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

References

•  From Hindley-Milner Types to First-Class Structures by Mark P. Jones, Haskell Workshop, 1995.
• ExistentialsVsPolymorphicComponents?

Pros

• 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.

Cons

• more complex denotational semantics