On Wed, Apr 22, 2009 at 3:30 PM, Peter Verswyvelen <bugf...@gmail.com>wrote:
> I was reading the explanation of GADTs on the > wiki<http://en.wikibooks.org/wiki/Haskell/GADT> , > and but can't make any sense of the examples. > Sure I understand what a GADT is, but I'm looking for practical examples, > and the ones on the wiki seem to show what you *cannot* do with them... > > For example, the article gives an alternative approach to the safeHead > function (see code below) > > But now that does not work either, since Cons x y never evaluates to > MarkedList x Safe, so safeHead (Cons 1 Nil) will give a type error... > Cons :: t -> MarkedList t y -> MarkedList t z Note the different variables y and z. Cons 42 y has type MarkedList Int a, for any type a, including Safe, NotSafe, and ElephantBanana. > Am I missing something or is this wikibook just confusing? > > Does anybody have good links to examples of GADTs? > > Yampa surely seems a good example, but it's a bit too advanced. > > data NotSafe > data Safe > > > data MarkedList :: * -> * -> * where > Nil :: MarkedList t NotSafe > Cons :: t -> MarkedList t y -> MarkedList t z > > > safeHead :: MarkedList x Safe -> x > safeHead (Cons x _) = x > > > silly 0 = Nil > silly 1 = Cons () Nil > silly n = Cons () $ silly (n-1) > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
