MPTCs and rigid variables

class Foo a b | a -> b
instance Foo Int String
bar :: Foo Int b => b
bar = "rargh"

Is there any reason why that shouldn't work? GHC gives one of its
silly "b is a rigid variable" errors (aside: that's a really confusing
error; I'd prefer something like Hugs's "Infered type is not general
enough").

--
-David House, dmhouse@...
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