[Haskell-cafe] nice simple problem for someone struggling....

[Jules Bean]

David Menendez wrote:
>> That's a reasonable thing to assume. It just happens that Haskell 
> doesn't work that way. There's an asymmetry between constructing and 
> pattern-matching, and it's one that many people have complained about.

With GADTs turned on (-XGADTS in 6.8, -fglasgow-exts in 6.6) pattern 
matchings will give rise to class contexts as you would naively expect.

Contexts on constructors aren't actualy haskell98, it is a bug that GHC 
6.6 accepts them without any extensions being activated. Or that's my 
understanding, see http://hackage.haskell.org/trac/ghc/ticket/1901

Jules