It's a named lambda, using self capture, and that's been in other languages for decades. Scheme is a good example, with letrec. Compile time recursive function

It's a named lambda, using self capture, and that's been in other languages for decades. Scheme is a good example, with letrec. Compile time recursive functions are likewise available in common lisp via recursive macros. Some schemes have them, too. And the syntax is much more elegant.