karahobny

;; α | Any | a'        => literally any type
;; Num { ℕ | ℤ | ... } => numerical tower, natural numbers, integers etc.
;; Fn                  => procedure/function
;; Ø? (null list predicate), Ø (null list), ¬ (negation, not)
;; hd (car = first of list), tl (cdr = all but first of list), :: (cons operator)

(define/c (foldl ((f : Fn) (n : α) (xs : List)) -> α)
  (case-with || (Ø? xs) => n
             || _       => (foldl f (f n (hd xs)) (tl xs))))