[Haskell-cafe] problem with type equality constraints
Manuel M T Chakravarty
chak at cse.unsw.edu.au
Sun Mar 16 22:43:12 EDT 2008
Ganesh Sittampalam:
> When I try to compile this code with ghc-6.9.20080310:
>
> module Test2 where
>
> type family Id a
> type instance Id Int = Int
> type instance Id (a, b) = (Id a, Id b)
>
> class Id a ~ ida => Foo a ida
>
> instance Foo Int Int
> instance (Foo a ida, Foo b idb) => Foo (a, b) (ida, idb)
>
> I get these errors:
>
> Test2.hs:12:0:
> Couldn't match expected type `ida' against inferred type `Id a'
> `ida' is a rigid type variable bound by
> the instance declaration at Test2.hs:12:16
> When checking the super-classes of an instance declaration
> In the instance declaration for `Foo (a, b) (ida, idb)'
>
> Test2.hs:12:0:
> Couldn't match expected type `idb' against inferred type `Id b'
> `idb' is a rigid type variable bound by
> the instance declaration at Test2.hs:12:27
> When checking the super-classes of an instance declaration
> In the instance declaration for `Foo (a, b) (ida, idb)'
>
> It seems to me that since Foo a ida and Foo b idb are superclassess,
> Id a ~ ida and Id b ~ idb should be known and so this should have
> worked - am I missing something?
Your are completely right. Unfortunately, superclass equalities (ie,
the Id a ~ ida in the class declaration of Foo) aren't fully
implemented yet. If I am not mistaken, superclass equalities, class
defaults for associated type families, and GADT data instances are the
three major features of type families/equality constraint saga that
aren't fully implemented yet.
Manuel
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