The parameters and result of functions in Haskell can be one “concerate” type, e.g., Bool, Int, etc; or polymorphic type, i.e., not a specified type, represented by a type variable, such as a, b, and so on.
Take length function as an example:
length :: [a] -> Int
The return value must be Int type, but the input parmater is a list which can contain any type. But if the polymorphic type is constrained as a set of types, e.g. (+):
(+) :: Num a => a -> a -> a
Num is called “type class”, it means not all types can be use in (+) except the ones which belong to Num. This “constrianed” polynomical type (Num a in (+)) is referred as “Ad hoc polymorphism”, while “no constrianed” one (a in length) is referred as “Parametric polymorphism”.