Monads in a software engineering context ("a particular set of rules for composition of two pieces of code"); defunctionalization; Lisp structural macros; fexpr

Monads in a software engineering context ("a particular set of rules for composition of two pieces of code"); defunctionalization; Lisp structural macros; fexprs; the Rust lifetime system; how to structure functional programs. All of these things had a delta in years between when I first encountered them and when I finally understood them, with repeated (4-8) spaced exposures over that time period contributing.