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(#%require racket/bool)
;; utils
(define apply-in-underlying-scheme apply)
(define (tagged-list? exp tag)
(if (pair? exp)
(eq? (car exp) tag)
#f))
(define (error reason . args)
(display "Error: ")
(display reason)
(for-each (lambda (arg)
(display " ")
(write arg))
args)
(newline)
(scheme-report-environment -1)) ;; we hope that this will signal an error
;; Eval
(define (meval exp env)
(cond ((self-evaluating? exp) exp)
((variable? exp) (lookup-variable-value exp env))
((quoted? exp) (text-of-quotation exp))
((assignment? exp) (eval-assignment exp env))
((definition? exp) (eval-definition exp env))
((if? exp) (eval-if exp env))
((and? exp) (eval-and (and-operands exp) env))
((or? exp) (eval-or (or-operands exp) env))
((let? exp) (meval (let->combination (cdr exp)) env))
((let*? exp) (meval (let*->nested-lets (cdr exp)) env))
((letrec? exp) (meval (letrec->let (cdr exp)) env))
((load? exp) (eval-load exp env))
((lambda? exp)
(make-procedure (lambda-parameters exp)
(lambda-body exp)
env))
((begin? exp)
(eval-sequence (begin-actions exp) env))
((cond? exp) (meval (cond->if exp) env))
((application? exp)
(mapply (meval (operator exp) env) ;; apply procedure (proc, args)
(list-of-values (operands exp) env)))
(else
(error "Unknown expression type -- EVAL" exp))))
;; Apply
(define (mapply procedure arguments)
(cond ((primitive-procedure? procedure)
(apply-primitive-procedure procedure arguments))
((compound-procedure? procedure)
(eval-sequence
(procedure-body procedure)
(extend-environment
(procedure-parameters procedure)
arguments
(procedure-environment procedure))))
(else
(error "Unknown procedure type -- APPLY" procedure))))
(define (list-of-values exps env)
(if (no-operands? exps)
'()
(cons (meval (first-operand exps) env)
(list-of-values (rest-operands exps) env))))
(define (eval-if exp env)
(if (true? (meval (if-predicate exp) env))
(meval (if-consequent exp) env)
(meval (if-alternative exp) env)))
(define (eval-sequence exps env)
(cond ((last-exp? exps) (meval (first-exp exps) env))
(else (meval (first-exp exps) env)
(eval-sequence (rest-exps exps) env))))
(define (eval-assignment exp env)
(set-variable-value! (assignment-variable exp)
(meval (assignment-value exp) env)
env)
'ok)
(define (eval-definition exp env)
(define-variable! (definition-variable exp)
(meval (definition-value exp) env)
env)
'ok)
;; Syntax
;; self-evaluating - only numbers and strings
(define (self-evaluating? exp)
(cond ((number? exp) #t)
((string? exp) #t)
(else #f)))
;; var = symbol
(define (variable? exp) (symbol? exp))
;; quotation
(define (quoted? exp)
(tagged-list? exp 'quote))
(define (text-of-quotation exp) (cadr exp))
;; assignment
(define (assignment? exp)
(tagged-list? exp 'set!))
