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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