316 lines
9.4 KiB
Scheme
316 lines
9.4 KiB
Scheme
(define (eval exp env)
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(cond ((self-evaluating? exp) exp)
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((variable? exp) (lookup-variable-value exp env))
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((quoted? exp) (text-of-quotation exp))
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((assignment? exp) (eval-assignment exp env))
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((definition? exp) (eval-definition exp env))
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((unless? exp) (eval (unless->combination exp) env))
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((if? exp) (eval-if exp env))
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((lambda? exp)
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(make-procedure (lambda-parameters exp)
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(lambda-body exp)
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env))
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((begin? exp)
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(eval-sequence (begin-actions exp) env))
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((cond? exp) (eval (cond->if exp) env))
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((application? exp)
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(apply (eval (operator exp) env)
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(list-of-values (operands exp) env)))
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(else
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(error "Unknown expression type -- EVAL" exp))))
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(define apply-in-underlying-scheme apply)
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(define (apply procedure arguments)
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(cond ((primitive-procedure? procedure)
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(apply-primitive-procedure procedure arguments))
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((compound-procedure? procedure)
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(eval-sequence
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(procedure-body procedure)
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(extend-environment
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(procedure-parameters procedure)
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arguments
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(procedure-environment procedure))))
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(else
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(error
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"Unknown procedure type -- APPLY" procedure))))
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(define (list-of-values exps env)
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(if (no-operands? exps)
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'()
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(cons (eval (first-operand exps) env)
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(list-of-values (rest-operands exps) env))))
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(define (eval-if exp env)
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(if (true? (eval (if-predicate exp) env))
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(eval (if-consequent exp) env)
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(eval (if-alternative exp) env)))
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(define (eval-sequence exps env)
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(cond ((last-exp? exps) (eval (first-exp exps) env))
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(else (eval (first-exp exps) env)
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(eval-sequence (rest-exps exps) env))))
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(define (eval-assignment exp env)
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(set-variable-value! (assignment-variable exp)
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(eval (assignment-value exp) env)
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env)
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'ok)
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(define (eval-definition exp env)
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(define-variable! (definition-variable exp)
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(eval (definition-value exp) env)
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env)
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'ok)
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(define (self-evaluating? exp)
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(cond ((number? exp) true)
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((string? exp) true)
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((eq? exp #t) true)
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((eq? exp #f) true)
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(else false)))
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(define (variable? exp) (symbol? exp))
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(define (quoted? exp)
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(tagged-list? exp 'quote))
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(define (text-of-quotation exp) (cadr exp))
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(define (tagged-list? exp tag)
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(if (pair? exp)
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(eq? (car exp) tag)
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false))
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(define (assignment? exp)
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(tagged-list? exp 'set!))
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(define (assignment-variable exp) (cadr exp))
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(define (assignment-value exp) (caddr exp))
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(define (definition? exp)
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(tagged-list? exp 'define))
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(define (definition-variable exp)
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(if (symbol? (cadr exp))
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(cadr exp)
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(caadr exp)))
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(define (definition-value exp)
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(if (symbol? (cadr exp))
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(caddr exp)
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(make-lambda (cdadr exp) ; formal parameters
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(cddr exp)))) ; body
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(define (lambda? exp) (tagged-list? exp 'lambda))
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(define (lambda-parameters exp) (cadr exp))
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(define (lambda-body exp) (cddr exp))
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(define (make-lambda parameters body)
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(cons 'lambda (cons parameters body)))
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(define (unless? exp) (tagged-list? exp 'unless))
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(define (if? exp) (tagged-list? exp 'if))
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(define (if-predicate exp) (cadr exp))
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(define (if-consequent exp) (caddr exp))
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(define (if-alternative exp)
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(if (not (null? (cdddr exp)))
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(cadddr exp)
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'false))
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(define (make-if predicate consequent alternative)
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(list 'if predicate consequent alternative))
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(define (begin? exp) (tagged-list? exp 'begin))
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(define (begin-actions exp) (cdr exp))
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(define (last-exp? seq) (null? (cdr seq)))
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(define (first-exp seq) (car seq))
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(define (rest-exps seq) (cdr seq))
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(define (sequence->exp seq)
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(cond ((null? seq) seq)
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((last-exp? seq) (first-exp seq))
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(else (make-begin seq))))
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(define (make-begin seq) (cons 'begin seq))
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(define (application? exp) (pair? exp))
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(define (operator exp) (car exp))
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(define (operands exp) (cdr exp))
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(define (no-operands? ops) (null? ops))
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(define (first-operand ops) (car ops))
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(define (rest-operands ops) (cdr ops))
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(define (cond? exp) (tagged-list? exp 'cond))
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(define (cond-clauses exp) (cdr exp))
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(define (cond-else-clause? clause)
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(eq? (cond-predicate clause) 'else))
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(define (cond-predicate clause) (car clause))
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(define (cond-actions clause) (cdr clause))
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(define (cond->if exp)
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(expand-clauses (cond-clauses exp)))
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(define (expand-clauses clauses)
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(if (null? clauses)
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'false ; no else clause
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(let ((first (car clauses))
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(rest (cdr clauses)))
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(if (cond-else-clause? first)
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(if (null? rest)
