Correction: I meant data Baz f a = Baz (Foo f => f a) (Dropped the 'forall', which would make the inner 'f' have nothing to do with the type parameter 'f' of 'Baz') On 2011-June-09 Thursday 01:07:13 Daniel Schüssler wrote:
Hello,
you might be thinking of this type?
{-# LANGUAGE Rank2Types #-}
class Foo f where foo :: a -> f a
data Baz f a = Baz (forall f. Foo f => f a)
instance Foo (Baz f) where foo a = Baz (foo a)
Maybe the difference between Bar and Baz ist best explained by writing it with an explicit class dictionary for Foo:
{-# LANGUAGE Rank2Types #-}
data FooDict f = FooDict { foo :: forall a. a -> f a }
data Bar f a = Bar (FooDict f) (f a)
data Baz f a = Baz (FooDict f -> f a)
fooDict_Baz :: FooDict (Baz f) fooDict_Baz = FooDict (\a -> Baz (\d -> foo d a))
-- fooDict_Bar :: FooDict (Bar f) -- fooDict_Bar = FooDict (\a -> Bar ? ?) -- Doesn't work - you'd have to create a 'FooDict f' and a 'f a' out of just an 'a'
Cheers, Daniel
On 2011-June-08 Wednesday 20:45:56 Guy wrote:
{- 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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