[plt-scheme] Very basic macro question

Thanks very much. This has put me in the right direction!

From: jos.koot at telefonica.net
To: johnyseventy at hotmail.com; noelwelsh at gmail.com
CC: plt-scheme at list.cs.brown.edu
Subject: Re: [plt-scheme] Very basic macro question
Date: Tue, 19 Jan 2010 19:42:27 +0100










You probably want match. Alernatively you can use 
syntax-case in a procedure for example:
 
#lang scheme
(define (trafo 
x)
 (syntax->datum
  (syntax-case x ()
   (id 
(identifier? #'id) #'(identifier id))
   ((a ...) #'(list a 
...))
   (else #'else))))
 
(trafo 'a) ; --> (identifier a)
(trafo '(a b 
c)) ; (list a b c)
(trafo "string") ; --> string
 
Jos

  ----- Original Message ----- 
  From: 
  johny 
  seventy 
  To: noelwelsh at gmail.com 
  Cc: plt-scheme at list.cs.brown.edu 
  
  Sent: Tuesday, January 19, 2010 7:26 
  PM
  Subject: RE: [plt-scheme] Very basic 
  macro question
  
> Date: Sun, 17 Jan 2010 18:42:32 +0000
> Subject: Re: 
  [plt-scheme] Very basic macro question
> From: noelwelsh at gmail.com
> To: johnyseventy at hotmail.com
> 
  CC: plt-scheme at list.cs.brown.edu
> 
  
> On Sun, Jan 17, 2010 at 2:22 PM, johny seventy <johnyseventy at hotmail.com> 
  wrote:
> > I understand why example
> > four works but is 
  there anyway I can do this without eval?
> 
> A transformer 
  binding and syntax-local-value.
> 
> It doesn't make sense to 
  parameterise a macro by a run-time value --
> macros are expanded before 
  run-time so they cannot depend on run-time
> values.
> 
> 
  N.

Thanks for the responses. I just want to clarify one more 
  thing.

I have an sexpr which is created dynamically at runtime and I 
  want to transform this sexpr. It seemed really easy to do the transformation 
  using a syntax-rules macro. So the problem is I have missed the boat at this 
  stage as macro expansion has already taken place. The only way I can see how 
  to invoke the macro at this stage is by using eval. I am not sure what you 
  mean by a transformer binding and syntax-local-value. Is there a simple 
  example of this in action I could take a look at?

Thanks for your 
  patience. 






  
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