[Haskell-cafe] Constraints on data-types, mis-feature?

[Jonathan Cast]

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