[Haskell-cafe] GADT on the wiki: I'm lost
bugfact at gmail.com
Wed Apr 22 17:30:35 EDT 2009
I was reading the explanation of GADTs on the
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...
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 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)
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