Arbitrary-rank polymorphism with RankNTypes

GHC’s type system supports arbitrary-rank explicit universal quantification in types through the use of the Rank2Types and RankNTypes language extensions.

RankNTypes

StackOverflow forces me to have one example. If this topic is approved, we should move this (opens new window) example here.

Syntax

• Arbitrary rank quantification is enabled with either the Rank2Types or RankNTypes language extension.
• With this extension enabled, the forall keyword can be used to add higher-rank quantification.