Sum types with functions

sum-fns.hs

-- We can have a sum type where each constructor is a function:
data F = A (String -> Int)
       | B (Int -> Int)
       
-- Now let's create a value of this type:
-- The `length` function has type `[a] -> Int` and since
-- `String` is a synonym for `[Char]` we can use `length` on strings
f :: F
f = A (length)

g :: F
g = B (+1)

-- How do we use a function of type F?
testF :: F -> Int
testF f = case f of
            A a -> a "hello world"
            B b -> b 5

testF f
-- Result: 11

testF g
-- Result: 6