luhn: add types and explicit args

Author: lpalma

exercises/luhn/src/Luhn.hs

@@ -1,11 +1,16 @@
 module Luhn (addends, checkDigit, checksum, create, isValid) where
 
-addends = error "You need to implement this function."
+addends :: Integer -> [Integer]
+addends n = error "You need to implement this function."
 
-checkDigit = error "You need to implement this function."
+checkDigit :: Integer -> Integer
+checkDigit n = error "You need to implement this function."
 
-checksum = error "You need to implement this function."
+checksum :: Integer -> Integer
+checksum n = error "You need to implement this function."
 
-create = error "You need to implement this function."
+create :: Integer -> Integer
+create n = error "You need to implement this function."
 
-isValid = error "You need to implement this function."
+isValid :: Integer -> Bool
+isValid n = error "You need to implement this function."