mutable type should be quantified and instantiated

[chadaustin]

This program fails to typecheck:

fun f() {
  return mutable []
}

fun main() {
  let a = f()
  a->append("hello")
  let b = f()
  b->append(1)
}

@andyfriesen any ideas?

[andyfriesen]

I think this is safe.

[chadaustin]

@andyfriesen Any gut sense as to why the type checker fails this program?

[andyfriesen]

Since the array is mutable, the type variable isn't being universally quantified. It's being made weakly polymorphic. This is the machinery that prevents us from making a forall a. IORef (Maybe a).

We need to think for a bit to really understand what the rules for weak polymorphism are if we want this case to work.