Edward,
it was my impression that you have to use ScopedTypeVariables or other
tricks to work with Proxy to.
But with Tagged the situation is similar:
{-# LANGUAGE ScopedTypeVariables #-}
typeOf :: forall a. Typeable a => a -> TypeRep
typeOf _ = unTagged (typeRep :: Tagged a TypeRep)
or without extensions:
typeOf :: Typeable a => a -> TypeRep
typeOf x = unTagged (t x)
where
t :: Typeable b => b -> Tagged b TypeRep
t _ = typeRep
Where is the "huge pain" you are talking about?
The pain is the need to define t.
With Proxy you can often use pre-existing combinators, because as a data type, it admits a ton of instances. In this case you can just use 'return' or 'pure'.
typeOf :: Typeable a => a -> TypeRep
typeOf = typeRep . pure
There are fewer combinators from commonly used classes for working with the left argument of a bifunctor, however.
-Edward