Suggestion: mask for effect aliases

[anfelor]

It is currently impossible to use an effect synonym like st in mask, but that means that one has to mask three effects instead of one. Similar code as in #153:

effect acquire<e>
  fun store-release(free : () -> e ()) : ()

fun run-dsl(dsl : () -> <acquire<e>|e> a, state : ref<h, list<() -> e ()>>) : <read<h>,write<h>,div|e> a
  with fun store-release(f)
    val r = !state
    set(state, Cons(f, r))
  mask<read>{ mask<write>{ mask<div>(dsl) } }

fun with-acquired(dsl : () -> acquire<e> a, inner : a -> e ()) : <div|e> ()
  with run
  val stref = ref(Nil)
  val a = mask<alloc>{ run-dsl(dsl, stref) }
  mask<read>{ mask<write>{ mask<alloc>{ mask<div>{ inner(a) }}}}}

Here it would be preferable to write mask<st> instead of three separate masks on read,write,alloc.

$ koka -l test.kk
compile: test.kk
loading: std/core
loading: std/core/types
loading: std/core/hnd
check  : test
test.kk(19, 9): error: Invalid type
  type context :         mask<st>(fn() { mask<div>(fn() { inner(a) }) }
  type         :         mask<st>(fn() { mask<div>(fn() { inner(a) }) }
  inferred kind: E
  expected kind: X
  because      : Can only inject effect constants (of kind X)

[daanx]

Ouch! thanks for the reports -- it is a busy time for me until ICFP but will look at this later :-)

One problem I can see is that div is not maskable as it never can be handled. (there is only one conceptual builtin handler for this that provides the fix operation)

[anfelor]

@daanx It's not that urgent, I just wanted to note down these issues here :)

[anfelor]

In Koka 3.1.1, writing mask<st> still fails with:

Invalid type
type context :   mask<st>
type         :        st
inferred kind: H -> E
expected kind: X
because      : Can only inject effect constants (of kind X)

while writing mask<st<h>> fails with:

Type.TypeVar.subNew.KindMismatch: length 1: (1033::KCon H |-> 107::KCon V)

CallStack (from HasCallStack):
  error, called at src/Common/Failure.hs:46:12 in koka-3.1.1-GrUKw6cowJ9E7PPCKFMvzQ:Common.Failure
  raise, called at src/Common/Failure.hs:32:5 in koka-3.1.1-GrUKw6cowJ9E7PPCKFMvzQ:Common.Failure
  failure, called at src/Common/Failure.hs:28:11 in koka-3.1.1-GrUKw6cowJ9E7PPCKFMvzQ:Common.Failure
  assertion, called at src/Type/TypeVar.hs:119:13 in koka-3.1.1-GrUKw6cowJ9E7PPCKFMvzQ:Type.TypeVar
  subNew, called at src/Kind/Infer.hs:1136:79 in koka-3.1.1-GrUKw6cowJ9E7PPCKFMvzQ:Kind.Infer

(notice that the latter exception is from an error call and does not render nicely in vscode).