Here's some explanation (not by me, found through search): https://blog.ocharles.org.uk/guest-posts/2014-12-20-scoped-type-variables.html https://blog.ocharles.

Here's some explanation (not by me, found through search): https://blog.ocharles.org.uk/guest-posts/2014-12-20-scoped-type-variables.html https://blog.ocharles.org.uk/guest-posts/2014-12-20-scoped-t... And among other things, it also enables pattern signatures. The official docs at https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/scoped_type_variables.html https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/scop... are generally worth a read, they start out with an example of what lexically scoped type variables allow, and follow with the design principles for the extension.