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.
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