[plt-scheme] How to make a setter out of an identifier

From: Jos Koot (jos.koot at telefonica.net)
Date: Tue Nov 3 20:42:49 EST 2009

This may be what you are looking for:

#lang scheme
(define kk 0)

(define-syntax (make-setter stx)
 (syntax-case stx ()
  ((_ id)
      (datum->syntax stx ; specify context
       (string->symbol ; we have no symbol-append, therefore use string-append.
        (string-append "set-" (symbol->string (syntax->datum #'id)) "!")))))
  #'(define-syntax setter
     (syntax-rules ()
      ((_ v) (set! id v))))))))

(make-setter kk)

(set-kk! 3)

kk ; --> 3

You have to use datum->syntax in order to introduce identifiers within their appropriate context.

Good luck, Jos

----- Original Message ----- 
From: "Cheng-Chang Wu" <chengchangwu at yahoo.com>
To: <plt-scheme at list.cs.brown.edu>
Sent: Wednesday, November 04, 2009 2:19 AM
Subject: [plt-scheme] How to make a setter out of an identifier


I am new to the brave scheme world, and trying hard to understand the secrets of macro.

I want to do the following: take an identifier and make a setter out of the identifier.

For example,

(define kk 0)
(make-setter kk)

should give me a set-kk! which can be used to set the value of kk.

The best macro I can image is

(define-syntax make-setter
  (lambda (x)
    (syntax-case x ()
        ((make-setter id)
                 (syntax k)
                 `(define (,(string->symbol (string-append "set-" (symbol->string 'id) "!")) a-value) (set! id a-value)))))
           (syntax def))))))

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