Unsoundness of mutable references

[elpinal]

It has turned out that mutable references in Bright ML is unsound. The following code is adopted from SML code in http://mlton.org/ValueRestriction and causes Bright ML crash.

val f 'a _ =
   let
      module R = Ref.Make {type t = option 'a}
      val r = R.make None
   in
   fun x ->
      let
         val y = R.get r
         val _ = R.set r $ Some x
      in
        match y with
          | None   -> x
          | Some y -> y
        end

val f = f ()

val _ = f 13
val _ = String.`<>` "" $ f "foo"

I had thought generative functors would rescue the interaction between polymorphism and mutable references, but that's wrong.