segfault bug(?) re: unsafe functions and parsing

[RyanBrewer317]

When I run the following code:

import std/text/parse

fun test(): parse char {
  fun test2(): parse char {
    char('r')
  }
  unsafe-no-div(test2)
}

pub fun main() {
  val code = "hi"
  val mb = parse(code.slice, test)
  println("success")
}

I get a segmentation fault, even though test2 is of course not divergent at all. Replacing unsafe-no-div with unsafe-no-ndet also leads to a segfault, but replacing it with unsafe-total results in printing "success," presumably because it gets rid of the parse effect. All three unsafe functions are of course implemented the same way, except for their typing.

I of course understand that using these unsafe functions are, well, unsafe, but this behavior is so odd that I figured I'd make an issue about it just in case it's caused by an unknown compiler bug.

I also want to add that the reason I got into this experimentation to begin with was wanting to build a recursive parser using the parser combinator library, then realizing that I would have to add the "div" effect to everything I'd written even when I knew that the top-level function expr-parsing function (which was the only recursive one) wasn't divergent. I knew that in std, functions like for use the unsafe functions to get the looping they need without getting the "div" effect. I say all of this just to say that this didn't come out of mindless messing-around with koka but an actual project, hopefully to show that if this is a bug, it's not a too-remote edge case.

[chtenb]

I'm running into the same or very similar issue. In my case it doesn't matter if I use unsafe-no-div or unsafe-total, both will crash.

Here is an example. We define a take function which should turn a divergent iterator into a non-divergent one. Furthermore, for some reason the to-list function also thinks it is divergent, requiring another unsafe-no-div there.

pub effect iter<a>
  ctl yield(elem : a) : ()

pub fun to-list(it : () -> iter<a> ()) : list<a>
  with unsafe-no-div()
  var result := Nil
  with fun yield(elem)
    result := Cons(elem, result)
  it()
  reverse(result) // List was accumulated in reverse order

pub fun take(n : int, it : () -> <div,iter<a>> ()) : iter<a> ()
  var count := 0
  with ctl yield(elem)
    if count < n then
      yield(elem) // Forward the yield to our new iterator
      count := count + 1
      resume(())
    else
      ()
  with unsafe-no-div()
  it()

And some test code

pub fun test-iter() : div list<int>
  fun fib-iter(n1 : int, n2 : int) : <div,iter<int>> ()
    val n3 = n1 + n2
    yield(n3)
    fib-iter(n2, n3)

  to-list
    take(100)
      fib-iter(1,1)

[Matthew-Mosior]

See https://github.com/koka-lang/koka/pull/321 as well (I was running into seg faults too).