Can higher order functions have generic types?

[matiwinnetou]

Aiken git commit: 0e65fec

Compiler is not happy that I put data to function input (v) which is effectively a generic... how to make it work?

fn new_leaf(key: Int, value: data, hash_fn: fn(v: data) -> Hash<alg, a>) -> Tree<data> {
  let hash: Hash<alg, a> = hash_fn(value)

  Node { key, value, hash, left: Empty, right: Empty }
}

error

mati@Mateuszs-MacBook-Pro collections % aiken check
    Compiling aiken-lang/stdlib main (/Users/mati/Devel/OpenSource/collections/build/packages/aiken-lang-stdlib)
    Compiling cardano-foundation/collections 0.0.0 (/Users/mati/Devel/OpenSource/collections)
Error: aiken::parser

  × While parsing files...
  ╰─▶ I found an unexpected token '":"'.

    ╭─[/Users/mati/Devel/OpenSource/collections/lib/collections/csmt.ak:31:1]
 31 │
 32 │ fn new_leaf(key: Int, value: data, hash_fn: fn(v: data) -> Hash<alg, a>) -> Tree<data> {
    ·                                                 ─
 33 │   let hash: Hash<alg, a> = hash_fn(value)
    ╰────
  help: Try removing it!
  

If I remove the data from fn I also run into issue:

mati@Mateuszs-MacBook-Pro collections % aiken check
    Compiling aiken-lang/stdlib main (/Users/mati/Devel/OpenSource/collections/build/packages/aiken-lang-stdlib)
    Compiling cardano-foundation/collections 0.0.0 (/Users/mati/Devel/OpenSource/collections)
Error: aiken::check::type_mismatch (link)

  × While trying to make sense of your code...
  ╰─▶ I struggled to unify the types of two expressions.

    ╭─[/Users/mati/Devel/OpenSource/collections/lib/collections/csmt.ak:32:1]
 32 │ fn new_leaf(key: Int, value: data, hash_fn: fn(v) -> Hash<alg, a>) -> Tree<data> {
 33 │   let hash: Hash<alg, a> = hash_fn(value)
    ·                                    ──┬──
    ·                                      ╰── expected type 'v'
 34 │
    ╰────
  help: I am inferring the following type:

            v

        but I found an expression with a different type:

            data

        Either, add type-annotation to improve my inference, or adjust the expression to have the expected type.


Summary
    1 error, 0 warnings

[KtorZ]

Hey! May you provide:

• The Git revision you're currently using?
• The error from the compiler you're running into?

Thanks 🙏

[matiwinnetou]

This seems to work.... but this is really confusing me why:

fn new_leaf(
  key: Int,
  value: data,
  hash_fn: fn(data) -> Hash<alg, a>,
) -> Tree<data> {
  let hash: Hash<alg, a> = hash_fn(value)

  Node { key, value, hash, left: Empty, right: Empty }
}

I actually have to specify type on higher order functions - as in input?

[KtorZ]

Yes, after : you only specify a type; For function, that means only the types of their arguments and their return types. There's no possibility to define labels there. This is something we could make clearer in the documentation I reckon.

[matiwinnetou]

OK. ignore this, it works when passing data but is confusing I have to say. I have to learn Aiken...

My expectation would be something like
v: data rather than only data

[matiwinnetou]

This works:

hash_fn: fn(data) -> Hash<alg, a>,

but I would expect something like this:

hash_fn: fn(v: data) -> Hash<alg, a>,

because data is type not value...

[rvcas]

Why would you expect labels in a functions type signature?