{-# LANGUAGE Haskell2010 #-}  -- reset extensions so we know what's going on
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
module M where

foo0 (f :: a -> b) (xs :: [a]) = case xs of
  [] -> []
  y:ys -> f y : foo0 @a @b f ys

foo1 :: (a -> b) -> [a] -> [b]
foo1 (f :: a -> b) (xs :: [a]) = case xs of
  [] -> []
  y:ys -> f y : foo1 @a @b f ys

foo2 :: forall a b. (a -> b) -> [a] -> [b]
foo2 (f :: a -> b) (xs :: [a]) = case xs of
  [] -> []
  y:ys -> f y : foo2 @a @b f ys

foo3 :: forall {a} {b}. (a -> b) -> [a] -> [b]
foo3 (f :: a -> b) (xs :: [a]) = case xs of
  [] -> []
  y:ys -> f y : foo3 @a @b f ys