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. > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe