A hack to require type applications, even when not necessary

RequireTypeAp.hs

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}


-- Don't export this, it's just to make it so the type family *could* have a different
-- result, even though it never will!
data Never
type family Identity a where
  Identity Never = ()
  Identity a = a

-- Set up the instance
class From a b where
  from :: a -> b
instance From a [a] where
  from = pure

-- The type fam makes (Identity b) ambigous until applied, even though `Identity b ~ b` always.
into :: forall b a. (From a (Identity b)) => a -> Identity b
into = from

-- This expression fails to compile without the type application
use :: String
use = into @String 'a'