1

这段代码在 Ikarus 的实现中vector-map：

     (let f ([p p] [v v] [i 0] [n (vector-length v)] [ac '()])
       (cond
         [($fx= i n) (ls->vec ac n)]
         [else 
          (f p v ($fxadd1 i) n (cons (p (vector-ref v i)) ac))]))]

为什么命名的 let 包含参数p,v和n？

的完整定义vector-map如下。

  (module (vector-map)
    (define who 'vector-map)
    (define (ls->vec ls n) 
      (let f ([v (make-vector n)]
              [n n]
              [ls ls])
        (cond
          [(null? ls) v]
          [else
           (let ([n ($fxsub1 n)])
             ($vector-set! v n ($car ls))
             (f v n ($cdr ls)))])))
    (define vector-map
      (case-lambda
        [(p v) 
         (unless (procedure? p) 
           (die who "not a procedure" p))
         (unless (vector? v) 
           (die who "not a vector" v))
         (let f ([p p] [v v] [i 0] [n (vector-length v)] [ac '()])
           (cond
             [($fx= i n) (ls->vec ac n)]
             [else 
              (f p v ($fxadd1 i) n (cons (p (vector-ref v i)) ac))]))]
        [(p v0 v1) 
         (unless (procedure? p) 
           (die who "not a procedure" p))
         (unless (vector? v0) 
           (die who "not a vector" v0))
         (unless (vector? v1) 
           (die who "not a vector" v1))
         (let ([n (vector-length v0)])
           (unless ($fx= n ($vector-length v1))
             (die who "length mismatch" v0 v1))
           (let f ([p p] [v0 v0] [v1 v1] [i 0] [n n] [ac '()])
             (cond
               [($fx= i n) (ls->vec ac n)]
               [else 
                (f p v0 v1 ($fxadd1 i) n 
                   (cons (p ($vector-ref v0 i) ($vector-ref v1 i)) ac))])))]
        [(p v0 v1 . v*) 
         (unless (procedure? p) 
           (die who "not a procedure" p))
         (unless (vector? v0) 
           (die who "not a vector" v0))
         (unless (vector? v1) 
           (die who "not a vector" v1))
         (let ([n (vector-length v0)])
           (unless ($fx= n ($vector-length v1))
             (die who "length mismatch" v0 v1))
           (let f ([v* v*] [n n])
                 (unless (null? v*) 
               (let ([a ($car v*)])
                 (unless (vector? a) 
                   (die who "not a vector" a))
                 (unless ($fx= ($vector-length a) n) 
                   (die who "length mismatch")))
               (f ($cdr v*) n)))
           (let f ([p p] [v0 v0] [v1 v1] [v* v*] [i 0] [n n] [ac '()])
             (cond
               [($fx= i n) (ls->vec ac n)] 
               [else 
                (f p v0 v1 v* ($fxadd1 i) n 
                   (cons 
                     (apply p ($vector-ref v0 i) ($vector-ref v1 i)
                       (let f ([i i] [v* v*]) 
                         (if (null? v*) 
                             '()
                             (cons ($vector-ref ($car v*) i) 
                                       (f i ($cdr v*))))))
                     ac))])))])))
4

1 回答 1

3

很难说出原因，因为它似乎没有必要，但可能是减少引用的自由变量数量的优化。在这种情况下，p、v和n成为 的词法变量f，不再需要自由变量引用。

但是，由于相应的自由变量实际上是词法变量，vector-map并没有进一步修改，因此编译器内部自动进行这种优化应该不难。现在，如果 Ikarus 没有编译器，那么这可能解释了手动优化。

于 2011-07-06T12:30:15.103 回答