# SICP: Metacircular Evaluator

SICP 的经典就不用说了，有史以来最值得一读的编程语言书目。

在SICP 4.1 Metacircular Evaluator中实现了一个用scheme实现的scheme解释器，以说明元语言级别的抽象概念，SICP以此章为基础，又在后续章节中延伸至Lazy Evaluation，amb，continuation和Logic Programing Prolog， 所以理解这个scheme版的元语言解释器对通晓SICP很重要。下面的代码主要是copy SICP上的，300行左右，在guile下运行成功（Lisp的一大特点就是代码即语法树，这个evaluator将会帮助初学者很好的理解这个概念）。

```;;; This is simple evaluator of lisp.
;;; SICP: 4.1 The Metacircular Evaluator
;;;

(define apply-in-underlying-scheme apply)

(define (true? x)
(not (eq? x #f)))
(define (false? x)
(eq? x #f))

(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)
(apply (eval (operator exp) env)
(list-of-values (operands exp) env)))
(else
(error "Unknown expression type -- EVAL" exp))))

(define (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)
'()
(cons (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)
(cond ((number? exp) #t)
((string? exp) #f)
(else #f)))

(define (variable? exp) (symbol? exp))

(define (quoted? exp)
(tagged-list? exp 'quote))

(define (text-of-quotation exp) (cadr exp))

(define (tagged-list? exp tag)
(if (pair? exp)
(eq? (car exp) tag)
#f))

(define (assignment? exp)
(tagged-list? exp 'set!))

(define (assignment-variable exp) (cadr exp))
(define (assignment-value exp) (caddr exp))

(define (definition? exp)
(tagged-list? exp 'define))

(define (definition-variable exp)
(if (symbol? (cadr exp))

(define (definition-value exp)
(if (symbol? (cadr exp))
(make-lambda (cdadr exp)   ; formal parameters
(cddr exp)))) ; body

(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)))

(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)))
'false))

(define (make-if predicate consequent alternative)
(list 'if predicate consequent alternative))

(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))

(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))

(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                          ; no else clause
(let ((first (car clauses))
(rest (cdr 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 (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))

(define (enclosing-environment env) (cdr env))
(define (first-frame env) (car env))
(define the-empty-environment '())

(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)))
(set-cdr! frame (cons val (cdr frame))))

(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? 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 'assoc assoc)
;<more primitives>
))
(define (primitive-procedure-names)
(map car
primitive-procedures))

(define (primitive-procedure-objects)
(map (lambda (proc) (list 'primitive (cadr proc)))
primitive-procedures))

(define (apply-primitive-procedure proc args)
(apply-in-underlying-scheme
(primitive-implementation proc) args))

(define (setup-environment)
(let ((initial-env
(extend-environment (primitive-procedure-names)
(primitive-procedure-objects)
the-empty-environment)))
(define-variable! 'true #t initial-env)
(define-variable! 'false #f 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 ((output (eval input the-global-environment)))
(announce-output output-prompt)
(user-print output)))
(driver-loop))
(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 (list 'compound-procedure
(procedure-parameters object)
(procedure-body object)
'<procedure-env>))
(display object)))

(driver-loop)```

```[root@localhost ~]# guile evaluator.scm

;;; M-Eval input:
(define (fact x)
(if (= x 0)
1
(* x (fact (- x 1)))))

;;; M-Eval value:
ok

;;; M-Eval input:
(fact 10)

;;; M-Eval value:
3628800```

SICP: Metacircular Evaluator

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