;;;;;;;; grammatical specification ;;;;;;;;;;;;;;;;

(define the-lexical-spec
  '((whitespace (whitespace) 
                skip)
    (id (letter (arbno (or letter digit "?")))
        make-symbol)
    (number ((or "" "-" "+") digit (arbno digit))
            make-number)))

(define the-grammar
  '((program (expression) a-program)

    (expression (number) lit-exp)
    (expression (id) var-exp)
    (expression
      (primitive "(" (separated-list expression ",") ")")
      primapp-exp)
    (expression ("let"
                 (arbno id "=" expression)
                 "in"
                 expression)
                let-exp)
    (expression ("proc"
                 "(" (separated-list id ",") ")"
                 expression)
                proc-exp)
    (expression ("(" expression (arbno expression) ")")
                app-exp)
    (expression ("if" expression
                 "then" expression
                 "else" expression)
                if-exp)
                 
    
    (primitive ("+")     add-prim)
    (primitive ("-")     subtract-prim)
    (primitive ("*")     mult-prim)
    (primitive ("add1")  incr-prim)
    (primitive ("sub1")  decr-prim)))

(sllgen:make-define-datatypes the-lexical-spec
                              the-grammar)

(define-datatype
 proc proc?
 (closure (ids (list-of symbol?))
          (body-exp expression?)
          (env environment?)))

;;;;;;;;;;;;;;;; the interpreter ;;;;;;;;;;;;;;;;

; eval-program : program -> expval
;
; Evaluates the given program, using an environment that
;  binds i, v, and x to 1, 5, and 10, respectively.
;
;   (eval-program (a-program (lit-exp 0))) = 0
;   (eval-program (a-program (var-exp 'a))) = error
;   (eval-program (a-program (var-exp 'x))) = 10
;   (eval-program (a-program (primapp-exp
;                             (add-prim)
;                             (list (lit-exp 1)
;                                   (lit-exp 2))))) = 3
;
(define eval-program 
  (lambda (pgm)
    (cases program pgm
      (a-program (body)
        (eval-expression body (empty-env))))))

; eval-expression : expression env -> expval
;
; Evaluates an expression in the given environment.
;
;   (eval-expression (lit-exp 0)
;                     (empty-env)) = 0
;   (eval-expression (var-exp 'x)
;                     (empty-env)) = error
;   (eval-expression (var-exp 'x))
;                     (extend-env '(i v x) 
;                                 '(1 5 10)
;                                 (empty-env))) = 10
;   (eval-expression (primapp-exp
;                     (add-prim)
;                     (list (lit-exp 1)
;                           (lit-exp 2)))
;                    (empty-env)) = 3
;   (eval-expression (let-exp
;                     (list 'x 'y)
;                     (list (lit-exp 10) (lit-exp 7))
;                     (primapp-exp (add-prim)
;                                  (var-exp 'x)
;                                  (var-exp 'y))))) = 17
;
(define eval-expression 
  (lambda (exp env)
    (cases expression exp
      (lit-exp (datum) datum)
      (var-exp (id) (apply-env env id))
      (primapp-exp (prim rands)
        (let ((args (eval-rands rands env)))
          (apply-primitive prim args)))
      (let-exp (ids exps body-exp) 
               (eval-expression
                body-exp ;; expression
                (extend-env
                 ids ;; list-of-sym
                 (eval-rands exps env)
                 env)))
      ;; Handle proc evaluation.
      ;;  Essentially, we just return the proc (its
      ;;  variables and body), but we also remember the
      ;;  current environment, to implement lexical scoping.
      (proc-exp (ids body-exp)
                (closure ids body-exp env))
      ;; Handle application
      (app-exp (rator rands)
               (let ([func (eval-expression rator env)] ; eval func expr
                     [args (eval-rands rands env)])     ; eval all arg exprs
                 (apply-proc func args)))               ; apply func
      (if-exp (test then else)
              (if (zero? (eval-expression test env))
                  (eval-expression else env)
                  (eval-expression then env))))))

; apply-proc : proc list-of-expressions -> expval
(define (apply-proc func args)
  (cases proc func
         ;; To apply a proc (a closure at this point),
         ;;  we extract the saved environment, extend it
         ;;  with bindings for the arguments, and evaluate
         ;;  the function body.
         (closure (ids body-exp env)
                  (eval-expression
                   body-exp
                   (extend-env
                    ids
                    args
                    env)))))

; eval-rands : list-of-expression env -> list-of-expval
(define eval-rands
  (lambda (rands env)
    (map (lambda (x) (eval-rand x env)) rands)))

; eval-rand : expression env -> expval
(define eval-rand
  (lambda (rand env)
    (eval-expression rand env)))

; apply-primitive : primitive list-of-expval -> expval
;  (apply-prim (add-prim) '(0 3)) = 3
;  (apply-prim (sub-prim) '(1 2)) = -1
(define apply-primitive
  (lambda (prim args)
    (cases primitive prim
      (add-prim  () (+ (car args) (cadr args)))
      (subtract-prim () (- (car args) (cadr args)))
      (mult-prim  () (* (car args) (cadr args)))
      (incr-prim  () (+ (car args) 1))
      (decr-prim  () (- (car args) 1)))))

;;;;;;;;;;;;;;;; environments ;;;;;;;;;;;;;;;;

;; Abstract envrionment datatype implementation.
;; >> >> >> Don't change this part! << << <<

;; <env> ::= '()
;;       ::= (cons (cons <symbol> <number>) <env>)

;; empty-env : -> env
(define (empty-env) '())

;; extend-env : list-of-symbol list-of-number env -> env
(define (extend-env names vals env)
  (letrec ([make-pairs
            ;; This local function takes a list of names
            ;; and a list of values and creates a list
            ;; of pairs of name and value.
	    (lambda (names vals)
	      (cond
	       [(null? names) '()]
	       [else (cons (cons (car names) (car vals))
			   (make-pairs (cdr names) (cdr vals)))]))])
    ;; Add new pairs to old environment:
    (append (make-pairs names vals) env)))

;; apply-env : env symbol -> num
(define (apply-env env s)
  (cond
   [(null? env) (eopl:error 'apply-env 
                            "no binding for variable: ~a" s)]
   [else (cond
           ;; Check whether the first binding is the one we're
           ;;  looking for
           [(eq? (car (car env)) s) (cdr (car env))]
           ;; If not, look in the rest of the environment:
           [else (apply-env (cdr env) s)])]))

(define (environment? v)
  (and (list? v)
       (or (null? v)
	   (and (pair? (car v))
		(symbol? (caar v))
		(environment? (cdr v))))))

;;;;;;;;;;;;;;;; top level ;;;;;;;;;;;;;;;;

; read-eval-print : -> [loops forever]
(define read-eval-print
  (lambda ()
    ((sllgen:make-rep-loop "-->"
                           eval-program
                           (sllgen:make-stream-parser
                            the-lexical-spec
                            the-grammar)))))