parametric polymorphism

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Polymorphism was first identified by Christopher Strachey in 1967 and developed by Hindley and Milner.

For example we could specify that the argument of the "head" function was a list without specifying a type for the elements of the list. In Haskell we would write:

	head :: [a] -> a

meaning head has type function from "list of a" to "a" where "a" is a type variable). This is known as parametric polymorphism. Polymorphic typing allows strong type checking as well as generic functions. ML in 1976 was the first language with polymorphic typing.