[foldr]

Out of curiosity, why do you find ScopedTypeVariables essential? I've written a fair amount of Haskell code and never once needed it.

[anyfoo]

Here's some explanation (not by me, found through search): 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/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.

[foldr]

They’re useful on occasion, but don’t seem anywhere close to essential.