These patterns might seem abstract and obscure at first, but have proven tremendously powerful in practice to me.

The current project I’m working on in Rust has a big recursive data type with quite some branches and different instantiations of recursive positions. Box/Vec/Option etc.

By parametrizing the enum with a type variable and tying the knot with a type level fixed point combinator gives you back the originally intended data type. But now suddenly you can put annotations in the recursive positions as well, use generic cata/para and other higher level combinators. Fun and powerful stuff!

(The lack of HKT in Rust can be challenging, but it’s mostly easy to work around it)