How do I enable language extensions from within GHCi?

What are language extensions in Haskell?

Language extensions are used to enable language features in Haskell that may seem useful in certain cases. They can be used to loosen restrictions in the type system or add completely new language constructs to Haskell. or (in GHC) using flags -X<Extension> .

What is language extensions?

An extension language is a programming language interpreter offered by an application program, so that users can write macros or even full-fledged programs to extend the original application.

:set -XRankNTypes. The -X part is a compiler switch, by the way, extension is called just RankNTypes.