Typesafe MRef with a regular monad

[Keith Wansbrough]

Ralf Hinze writes:

> Why is that? Ok, here is my second implementation. It uses the
> Dynamic module from our HW2002 paper. A key is a pair consisting
> of the actual key and a type representation.

[..]

> > update                        :: (Typable b) => FM k -> Key k a -> b -> (FM k, Key k b)
> > update (FM bs) (Key k _) b    =  (FM ((k, Dyn rep b) : bs), Key k rep)
> 
> Does this fit the bill?

No, because update shouldn't return a new key, it should allow reuse
of the same key.  Restating Simon PJ's original signature, and adding
update:

   module TypedFM where
	data FM k     -- Abstract; finite map indexed bykeys of type k
	data Key k a  -- Abstract; a key of type k, indexing a value of type a
 
	empty :: FM k
	insert :: Ord k => FM k -> k -> a -> (FM k, Key k a)
	lookup :: Ord k => FM k -> Key k a -> Maybe a
        update :: Ord k => FM k -> Key k a -> a -> FM k

If updating gives you a new key, then you might as well just store the
value in the key.  Instead, you keep the same key; and so you'd better
remain type-compatible.

--KW 8-)