How to relate to type from outer context

Question

Let us consider the following code snippet:

blah :: a -> b -> a
blah x y = ble x where
    ble :: b -> b
    ble x = x

This compil

Answer 1

If you don't want to use ScopedTypeVariables, you can use the good ole fashion asTypeOf function.

-- defined in Prelude
asTypeOf :: a -> a -> a
x `asTypeOf` y = x

---

blah :: a -> b -> a
blah x y = ble x where
    ble x = x `asTypeOf` y

Of course, this won't compile because of the type error.

Update:

I would like to point out that sometimes you might have to be a little crafty to do what you want with asTypeOf. Take the following example that superflously uses asTypeOf because I don't want to think of a case that actually needs asTypeOf. Similar solutions would work the same for real world cases.

foo :: Bounded a => Maybe a -> a
foo m = x
  where
    x = maxBound -- Q: how do I make (x :: a) when given (Maybe a)?
    _ = Just x `asTypeof` m -- A: witchcraft!