Implicits

[b-studios]

First draft of implementing implicits in terms of effect handlers

This PR is just for discussion and not meant to be merged

[b-studios]

The current design allows users to write programs like (I'll keep this updated)

implicit ^x : int
implicit ^y : int
implicit ^foo<a> : (a, a)

fun test() : ^x int {
  ^x + ^x
}

fun test2() {
  val ^x = { "hello".print; ^x + 4 }
  test()
}

fun res() {
  val ^x = 3
  test2()
}

val manual = handle(test) { ^x() -> resume(4) }

fun test3() {
  ^x + ^y
}

fun test4() : < ^foo<int> > int {
  ^foo.fst + ^foo.snd
}

if operations look like implicit parameters, we can use the sugar on them

effect manualImplicit {
  fun ^man() : int
}

fun test5() {
  ^man + ^x
}

fun test5handled() {
  val ^man = 42
  val ^x = 2
  test5()
}

[b-studios]

I now turned implicits into legal identifiers. Using the ? prefix however conflicted with generated names. As a workaround I just picked the ^ prefix for now. I updated the above code example accordingly.

Of course the syntax can only be temporary as it also conflicts with exponentials.

[b-studios]

I added with-expression syntax since the implicit val bindings get quite clunky very fast:

with (^x = 4, ^y = ^x+2) test3()

for

{
val ^x = 4
val ^y = ^x + 2
test3()
}

[b-studios]

Ok, I switched back to ?x syntax. To avoid names that include -? I changed makeHiddenName to replace the ? by implicit- so operation name look like .op-implicit-x. Of course, there still can be name clashes if users define operations implicit-x.