favour of #943, specifically the first section "Functions"