m (not a stub)
(Encoding of existentials in terms of higher rank types)
Revision as of 06:46, 11 November 2008
Normal Haskell '98 types are considered Rank-1 types. A Haskell '98 type signature such as
implies that the type variables are universally quantified like so:
is also a Rank-1 type because it is equivalent to the previous signature.However, a
Rank-N type reconstruction is undecidable in general, and some explicit type annotations are required in their presence.
Rank-2 or Rank-N types may be specifically enabled by the language extensions
2 Relation to Existentials
In order to unpack an existential type, you need a polymorphic function that works on any type that could be stored in the existential. This leads to a natural relation between higher-rank types and existentials; and an encoding of existentials in terms of higher rank types in continuation-passing style.
In general, you can replace
3 Also see
Rank-N types on the Haskell' website.