Arbitrary-rank polymorphism with RankNTypes

Introduction#

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

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.

RankNTypes

StackOverflow forces me to have one example. If this topic is approved, we should move this example here.