comprehension.fnl

(fn comprehend [init update ...]
    (let [clauses [...] n (length clauses) result (gensym)]
      (tset clauses n (update result (. clauses n)))
      (for [i (- n 1) 1 -1] (table.insert (. clauses i) (. clauses (+ i 1))))
      `(do (var ,result ,init) ,(. clauses 1) ,result)))

(fn tbl [...]
    (comprehend {}
                (fn [result expr] `(match ,expr (k# v#) (tset ,result k# v#)))
                ...))

(fn seq [...]
    (comprehend []
                (fn [result expr] `(tset ,result (+ (length ,result) 1) ,expr))
                ...))

(fn sum [...]
    (comprehend 0
                (fn [result expr] `(set ,result (+ ,result ,expr)))
                ...))

{:tbl tbl :sum sum :seq seq}