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A value is polymorphic if, depending on the context where it's used, it can take on more than one type.

There are different kinds of polymorphism.

  1. Parametric polymorphism; mostly found in functional languages
  2. Ad-hoc polymorphism or overloading
  3. Inclusion polymorphism; mostly found in object oriented languages


foldr :: forall a b. (a -> b -> b) -> b -> [a] -> b

foldr is a parametric polymorphic function. When actually used, it may take on any of a variety of types, for example:

::(Char ->Int->Int)->Int->String->Int

An "integer literal" is a parametric polymorphic data type:

1 :: forall t. (Num t) => t