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Re: [Chicken-users] Functions with lots of parameters
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From: |
John Cowan |
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Subject: |
Re: [Chicken-users] Functions with lots of parameters |
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Date: |
Fri, 29 Sep 2006 17:05:44 -0400 |
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User-agent: |
Mutt/1.3.28i |
Alejandro Forero Cuervo scripsit:
> [Lots of procedure arguments makes for a hard problem, especially when
> passed around from a procedure to its callees. Records are a possibility,
> but they aren't very extensible, and preplanning everything we might
> want is hard. Dynamically scoped variables would solve the problem,
> but that's not the Scheme way.]
Fortunately, SRFI-39, Parameter objects, gets you what you want.
The general idea is that you use:
(define foo (make-parameter 10))
to create a dynamic variable named foo with initial value 10. You can get
the value of foo by saying "(foo)" and change its value to 15 by saying
"(foo 15)".
More usefully, the special form "parameterize" is a dynamic-extent
equivalent of "let":
(parameterize ((foo 15)) <body>)
will cause "(foo)" to evaluate to 15 in the *dynamic* extent of "<body>".
Parameters actually have more flexibility than this: the full details
are available at http://srfi.schemers.org/srfi-39/srfi-39.html .
Even more fortunately, SRFI-39 is built into Chicken. You can get
portability by saying "(require-extension (srfi 39))", although Chicken
does not require this.
--
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