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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