Yeah that's what I also thought. I tried it first in GHCi, and I got a type error. I tried it again, and it works now. I must have forgotten to reload the file or something, cause I made the wrong conclusion. Duh, mea culpa. On Wed, Apr 22, 2009 at 11:48 PM, Luke Palmer <lrpal...@gmail.com> wrote:
> 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 >> >> >
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