(define (assignment-variable exp) (cadr exp))
(define (assignment-value exp) (caddr exp))
;; definition of vars and procs
(define (definition? exp)
(tagged-list? exp 'define))
(define (definition-variable exp)
(if (symbol? (cadr exp))
(cadr exp) ;; define variable
(caadr exp))) ;; define procedure
(define (definition-value exp)
(if (symbol? (cadr exp))
(caddr exp) ;; value for variable
(make-lambda (cdadr exp) ;; value for procedure
(cddr exp))))
;; lambdas
(define (lambda? exp) (tagged-list? exp 'lambda))
(define (lambda-parameters exp) (cadr exp))
(define (lambda-body exp) (cddr exp))
(define (make-lambda parameters body)
(cons 'lambda (cons parameters body)))
;; and
(define (and? exp) (tagged-list? exp 'and))
(define (and-operands exp) (cdr exp))
(define (eval-and exp env)
(cond ((null? exp) 'true)
((false? (meval (car exp) env)) 'false)
(else (eval-and (cdr exp) env))))
;; or
(define (or? exp) (tagged-list? exp 'or))
(define (or-operands exp) (cdr exp))
(define (eval-or exp env)
(cond ((null? exp) 'false)
((true? (meval (car exp) env)) 'true)
(else (eval-or (cdr exp) env))))
;; if stuff
(define (if? exp) (tagged-list? exp 'if))
(define (if-predicate exp) (cadr exp))
(define (if-consequent exp) (caddr exp))
(define (if-alternative exp)
(if (not (null? (cdddr exp)))
(cadddr exp)
'false))
(define (make-if predicate consequent alternative)
(list 'if predicate consequent alternative))
;; let stuff
(define (let? exp)
(tagged-list? exp 'let))
(define (let-body exp) (cdr exp))
(define (let-vars-and-exps exp) (car exp))
(define (let-vars exp)
(if (null? exp)
'()
(cons (caar exp) (let-vars (cdr exp)))))
(define (let-exps exp)
(if (null? exp)
'()
(cons (cadar exp) (let-exps (cdr exp)))))
(define (let->combination exp)
(cons (make-lambda (let-vars (let-vars-and-exps exp)) (let-body exp))
(let-exps (let-vars-and-exps exp))))
;; let* stuff
(define (let*? exp)
(tagged-list? exp 'let*))
(define (let*-body exp) (cdr exp))
(define (let*-vars-and-exps exp) (car exp))
(define (let*->nested-lets exp)
(define (helper var-exps body)
(if (null? (cdr var-exps))
(cons 'let (cons var-exps body)) ;; final let
(list 'let (list (car var-exps))
(helper (cdr var-exps) body))))
(helper (let*-vars-and-exps exp) (let*-body exp)))
;; letrec stuff
(define (letrec? exp) (tagged-list? exp 'letrec))
(define (letrec-vars-and-exps exp) (car exp))
(define (letrec-body exp) (cdr exp))
(define (letrec->let exp)
(list 'let
(map (lambda (var-exp) (list (car var-exp) ''*unassigned)) (letrec-vars-and-exps exp))
(make-begin (append (map (lambda (var-exp) (list 'set! (car var-exp) (cadr var-exp))) (letrec-vars-and-exps exp))
(letrec-body exp)))))
;; begin
(define (begin? exp) (tagged-list? exp 'begin))
(define (begin-actions exp) (cdr exp))
(define (last-exp? seq) (null? (cdr seq)))
(define (first-exp seq) (car seq))
(define (rest-exps seq) (cdr seq))
(define (sequence->exp seq)
(cond ((null? seq) seq)
((last-exp? seq) (first-exp seq))
(else (make-begin seq))))
(define (make-begin seq) (cons 'begin seq))
;; procedure invocation
(define (application? exp) (pair? exp))
(define (operator exp) (car exp))
(define (operands exp) (cdr exp))
(define (no-operands? ops) (null? ops))
(define (first-operand ops) (car ops))
(define (rest-operands ops) (cdr ops))
;; compound cond
(define (cond? exp) (tagged-list? exp 'cond))
(define (cond-clauses exp) (cdr exp))
(define (cond-else-clause? clause)
(eq? (cond-predicate clause) 'else))
(define (cond-predicate clause) (car clause))
(define (cond-actions clause) (cdr clause))
(define (cond->if exp)
(expand-clauses (cond-clauses exp)))
(define (expand-clauses clauses)
(if (null? clauses)
'false
(let ((first (car clauses))
(rest (cdr clauses)))
(if (cond-else-clause? first)
(if (null? rest)
(sequence->exp (cond-actions first))
(error "Clause else not the last" clauses))
(make-if (cond-predicate first)
(sequence->exp (cond-actions first))
(expand-clauses rest))))))
;; load
(define (load? exp) (tagged-list? exp 'load))
(define (eval-load exp env)
(meval (read (open-input-file (cadr exp))) env))
