Safe and sound STRef [Was Implementing RefMonads in Haskell without ST,IO]

[Keith Wansbrough]

> The following code shows a safe and sound implementation of a
> polymorphic heap with references and updates. The heap is capable of
> storing of polymorphic, functional and IO values. All operations are
> *statically* checked. An attempt to alter a heap reference with a
> value of a mismatched type leads to a _compile-time_ error. Everything
> is implemented in Haskell98 + multiparameter classes with functional
> dependencies + overlapping instances.

The problem you mention later, that the type of the heap returned is
different from the type of the heap passed, is fatal.  The following
expression is untypeable:

  let heap = init_gh in
  let (mr,heap1) = if 1<2 then
                     let (xr,h) = alloc_gh 42 heap in (Just xr,h)
                   else
                     (Nothing,heap) in
  case mr of
    Nothing -> ""
    Just r  -> show (fetch_gh r heap1)

Heaps should be more dynamic than this; the (type of the) *reference*
should encode the type it points to, but the (type of the) *heap*
should not.

The question is still open...

--KW 8-)