Implement letfn*

[frenchy64]

Needed

• Add a new letfn expression (follow let)
  • Needs new header
  • Needs to be added to the variant in expression.hpp
  • Will fail to compile until there are overloads for eval and gen (the next step)
• Add eval and codegen support
  • For eval just wrap the expression and call it, like we do with let normally. This will require codegen by letfn by wrapping it in a fn and the evaluating that
• Ensure letfn macro is added and working
• Add tests under test/jank/form/letfn

References

• https://www.nils-blum-oeste.net/letfn-drilldown-a-clojure-special-form-in-detail/

[frenchy64]

I can look into this.

[jeaye]

Added some additional info.

[frenchy64]

Thanks @jeaye! I started with the boilerplate and hit #260. I was hoping to get some pointers on how to implement recursion without vars but now I'm not sure if there is an existing example.

EDIT: I think I'm back on track, found an example to go off:

clojure.core=> ((fn a [] (when (< 0.5 (rand)) (println "iter") (a))))
iter
iter
iter
iter
nil