;; CSE341, Programming Languages, Homework 5

#lang racket
(provide (all-defined-out)) ;; so we can put tests in a second file

;; definition of structures for MUPL programs - Do NOT change
;; (struct var       (string)              #:transparent) ;; a variable, e.g., (var "foo")
;; (struct int       (num)                 #:transparent) ;; a constant number, e.g., (int 17)
;; (struct mtrue     ()                    #:transparent) ;; boolean constant true
;; (struct mfalse    ()                    #:transparent) ;; boolean constant false
;; (struct add       (e1 e2)               #:transparent) ;; add two expressions
;; (struct isgreater (e1 e2)               #:transparent) ;; if e1 > e2 then (mtrue) else (mfalse)
;; (struct mif       (e1 e2 e3)            #:transparent) ;; if e1 then e2 else e3
;; (struct fun       (nameopt formal body) #:transparent) ;; a recursive(?) 1-argument function
;; (struct call      (funexp actual)       #:transparent) ;; function call
;; (struct mlet      (var e body)          #:transparent) ;; a local binding (let var = e in body) 
;; (struct apair     (e1 e2)               #:transparent) ;; make a new pair
;; (struct first     (e)                   #:transparent) ;; get first part of a pair
;; (struct second    (e)                   #:transparent) ;; get second part of a pair
;; (struct munit     ()                    #:transparent) ;; unit value -- good for ending a list
;; (struct ismunit   (e)                   #:transparent) ;; if e1 is unit then (mtrue) else (mfalse)

;; a closure is not in "source" programs; it is what functions evaluate to
(struct closure (env fun) #:transparent) 

;(define foo-mupl
;  (mif (mtrue) (int 0) (int 3)))

(define foo
  (if #t 0 3))

(define bar
  (if #t 0 3))

(define bar-quoted
  '(if #t 0 3))

(define (eval-under-env env e)
  (cond [(symbol? e) (cdr (assoc e env))]
        [(eq? e #t) e]
        [(eq? e #f) e]
        [(number? e) e]
        [(and (list? e) (not (null? e))
              (eq? (car e) 'if))
         (if (eval-under-env env (cadr e))
             (eval-under-env env (caddr e))
             (eval-under-env env (cadddr e)))]
        [else (error "unrecognized expression" e)]))