Exotic Programming Ideas, Part 3: Effect Systems

Re:Exotic Programming Ideas, Part 3: Effect Systems by smegma2 on Sunday November 22, 2020 @01:24PM (#25179627)

I was curious about this function:

    fun addRefs(
      a : forall<h> ref<h,int>, 
      b : forall<h> ref<h,int> 
    ) : total ()
    {
      a := 10;
      b := 20;
      return (!a + !b);
    }

Why is it total instead of st<h>? Won't this have a side effect of setting the references?

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Re:Exotic Programming Ideas, Part 3: Effect Systems by daanx on Sunday November 22, 2020 @01:30PM (#25179695)

Ah, I think Stephen meant to write the following:

  fun add-refs( a : ref<h,int>, b : ref<h,int> ) : st<h> int {
    a := 10
    b := 20
    (!a + !b)
  }

where indeed the effect is `st<h>` as the updates are observable. How the function was written before, the two arguments use a "rank-2" polymorphic type and the heaps are fully abstract -- in that case it would be unobservable but you cannot create such values :-)

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