import Prelude hiding (length)

-- The length of a list:
length :: [a] -> Int     -- a is a type variable/parameter
length []     = 0
length (_:xs) = 1 + length xs

-- Concrete types:
--length :: [Int] -> Int   because a |-> Int
--length :: [Bool] -> Int  because a |-> Bool
--length :: [Char] -> Int  because a |-> Char

ll = length [2,3,4] + length "abc"

--mf = foldr 0 (+) [1,2,3]
mf = foldr (+) 0 [1,2,3]

-- Most general types might indicate programming errors:
revf []     = []
revf (x:xs) = revf xs ++ x

f :: (a -> a) -> (a -> a)
f g = g g