interp.scm

(load "pmatch.scm")

(define eval-expr
  (lambda (expr env)
    (pmatch expr
      [,n (guard (number? n)) n]
      [,x (guard (symbol? x))
          (env x)]
      [(lambda (,x) ,body)
       (lambda (arg)
         (eval-expr
           body
           (lambda (var)
             (if (eq? x var)
               arg
               (env var))
             )))]
      [(,rator ,rand)
       ((eval-expr rator env)
        (eval-expr rand env))])))
(define evalo
  (lambda (expr)
    (eval-expr
      expr
      (lambda
        (var)
        (error 'lookup "unbound")))))

;; taken from "The Most Beautiful Program Ever Written" by William Byrd, presented at PWL NYC
;; https://youtu.be/OyfBQmvr2Hc