pattern-matching extension?

[Derek Elkins]

On Mon, 8 Dec 2003 15:37:46 +1100
Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:

> On 05-Dec-2003, Derek Elkins <ddarius at hotpop.com> wrote:
> > "Abraham Egnor" <aegnor at antioch-college.edu> 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".