On Mon, 8 Dec 2003 15:37:46 +1100 Fergus Henderson <[EMAIL PROTECTED]> wrote:
> On 05-Dec-2003, Derek Elkins <[EMAIL PROTECTED]> wrote: > > "Abraham Egnor" <[EMAIL PROTECTED]> wrote: > > > I've occasionally wanted some sort of equivalent of an instanceOf > > > function in haskell, i.e. one that would let me define a function > > > that could dispatch on the type of its argument as well as the > > > value. One option I've seen for this is > > > "http://okmij.org/ftp/Haskell/class-based-dispatch.lhs";, but that > > > unfortunately has the downside of requiring you to write both a > > > constructor for PACK and an instance of Packable for each type > > > you'd like to dispatch on. > > > > > > The thought occurred to me that it is (intuitively) natural to do > > > this via extending the pattern-matching facility to include types > > > as well as literal values, i.e. something like: > > > > > > f :: a -> String > > > f (a :: Int) = "got an int, incremented: "++(show (a+1)) > > > f (a :: Show q => q) = "got a showable: "++(show a) > > > f _ = "got something else" > > > > > > This has a couple of nice features - it's a simple extension of > > > the syntax, and acts as a sort of type-safe typecast. However, I > > > have zero knowledge of how hard it would be to implement this, and > > > there may be theoretical difficulties I haven't seen. Thoughts? > ... > > data Showable = forall a. Show a => Showable a > > > > instance Show Showable where > > show (Showable showable) = show showable > > > > -- extension: pattern guards (I think Hugs has them but I don't > > think-- NHC does. They are only used for prettiness here.) > > f :: Dynamic -> String > > f val | Just (a :: Int) <- fromDynamic val > > = "got an int, incremented: "++show (a+1) > > | Just (a :: Showable) <- fromDynamic val > > = "got a showable: "++show a > > Casting to a "Showable" is not the same as a dynamic class cast. I agree. In the "..." above, I say "What you could do today". _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
