(a real description of rank n types)
Revision as of 12:48, 26 August 2007
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 Also see
Rank-N types on the Haskell' website.