[Haskell-cafe] nice simple problem for someone struggling....
jules at jellybean.co.uk
Fri Dec 21 14:38:40 EST 2007
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
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