I don't know if this is a bug or not, but the translation to type
families works:
class Foo a where
type FooT a :: *
instance Foo Int where
type FooT Int = Float
f :: Int -> FooT Int
f = undefined
g :: Int -> Float
g = undefined
h :: Int -> FooT Int
h = g
You don't even need the class contexts.
Erik
On Thu, Sep 13, 2012 at 11:18 AM, satvik chauhan
<mystic.sat...@gmail.com> wrote:
> Consider the code below :
>
> {-# LANGUAGE
> MultiParamTypeClasses,FlexibleInstances,FunctionalDependencies,UndecidableInstances,FlexibleContexts
> #-}
> class Foo a c | a -> c
> instance Foo Int Float
> f :: (Foo Int a) => Int -> a
> f = undefined
>
> Now when I see the inferred type of f in ghci
>
>> :t f
>
>> f :: Int -> Float
>
> Now If I add the following code
>
> g :: Int -> Float
> g = undefined
>
> h :: (Foo Int a) => Int -> a
> h = g
>
>
> I get the error
>
> Could not deduce (a ~ Float)
>
>
> I am not able to understand what has happened here ? The restriction "Foo
> Int a" should have restricted the type of h to "Int -> Float" as shown in
> the inferred type of f.
>
>
> - Satvik
>
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