[Haskell-cafe] How Type inference work in presence of Functional Dependencies

[Erik Hesselink]

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.satvik at 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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