;; data structures
(define (true? x)
(not (eq? x false)))
(define (false? x)
(eq? x false))
(define (make-procedure parameters body env)
(list 'procedure parameters body env))
(define (compound-procedure? p)
(tagged-list? p 'procedure))
(define (procedure-parameters p) (cadr p))
(define (procedure-body p) (caddr p))
(define (procedure-environment p) (cadddr p))
;; environment
;; environment is the list of frames
(define (enclosing-environment env) (cdr env))
(define (first-frame env) (car env))
(define the-empty-environment '())
;; frame is the two lists: list of variables and list of values
(define (make-frame variables values)
(cons variables values))
(define (frame-variables frame) (car frame))
(define (frame-values frame) (cdr frame))
(define (add-binding-to-frame! var val frame)
(set-car! frame (cons var (car frame))) ;; mutable append (frame = x + frame)
(set-cdr! frame (cons val (cdr frame))))
;; create new frame(vars, vals) and ptr to base env
(define (extend-environment vars vals base-env)
(if (= (length vars) (length vals))
(cons (make-frame vars vals) base-env)
(if (< (length vars) (length vals))
(error "Too many arguments supplied" vars vals)
(error "Too few arguments supplied" vars vals))))
(define (lookup-variable-value var env)
(define (env-loop env)
(define (scan vars vals)
(cond ((null? vars)
(env-loop (enclosing-environment env)))
((eq? (car vars) '*unassigned)
(error "Unassigned variable -- LOOKUP" var))
((eq? var (car vars))
(car vals))
(else (scan (cdr vars) (cdr vals)))))
(if (eq? env the-empty-environment)
(error "Unbound variable" var)
(let ((frame (first-frame env)))
(scan (frame-variables frame)
(frame-values frame)))))
(env-loop env))
(define (set-variable-value! var val env)
(define (env-loop env)
(define (scan vars vals)
(cond ((null? vars)
(env-loop (enclosing-environment env)))
((eq? var (car vars))
(set-car! vals val))
(else (scan (cdr vars) (cdr vals)))))
(if (eq? env the-empty-environment)
(error "Unbound variable -- SET!" var)
(let ((frame (first-frame env)))
(scan (frame-variables frame)
(frame-values frame)))))
(env-loop env))
(define (define-variable! var val env)
(let ((frame (first-frame env)))
(define (scan vars vals)
(cond ((null? vars)
(add-binding-to-frame! var val frame))
((eq? var (car vars))
(set-car! vals val))
(else (scan (cdr vars) (cdr vals)))))
(scan (frame-variables frame)
(frame-values frame))))
(define (primitive-procedure? proc)
(tagged-list? proc 'primitive))
(define (primitive-implementation proc) (cadr proc))
(define primitive-procedures
(list (list 'car car)
(list 'cdr cdr)
(list 'cons cons)
(list 'null? null?)
(list '+ +)
(list '- -)
(list '/ /)
(list '* *)
(list '= =)
(list '< <)
(list '> >)
(list '<= <=)
(list '>= >=)
))
(define (primitive-procedure-names)
(map car primitive-procedures))
(define (primitive-procedure-objects)
(map (lambda (proc) (list 'primitive (cadr proc))) primitive-procedures))
(define (setup-environment)
(let ((initial-env
(extend-environment (primitive-procedure-names)
(primitive-procedure-objects)
the-empty-environment)))
(define-variable! 'true true initial-env)
(define-variable! 'false false initial-env)
initial-env))
(define the-global-environment (setup-environment))
(define (apply-primitive-procedure proc args)
(apply-in-underlying-scheme (primitive-implementation proc) args))
;; driver loop
(define input-promt ";;; M-Eval input:")
(define output-promt ";;; M-Eval value:")
(define (driver-loop)
(promt-for-input input-promt)
(let ((input (read)))
(let ((output (meval input the-global-environment)))
(announce-output output-promt)
(user-print output)))
(driver-loop))
(define (promt-for-input string)
(newline) (newline) (display string) (newline))
(define (announce-output string)
(newline) (display string) (newline))
(define (user-print object)
(if (compound-procedure? object)
(display (list 'compound-procedure
(procedure-parameters object)
(procedure-body object)
'<procedure-env>))
(display object)))
the-global-environment
(driver-loop)
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