lisp语法分析与执行分离

lisp语法分析与执行分离

《sicp》4.1.7节读书笔记

这节中将书中之前实现的解释器做了改进,将语法分析于执行分离.这样做能提高很多效率,比如我们需定义一个过程,然后执行该过程多次.分情况说明:

1.语法分析于执行未分离

每次执行都会进行语法分析

2.语法分析于执行分离.

在定义该过程时,已经完成语法分析,该过程名保存在环境中的相应约束
就是,语法分析生成的(lambda (env) …).所以当执行该过程时,只需传入相应环境就可以完成执行,这就实现了一次分析,供永久执行.

实际例子:

输入语句

(define (f x) (+ x x))
(f 4)

分别分析以上两种情况的执行过程

//lisp解释器的语法分析和执行未分离的情况

输入: (define (f x) (+ x x))

==> (eval ('define ('f 'x) ('+ 'x 'x))  env)		
==> (eval-definition ('define ('f 'x) ('+ 'x 'x)) env )
==> (define-variable! (definition-variable ('define ('f 'x) ('+ 'x 'x)))
			          (eval (definition-value ('define ('f 'x) ('+ 'x 'x))) env)
					  env)
		;其中的 (definition-variable ('define ('f 'x) ('+ 'x 'x)) ) 返回为 'f  
                (eval (definition-value ('define ('f 'x) ('+ 'x 'x))) env)  返回为 ('procedure ('x) ('+ 'x 'x) env) 
//所以在这里执行完毕后 env环境中,多了一条f函数的约束.
('f ('procedure ('x) ('+ 'x 'x) env) env )

输入: (f 4)

==> (eval ('f 4) env)
==> (apply (eval 'f env) 4) 
		;其中的(eval 'f env) 返回 ('procedure ('x) ('+ 'x 'x) env)

==>(apply ('procedure ('x) (+ 'x 'x) env)  4)
==>(eval-sequence ('+ 'x 'x) (('x:4) (env)) )
		;其中(('x:4) (env)) 是执行(f 4)后创建的新环境
==>(eval ('+ 'x 'x) (('x:4) (env)))
==>(apply (eval '+ (('x:4) (env))) (4 4))	
		;其中(eval '+ (('x:4) (env))) 返回 ('primitive +)       (假设初始环境含有 ""+"" 基本过程)
==>(apply ('primitive +) (4 4))
==>(apply-primitive-procedure ('primitive) (4 4))

==>(apply-in-underlying-scheme
   + (4  4))
==>8
如果再输入一次(f 4)仍然将执行以上分析和执行过程,可见每执行次(f 4)解释器都将分析一次 f 函数,然后执行


//lisp解释器的语法分析与执行分开的情况

输入 (define (f x) (+ x x))

==>(eval  ('define ('f 'x) ('+ 'x 'x)  begin_env)
==>((analyze ('define ('f 'x) ('+ 'x 'x))) begin_env)
   ;;;执行 (analyze ('define ('f 'x) ('+ 'x 'x))) 
   ==>(analyze ('define ('f 'x) ('+ 'x 'x)) )
   ==>(analyze-definition ('define ('f 'x) ('+ 'x 'x)))
       {
	   (let ((var 'f)
		   (vproc (analyze ('lambda ('x 'x) ('+ 'x 'x))))
	     (lambda (env)
		    (define-variable! var (vproc env) env)
			'ok))
	   }
	  ;;;执行 (vproc (analyze ('lambda  ('x 'x) ('+ 'x 'x)))))
      ==>(analyze('lambda ('x 'x) ('+ 'x 'x)))
	  ==>(analyze-lambda ('lambda ('x 'x) ('+ 'x 'x)))
	     {
		     (let ((vars ('x 'x))
				  ( bproc (analyze-sequence  ( ('+ 'x 'x)) )))
			     (lambda (env) (make-procedure ('x 'x) brpoc env)))
		 }
		 ;;执行bproc
         ==>(analyze-sequence  (('+ 'x 'x)))
		 ==>{ (analyze ('+ 'x 'x))
	           (analyze-application ('+ x x)){
	        (let ((fproc (analyze '+))
				 (aprocs (map analyze ('x 'x))))
			(lambda (env)
			  (execute-application (fproc env)
			                       (map (lambda (aproc) (aproc env) aprocs))))
		    }
		    ;执行(analyze '+)
		    ==>(analyze-variable '+)
		    返回 :(lambda (env) (lookup-variable-value '+ env))
		    ;执行(map analyze ('x 'x))
		    返回 :((lambda (env) (look-variable-value 'x env)) (lambda(env) (look-variable-value 'x env)))
         返回:bproc:
	     (lambda(env) 
	     (execute-application ((lambda (env) (lookup-variable-value '+ env)) env) 
		 (map (lambda (aproc) (aproc env)) ((lambda (env) (look-variable-value 'x env)) (lambda(env) (look-variable-value 'x env))))
		 ))


		   }}
	    ==>返回:vproc=
		     (lambda (env)  (make-procedure ('x 'x) brpoc env))
			 ==>(make-procedure ('x 'x) brpoc env)
			 ==>返回:('procedure ('x 'x)  brpoc env)
	==>返回:
	(lambda(env)
	        (define-varibale! 'f) (vproc env) env)
;;代入 begin_env
==>((lambda(env) (define-varibale! 'f (vproc env) env) begin_env))
==>(define-varibale! 'f (vproc begin_env) begin_env)
      ==>(vproc begin_env)
		 f   :   ('procedure ('x 'x) brpoc begin_env)
最终:在 begin_env 中
f=('procedure 
	('x 'x)
	(lambda(env) 
	   (execute-application 
		   ((lambda (env) (lookup-variable-value '+ env)) env) 
	       (map (lambda (aproc) (aproc env)) ((lambda (env) (look-variable-value 'x env)) (lambda(env) (look-variable-value 'x env)))))))



//输入 (f 4)
==>(eval ('f 4) changed_env)
==>((analyze ('f 4)) changed_env) 
  ==>(analyze-application ('f 4))
  ==>{
	 (let ( (fproc (lambda(env) (lookup-varible-value 'f env)))
			(approcs (lambda(env) 4))
			(lambda(env)
			  (execute-application (fproc env)
			                        (map (lambda (aproc) (aproc env)) aproc)))))
  }
==>代入changed_env
==> (execute-application (fproc changed_env)  4)
==> (execute-application
			 ('procedure 
	         ('x 'x)
	         (lambda(env) 
	         (execute-application 
		     ((lambda (env) (lookup-variable-value '+ env)) env) 
	         (map (lambda (aproc) (aproc env)) ((lambda (env) (look-variable-value 'x env)) (lambda(env) (look-variable-value 'x env)))))))
			 4)
==>((lambda(env) 
	         (execute-application 
		     ((lambda (env) (lookup-variable-value '+ env)) env) 
	         (map (lambda (aproc) (aproc env)) ((lambda (env) (look-variable-value 'x env)) (lambda(env) (look-variable-value 'x env)))))))
             
		     ('x:4 (begin_env)))
==>(execute-application ('primitive +) (4 4))
==>(apply-in-underlying-scheme
   + (4  4))
==>8