Headline

The ability of program fragments to operate on elements of several types

Illustration

Consider the type of list append in Language:Haskell:

(++) :: [a] -> [a] -> [a]

This type signature uses a type variable a to express that list append is polymorphic in the element type a. The operation can be applied for as long as the element type of both operand lists for an append are the same. We also speak of parametric polymorphism in this case.

Consider the type of addition in Language:Haskell:

(+) :: Num a => a -> a -> a

This type signature uses a type constraint on the operand type of addition to express that only "suitable" types (i.e., type-class instances of Num) can be used for addition. We also speak of type-class polymorphism or more generally of bounded polymorphism in this case. Languages with subtyping may also use types in a subtyping hierarchy for bounds.

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Ralf Lämmel edited this article at Thu, 25 May 2017 23:09:03 +0200
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