Existential Quantification

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Brief Explanation

References

•  Polymorphic Type Inference and Abstract Data Types by K. Läufer and M. Odersky, in TOPLAS, Sep 1994.
•  GHC documentation

Variations

• Hugs allows existential quantification for newtype declarations, as long as there are no class constraints.

  newtype T = forall a. C a
  

  GHC and Nhc98 do not.

• Hugs and Nhc98 allow matching on an existentially quantified constructor in a pattern binding declaration, except at the top level.

  data T = forall a. C (a -> Int) a
  foo t = let C f x = t in f x
  

  GHC does not allow such matching.

• None of the implementations can derive instances for existentially quantified types, but this could be relaxed, e.g

  data T = forall a. Show a => C [a]
          deriving Show
  

• GHC 6.5 allows fields with existentially quantified types, though selectors may only be used if their type does not include the quantified variable.

  data T = forall a. C { f1 :: a, f2 :: Int }
  

Pros

Cons