# Type Inference Infelicity

**Ashley Yakeley
**
ashley@semantic.org

*Wed, 19 Mar 2003 01:42:41 -0800*

This should compile, shouldn't it?
-- ghc -c -fglasgow-exts TestInfer.hs
module TestInfer where
{
class C t a b | t a -> b;
instance C Char a Bool;
data P t a = forall b. (C t a b) => MkP b;
data Q t = MkQ (forall a. P t a);
f' :: Q Char;
f' = MkQ (MkP True :: forall a. P Char a);
f :: Q Char;
f = MkQ (MkP True);
}
GHC 5.04.2 for MacOS X complains about f, but not about f':
TestInfer.hs:15:
Could not deduce (C t a Bool) from the context ()
Probable fix:
Add (C t a Bool) to the When generalising the type of an
expression
Or add an instance declaration for (C t a Bool)
arising from use of `MkP' at TestInfer.hs:15
In the first argument of `MkQ', namely `(MkP True)'
In the definition of `f': MkQ (MkP True)
--
Ashley Yakeley, Seattle WA