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eval-apply.rkt
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eval-apply.rkt
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#lang racket
(require racket/mpair)
(include "mpair-extension.rkt")
(define (eval 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))
((lambda? exp)
(make-procedure (lambda-parameters exp)
(lambda-body exp)
env))
((begin? exp)
(eval-sequence (begin-actions exp) env))
((cond? exp) (eval (cond->if exp) env))
((application? exp)
(metacircular-apply (eval (operator exp) env)
(list-of-values (operands exp) env)))
(else
(error "Unknown expression type -- EVAL" exp))))
(define (metacircular-apply 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)
'()
(mcons (eval (first-operand exps) env)
(list-of-values (rest-operands exps) env))))
(define (eval-if exp env)
(if (true? (eval (if-predicate exp) env))
(eval (if-consequent exp) env)
(eval (if-alternative exp) env)))
(define (eval-sequence exps env)
(cond ((last-exp? exps) (eval (first-exp exps) env))
(else (eval (first-exp exps) env)
(eval-sequence (rest-exps exps) env))))
(define (eval-assignment exp env)
(set-variable-value! (assignment-variable exp)
(eval (assignment-value exp) env)
env)
'ok)
(define (eval-definition exp env)
(define-variable! (definition-variable exp)
(eval (definition-value exp) env)
env)
'ok)
(define (self-evaluating? exp)
(or (number? exp) (string? exp)))
(define (variable? exp) (symbol? exp))
(define (quoted? exp)
(tagged-list? exp 'quote))
(define (text-of-quotation exp) (mcadr exp))
(define (tagged-list? exp tag)
(if (mpair? exp)
(eq? (mcar exp) tag)
false))
(define (assignment? exp)
(tagged-list? exp 'set!))
(define (assignment-variable exp)
(mcadr exp))
(define (assignment-value exp)
(mcaddr exp))
(define (definition? exp)
(tagged-list? exp 'define))
(define (definition-variable exp)
(if (symbol? (mcadr exp))
(mcadr exp)
(mcaadr exp)))
(define (definition-value exp)
(if (symbol? (mcadr exp))
(mcaddr exp)
(make-lambda (mcdadr exp)
(mcddr exp))))
(define (lambda? exp)
(tagged-list? exp 'lambda))
(define (lambda-parameters exp) (mcadr exp))
(define (lambda-body exp) (mcddr exp))
(define (make-lambda parameters body)
(mcons 'lambda (mcons parameters body)))
(define (if? exp) (tagged-list? exp 'if))
(define (if-predicate exp) (mcadr exp))
(define (if-consequent exp) (mcaddr exp))
(define (if-alternative exp)
(if (not (null? (mcdddr exp)))
(mcadddr exp)
'false))
(define (make-if predicate consequent alternative)
(mlist 'if predicate consequent alternative))
(define (begin? exp) (tagged-list? exp 'begin))
(define (begin-actions exp) (mcdr exp))
(define (last-exp? seq) (null? (mcdr seq)))
(define (first-exp seq) (mcar seq))
(define (rest-exps seq) (mcdr seq))
(define (sequence->exp seq)
(cond ((null? seq) seq)
((last-exp? seq) (first-exp seq))
(else (make-begin seq))))
(define (make-begin seq) (mcons 'begin seq))
(define (application? exp) (mpair? exp))
(define (operator exp) (mcar exp))
(define (operands exp) (mcdr exp))
(define (no-operands? ops) (null? ops))
(define (first-operand ops) (mcar ops))
(define (rest-operands ops) (mcdr ops))
(define (cond? exp) (tagged-list? exp 'cond))
(define (cond-clauses exp) (mcdr exp))
(define (cond-else-clause? clause)
(eq? (cond-predicate clause) 'else))
(define (cond-predicate clause) (mcar clause))
(define (cond-actions clause) (mcdr clause))
(define (cond->if exp)
(expand-clauses (cond-clauses exp)))
(define (expand-clauses clauses)
(if (null? clauses)
'false
(let ((first (mcar clauses))
(rest (mcdr clauses)))
(if (cond-else-clause? first)
(if (null? rest)
(sequence->exp (cond-actions first))
(error "ELSE clause isn't last -- COND->IF"
clauses))
(make-if (cond-predicate first)
(sequence->exp (cond-actions first))
(expand-clauses rest))))))
(define (true? x)
(not (eq? x false)))
(define (false? x)
(eq? x false))
(define (make-procedure parameters body env)
(mlist 'procedure parameters body env))
(define (compound-procedure? p)
(tagged-list? p 'procedure))
(define (procedure-parameters p) (mcadr p))
(define (procedure-body p) (mcaddr p))
(define (procedure-environment p) (mcadddr p))
; procedures for handling environments
; environments are essentially lists of frames
(define (enclosing-environment env) (mcdr env))
(define (first-frame env) (mcar env))
(define the-empty-environment '())
(define (extend-environment vars vals base-env)
(if (= (mlength vars) (mlength vals))
(mcons (make-frame vars vals) base-env)
(if (< (mlength vars) (mlength vals))
(error "Too many arguments supplied" vars vals)
(error "Too few arguments supplied" vars vals))))
; procedures for handling frames containing variable to
; value bindings
(define (make-frame variables values)
(mcons variables values))
(define (frame-variables frame) (mcar frame))
(define (frame-values frame) (mcdr frame))
(define (add-binding-to-frame! var val frame)
(set-mcar! frame (mcons var (mcar frame)))
(set-mcdr! frame (mcons val (mcdr frame))))
; procedures for looking, setting, and defining variables
; in an environment
(define (lookup-variable-value var env)
(define (env-loop env)
(define (scan vars vals)
(cond ((null? vars)
(env-loop (enclosing-environment env)))
((eq? var (mcar vars))
(mcar vals))
(else (scan (mcdr vars) (mcdr 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 (mcar vars))
(set-mcar! vals val))
(else (scan (mcdr vars) (mcdr 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 (mcar vars))
(set-mcar! vals val))
(else (scan (mcdr vars) (mcdr vals)))))
(scan (frame-variables frame)
(frame-values frame))))
(define (primitive-procedure? proc)
(tagged-list? proc 'primitive))
(define (primitive-implementation proc) (mcadr proc))
(define primitive-procedures
(mlist (mlist 'car car)
(mlist 'cdr cdr)
(mlist 'cons cons)
(mlist 'null? null?)
(mlist 'list list)
(mlist '+ +)
(mlist '* *)
(mlist '/ /)
(mlist '- -)
(mlist '= =)))
(define (primitive-procedure-names)
(mmap mcar primitive-procedures))
(define (primitive-procedure-objects)
(mmap (lambda (proc) (mlist 'primitive (mcar (mcdr proc))))
primitive-procedures))
(define (apply-primitive-procedure proc args)
(apply
; The primitive apply only accepts immutable lists
; thus, the need to convert
(primitive-implementation proc) (mlist->list args)))
(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 input-prompt ";;; M-Eval input:")
(define output-prompt ";;; M-Eval value:")
(define (driver-loop)
(prompt-for-input input-prompt)
(let ((input (read)))
(if (not (eof-object? input))
(let ((output (eval (list->mlist-r input) the-global-environment)))
(announce-output output-prompt)
(user-print output)
(driver-loop))
(display "EOF Encountered. Terminating.."))))
(define (prompt-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 (mlist 'compound-procedure
(procedure-parameters object)
(procedure-body object)
'<procedure-env>))
(display object)))
(driver-loop)