Superclass functional dependencies are ignored during instance selection
The following example compiles with ghc-6.12.3, but not with ghc-7.0.0.20100924:
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
module Ambiguity where
class C1 a b | b -> a
class (C1 a b) => C2 a b where
foo :: b -> b
data A = A
data B = B
instance C1 A B
instance C2 A B
-- this is accepted by both 6.12.3 and 7
runFoo :: C2 a b => b -> b
runFoo = foo
-- this is accepted by 6.12.3, but not by 7
runFoo2 :: B -> B
runFoo2 = foo
A straightforward fix is to add the b -> a
fundep to the C2
class as well.
Trac metadata
Trac field | Value |
---|---|
Version | 7.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |