The type of a fully applied polymorphic function, e.g. The result is the function id itself, which can be further applied to any value: > (id id) 4 We can also call id on the function id itself. We can call this function on values of arbitrary types: > id True This function is parametrically polymorphic since there are no further restrictions on the input parameter - it is returned unchanged from the function. In Haskell, concrete types (such as Integer) start with a capital letter, and type variables start with a lowercase letter.
Here, t is called a type variable, meaning it can be applied to any arbitrary type. The simplest example of a polymorphic function is id (which exists in the standard library), which returns its argument: > id x = x Examples above, for using the + operator with Int and Double types is an example of ad hoc polymorphism, since the implementation of addition for those types behaves differently. The former means that the function implementation is identical for all types which this function can be used with, the latter implies that for each type used with this function, there exists a special implementation, specific to the type. There are two kinds of polymorphism: parametric polymorphism and ad hoc polymorphism. For example, the + operator may be used on values of type Int, returning an Int value, or Double values, returning a Double. Programming fundamentals Parametric polymorphismĪ function is said to be polymorphic if it can be used on values of different types. Read the first part in the Introduction module. This is the second module (out of 5) of my English summary of the Haskell MOOC on Stepik, available only in Russian.
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