madlisp/mad/eval.mad

;;
;; MadLisp interpreter written in... MadLisp?
;;
;; Inspired by "The Most Beautiful Program Ever Written" by William Byrd:
;; https://youtu.be/OyfBQmvr2Hc
;;
;; Currently supports very minimal functionality. Note that the expression
;; to evaluate must be quoted, because otherwise it is evaluated before
;; eval-expr!
;;
;; Some examples:
;; > (eval-expr '(+ 1 2 (* 3 4)) (make-envr))
;; 15
;; > (eval-expr '((fn (a) (* a 2)) 7) (make-envr))
;; 14
;;

(defn eval-list (lst envr)
  (case (let (item (first lst)) (if (symbol? item) (str item) "other"))
    ["if" (if (eval-expr (second lst) envr)
              (eval-expr (get lst 2) envr)
              (eval-expr (get lst 3) envr))]
    ["fn" (let (x (first (second lst)) body (get lst 2))
            (fn (arg) (eval-expr body
              (fn (y) (if (= x y) arg (envr y))))))]
    [else (apply (eval-expr (first lst) envr)
            (map (fn (a) (eval-expr a envr)) (tail lst)))]))

(defn eval-expr (expr envr)
  (cond
    [(symbol? expr) (envr expr)]
    [(list? expr) (if (empty? expr) expr (eval-list expr envr))]
    [else expr]))

(defn make-envr ()
  (fn (key) (case key
    (+ +) (- -) (* *) (/ /)
    (else (throw (str key " not found in envr"))))))