Is it possible to handle polymorphic effect operations?

[schuelermine]

Consider, for instance:

effect log
  fun log(value : a, show : a -> string) : ()

fun log-nine()
  log(9, show)

fun main()
  with handler
    fun log(value : a, show : a -> string)
      value.show.println
  log-nine()

This does not compile, complaining that a is unbound.
How can I bind a? forall in each argument type obviously will not work,
and log<a> is not permitted either, and handler<a> will not compile.

[TimWhiting]

This works with type inference:

  fun log(value, show)
    value.show.println

or with explicit some annotations.

  fun log(value: some<a> a, show: some<a> a -> string)
    value.show.println

However, I would expect to be able to do

  fun log(value: forall<a> a, show: a -> string)
    value.show.println

And have the forall be lifted (promoted) to quantify over the full function signature instead of the single parameter (like it does for regular function definitions). This doesn't work currently. It seems like promotion is not happening for these signatures, while it does for all other signatures. I believe this is the problem with the simple annotation in the original post as well. This is a problem in the parser, which we should align with regular function declarations and also allow the other attempted solution:

  fun log<a>(value: a, show: a -> string)
    value.show.println

The problem with

  fun log(value: forall<a> a, show: forall<a> a -> string)
    value.show.println

Is that you would be introducing a "new" unrelated a for each parameter. Which is why some<a> should be used in this case.

[schuelermine]

The same error (I think) occurs when using some

[schuelermine]

$ koka --version
Koka 3.1.2, 00:00:00 Jan  1 1980 (ghc release version)
version: 3.1.2
bin    : /nix/store/gbbd85w5fffi2iylxm022105xfaabkq3-koka-3.1.2/bin
lib    : /nix/store/gbbd85w5fffi2iylxm022105xfaabkq3-koka-3.1.2/lib/koka/v3.1.2
share  : /nix/store/gbbd85w5fffi2iylxm022105xfaabkq3-koka-3.1.2/share/koka/v3.1.2
output : .koka/v3.1.2/cc-debug-2c37dd
cc     : /nix/store/dc6bahp3f5af2rxz3pal9m3kp4vx4rpy-gcc-wrapper-14.2.1.20250322/bin/cc
Copyright 2019-2024, Microsoft Research, Daan Leijen, and others.
This program is free software; see the source for copying conditions.
This program is distributed in the hope that it will be useful,
but without any warranty; without even the implied warranty
of merchantability or fitness for a particular purpose.

[TimWhiting]

Interesting, it definitely works on the latest dev release:
Download the .tar.gz and install script here:
https://github.com/koka-lang/koka/releases/tag/v3.1.3-alpha14
Then ./install.sh path/to/tar.gz.

[TimWhiting]

Also, I was going to suggest that maybe a more natural way of approaching this issue is to determine the "minimal" interface for the effect, and create helper functions to provide a more rich interface for using the effect.

effect log
  fun elog(value: string) : ()

fun log(value: a, ?show: a -> string): log ()
  elog(value.show)

fun log-nine()
  log(9)

fun main()
  with handler
    fun elog(value : string)
      value.println
  log-nine()

[schuelermine]

That’s a good idea; I was presenting this not because it is practical, but because it illustrates the issue. I was playing around with effects without a specific goal in mind.