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One problem on one of my practice exams is centered around message passing and data directed program. It asks for a procedure that utilizes the table initializing these install packages:

(define (attach-tag tag data) (cons tag data))
(define (get-tag data) (car data))
(define (get-contents data) (cdr data))

(define (install-metric-package)
; internal procedures
(define (get-kilometers d) d)
(define (get-miles d) (/ d 1.6))
(define (make-from-kilometers d) d)
(define (make-from-miles d) (* d 1.6))
; install metric packages
(2d-put! 'get-kilometers 'metric get-kilometers)
(2d-put! 'get-miles 'metric get-miles)
(2d-put! 'make-from-kilometers 'metric
   (lambda(d) (attach-tag 'metric (make-from-kilometers d))))
(2d-put! 'make-from-miles 'metric
   (lambda(d) (attach-tag 'metric (make-from-miles d))))
'done)

(define (install-english-package)
; internal procedures
(define (get-kilometers d) (* d 1.6))
(define (get-miles d) d)
(define (make-from-kilometers d) (/ d 1.6))
(define (make-from-miles d) d)
; install english packages
(2d-put! 'get-kilometers 'english get-kilometers)
(2d-put! 'get-miles 'english get-miles)
(2d-put! 'make-from-kilometers 'english
   (lambda(d) (attach-tag 'english (make-from-kilometers d))))
(2d-put! 'make-from-miles 'english
   (lambda(d) (attach-tag 'english (make-from-miles d))))
'done)

and allows the generic operators to work. Normally, I would have some code to show effort on my part, but for about a day, I've been completely stumped on how to even BEGIN with this one. All I'm given to start with is this: 

(define (generic-op operator object)

I do remember having a lab on this, but it was basically taking an already-existing generic procedure and creating simple procedures to get values already contained within it. If anyone can shed some light on how to approach this, I would appreciate it greatly. Again, I'm sorry that I have nothing to show on my part, but I honestly do not know what to do here.

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手順は、2d-put!テーブルにエントリを追加するだけです。コードを実行するinstall-*-packageと、次のよう[proc]になります（指定されたコードで定義されているプロシージャの1つはどこにありますか）。

                      | 'metric | 'english |
'make-from-kilometers | [proc]  |  [proc]  |
'make-from-miles      | [proc]  |  [proc]  |
'get-kilometers       | [proc]  |  [proc]  |
'get-miles            | [proc]  |  [proc]  |

次に2d-get、の本文で使用してgeneric-op、タグ付けされたデータの一部でプロシージャを呼び出すたびに、プログラムが使用するプロシージャのバージョン（メートル法/英語）を認識できるようにします。

(define (generic-op operator object)
    ((2d-get [first-tag] [second-tag]) [data]))

この形式では、[first-tag]は演算子を指定するために使用される記号になります。たとえば'get-miles、は、指定された手順で[second-tag]アクセスできるデータに添付された記号であり、実際に操作を実行する番号です。与えられた手順でアクセスできます。これらすべてを、次のような最終的な答えにまとめます。get-tag[data]get-contents

(define (generic-op operator object)
    ((2d-get operator (get-tag object)) (get-contents object)))
于 2012-11-19T22:09:14.403 に答える