On Fri, 30 May 2003 00:00:26 -0500 "Tim Sweeney" <[EMAIL PROTECTED]> wrote:
> Hi Derek, > > How can one implement RefMonad below directly in Haskell? > > class Monad m => RefMonad m r | m -> r where > newRef :: a -> m (r a) > readRef :: r a -> m a > writeRef :: r a -> a -> m () > > I've been able to implement a monad that encapsulates "references to > integers", by creating a monad that passes "the heap" (a function > int->int, from heap indices to integer values stored in the heap) in > and out of functions. That was pretty straightforward, and the monad > makes everything look like an imperative language that supports > updatable references to integers (and only integers). > > But how can one implement RefMonad to support references of all > possible types simultaneously? There isn't without an unsafe coerce function. However, you can get all the types that are an instances of a class, e.g. Typeable. As Hal Daume said, you can use Dynamics. GHC's implementation does use an unsafeCoerce function but "A Lightweight Implementation of Generics and Dynamics" (or something very close to that) shows that such a function is unnecessary and only existential quantification is needed to make Dynamics. If you want to be able to change the type you store in/at a reference then this is your only real option. However, likely you want typed references. Dynamics seems unnecessary/overkill for this (though it may be). You know what type you want, you aren't going to mistake it, so you shouldn't need to carry around a TypeRep. Anyways, you may want to check out "A Lightweight Implementation...", it has some interesting type stuff that may give you ideas. _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
