How to use Proxy properly?

Question

Why this code doesn't work?

class Foo a where 
    foo :: Proxy a -> Int

bar :: Foo a => a -> Int
bar _ = foo (Proxy :: Proxy a)

It fails to compile with the message:

Could not deduce (Foo a0) arising from a use of `foo'
from the context (Foo a)
  bound by the type signature for bar :: Foo a => a -> Int
The type variable `a0' is ambiguous
In the expression: foo (Proxy :: Proxy a)
In an equation for `bar': bar _ = foo (Proxy :: Proxy a)

I tried it with and without ScopedTypeVariables extension

Answer 1

You need ScopedTypeVariables and forall introduction to get a scoped type variable:

{-# LANGUAGE ScopedTypeVariables #-}

bar :: forall a. Foo a => a -> Int
bar _ = foo (Proxy :: Proxy a)