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let区别（关于racket和r5rs)

2013-11-11 08:48  youxin  阅读(817)  评论(0)  编辑  收藏  举报

R5RS is the Revised5 Report on the Algorithmic Language Scheme.参考http://www.schemers.org/Documents/Standards/R5RS/。

racket let 用法：

 

 

(let ([id val-expr] ...) body ...+)
(let proc-id ([id init-expr] ...) body ...+)
The first form evaluates the val-exprs left-to-right, creates a newlocation for each id, and places the values into the locations. It then evaluates the bodys, in which the ids are bound. The last bodyexpression is in tail position with respect to the let form. The ids must be distinct according to bound-identifier=?.

 

 

Examples:
> (let ([x 5]) x)

5
> (let ([x 5])
    (let ([x 2]
          [y x])
      (list y x)))

'(5 2)

 

The second form evaluates the init-exprs; the resulting values become arguments in an application of a procedure (lambda (id ...) body ...+), where proc-id is bound within the bodys to the procedure itself.

 

Example:
> (let fac ([n 10])
    (if (zero? n)
        1
        (* n (fac (sub1 n)))))

3628800

syntax

(let* ([id val-expr] ...) body ...+)
Like let, but evaluates the val-exprs one by one, creating a locationfor each id as soon as the value is available. The ids are bound in the remaining val-exprs as well as the bodys, and the ids need not be distinct; later bindings shadow earlier bindings.

 

 

Example:
> (let* ([x 1]
         [y (+ x 1)])
    (list y x))

'(2 1)

 

r5rs:

library syntax:  (let <bindings> <body>) 

Syntax: <Bindings> should have the form ((<variable1> <init1>) ...),

where each <init> is an expression, and <body> should be a sequence of one or more expressions. It is an error for a <variable> to appear more than once in the list of variables being bound.

Semantics: The <init>s are evaluated in the current environment (in some unspecified order), the <variable>s are bound to fresh locations holding the results, the <body> is evaluated in the extended environment, and the value(s) of the last expression of <body> is(are) returned. Each binding of a <variable> has <body> as its region.

(let ((x 2) (y 3))
  (* x y))                              ===>  6

(let ((x 2) (y 3))
  (let ((x 7)
        (z (+ x y)))
    (* z x)))                           ===>  35

See also named let, section 4.2.4.

 

 

 

 

library syntax:  (let* <bindings> <body>) 

Syntax: <Bindings> should have the form ((<variable1> <init1>) ...),

and <body> should be a sequence of one or more expressions.

Semantics: Let* is similar to let, but the bindings are performed sequentially from left to right, and the region of a binding indicated by(<variable> <init>) is that part of the let* expression to the right of the binding. Thus the second binding is done in an environment in which the first binding is visible, and so on.

(let ((x 2) (y 3))
  (let* ((x 7)
         (z (+ x y)))
    (* z x)))                     ===>  70