Version 4 (modified by ross@…, 12 years ago) (diff)

Polymorphic Components

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Brief Explanation

Types of data constructor arguments are allowed local universal quantification (forall) and contexts constraining universally quantified type variables, e.g.

newtype Swizzle = MkSwizzle (forall a. Ord a => [a] -> [a])

The constructor then has a rank-2 type:

MkSwizzle :: (forall a. Ord a => [a] -> [a]) -> Swizzle



  • type inference seems to be a simple extension of Hindley-Milner.
  • large increment in expressiveness: types become impredicative, albeit with an intervening data constructor, enabling Church encodings and similar System F tricks.
  • used by the ReadP type.


  • more complex denotational semantics