On Monday 09 July 2007, Daniil Elovkov wrote:
Hello
In the archives of haskell-cafe I found a mention of constraints on datatypes as a mis-feature of Haskell. In particular, that they're not propagated well. Can someone elaborate on that?
Also, are they still considered a mis-feature with the emergence of GADTs ?
If I have
data GADT a where ... Alt :: (a -> b -> c) -> GADT a -> GADT b -> GADT c ...
and I want to constrain all a, b, c.
Would it be better to expose all of them as type vars, rather than constrain in-place? It would lead to a rather verbose code.
GADTs don't change anything (at least, not the last time I checked). If you say class C a where ... data GADT a where ... Alt :: (C a, C b, C c) => (a -> b -> c) -> GADT a -> GADT b -> GADT c ... when you pattern match on Alt, the compiler finds the instances for C a and C b, but the constraint C c is ignored. So constraints on data types work exactly the same way they always have, and the standard arguments against them all still work. (Although now I think the status of this `feature' can be down-graded to wart: after all, if you say newtype Id a = Id a data GADT a where ... Alt :: (C a, C b, C c) => (a -> b -> c) -> GADT (Id a) -> GADT (Id b) -> GADT (Id c) ... pattern-matching on Alt introduces all three constraints into the current context. . .) Jonathan Cast http://sourceforge.net/projects/fid-core http://sourceforge.net/projects/fid-emacs