Typesafe MRef with a regular monad

[Ralf Hinze]

Am Freitag, 6. Juni 2003 16:09 schrieb Simon Peyton-Jones:
> You can't overwrite an entry with a value of a different type, because
> the keys are typed!  Any more than you can overwrite an IORef with a
> value of a different type.
> S

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.

> module TypedFM
> where
> import Prelude hiding (lookup)
> import qualified Prelude
> import Dynamics

> data FM k                     =  FM [(k, Dynamic)]
> data Key k a                  =  Key k (Type a)

> empty                         :: FM k
> empty                         =  FM []

> insert                        :: (Typable a) => FM k -> k -> a -> (FM k, Key k a)
> insert (FM bs) k a            =  (FM ((k, Dyn rep a) : bs), Key k rep)

> lookup                        :: (Eq k) => FM k -> Key k a -> Maybe a
> lookup (FM bs) (Key k rep)    =  case Prelude.lookup k bs of
>                                  Nothing -> Nothing
>                                  Just dy -> cast dy rep

> 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?

Cheers, Ralf