{- continuing discussion from beginners@ -}
I have code such as
class Foo f where
foo :: a -> f a
data Bar f a = Foo f => Bar {bar :: f a}
instance Foo (Bar f) where
foo a = Bar $ foo a
GHC insists that I put Foo f => on the instance declaration, even though the
constructor for Bar implies this.
Is there any reason why GHC cannot infer this constraint from the Bar constructor? One issue raised in the beginners
thread is that
undefined :: Bar f a
is not Foo f, but as undefined cannot be evaluated, this would not appear to be
a problem.
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