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(sequence->exp (cond-actions first))
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(error "ELSE clause isn't last -- COND->IF"
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clauses))
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(make-if (cond-predicate first)
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(sequence->exp (cond-actions first))
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(expand-clauses rest))))))
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(define (true? x)
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(not (eq? x false)))
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(define (false? x)
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(eq? x false))
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(define (make-procedure parameters body env)
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(list 'procedure parameters body env))
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(define (compound-procedure? p)
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(tagged-list? p 'procedure))
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(define (procedure-parameters p) (cadr p))
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(define (procedure-body p) (caddr p))
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(define (procedure-environment p) (cadddr p))
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(define (enclosing-environment env) (cdr env))
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(define (first-frame env) (car env))
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(define the-empty-environment '())
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(define (make-frame variables values)
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(cons variables values))
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(define (frame-variables frame) (car frame))
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(define (frame-values frame) (cdr frame))
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(define (add-binding-to-frame! var val frame)
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(set-car! frame (cons var (car frame)))
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(set-cdr! frame (cons val (cdr frame))))
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(define (extend-environment vars vals base-env)
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(if (= (length vars) (length vals))
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(cons (make-frame vars vals) base-env)
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(if (< (length vars) (length vals))
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(error "Too many arguments supplied" vars vals)
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(error "Too few arguments supplied" vars vals))))
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(define (lookup-variable-value var env)
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(define (env-loop env)
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(define (scan vars vals)
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(cond ((null? vars)
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(env-loop (enclosing-environment env)))
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((eq? var (car vars))
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(car vals))
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(else (scan (cdr vars) (cdr vals)))))
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(if (eq? env the-empty-environment)
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(error "Unbound variable" var)
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(let ((frame (first-frame env)))
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(scan (frame-variables frame)
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(frame-values frame)))))
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(env-loop env))
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(define (set-variable-value! var val env)
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(define (env-loop env)
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(define (scan vars vals)
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(cond ((null? vars)
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(env-loop (enclosing-environment env)))
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((eq? var (car vars))
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(set-car! vals val))
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(else (scan (cdr vars) (cdr vals)))))
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(if (eq? env the-empty-environment)
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(error "Unbound variable -- SET!" var)
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(let ((frame (first-frame env)))
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(scan (frame-variables frame)
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(frame-values frame)))))
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(env-loop env))
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(define (define-variable! var val env)
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(let ((frame (first-frame env)))
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(define (scan vars vals)
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(cond ((null? vars)
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(add-binding-to-frame! var val frame))
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((eq? var (car vars))
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(set-car! vals val))
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(else (scan (cdr vars) (cdr vals)))))
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(scan (frame-variables frame)
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(frame-values frame))))
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(define (primitive-procedure-objects)
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(map (lambda (proc) (list 'primitive (cadr proc)))
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primitive-procedures))
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(define primitive-procedures
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(list (list 'car car)
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(list 'cdr cdr)
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(list 'cons cons)
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(list 'null? null?)
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(list 'display display)
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(list 'equal? equal?)
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(list 'newline newline)
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(list '= =)
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(list '+ +)
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(list '* *)
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(list '/ /)
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(list '- -)
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(list '< <)
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;;<more primitives>
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))
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(define (primitive-procedure-names)
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(map car
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primitive-procedures))
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(define (setup-environment)
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(let ((initial-env
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(extend-environment (primitive-procedure-names)
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(primitive-procedure-objects)
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the-empty-environment)))
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(define-variable! 'true true initial-env)
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(define-variable! 'false false initial-env)
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initial-env))
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(define the-global-environment (setup-environment))
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(define (primitive-procedure? proc)
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(tagged-list? proc 'primitive))
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(define (primitive-implementation proc) (cadr proc))
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(define (apply-primitive-procedure proc args)
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(apply-in-underlying-scheme
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(primitive-implementation proc) args))
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(define input-prompt ";;; M-Eval input:")
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(define output-prompt ";;; M-Eval value:")
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(define (driver-loop)
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(prompt-for-input input-prompt)
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(let ((input (read)))
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(let ((output (eval input the-global-environment)))
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(announce-output output-prompt)
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(user-print output)))
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(driver-loop))
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(define (prompt-for-input string)
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(newline) (newline) (display string) (newline))
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(define (announce-output string)
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(newline) (display string) (newline))
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(define (user-print object)
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(if (compound-procedure? object)
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(display (list 'compound-procedure
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(procedure-parameters object)
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(procedure-body object)
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'<procedure-env>))
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(display object)))
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(define (eval-force exp env)
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(let ((output (actual-value exp env)))
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output))
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(define the-global-environment (setup-environment))
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;(driver-loop)
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