[Haskell-cafe] GADT on the wiki: I'm lost

[Luke Palmer]

On Wed, Apr 22, 2009 at 3:30 PM, Peter Verswyvelen <bugfact at 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 at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20090422/878f0f06/attachment.htm