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Re: How many arguments to a macro transformer?



 
 Thanks again for the comments, Keith.  For those who may be puzzled,
 they refer to an updated version of the SRFI that is available at 
 
   http://www.het.brown.edu/people/andre/macros/srfi-72.html
   
 until the editor has had a chance to update the official page.  

 > I got the new version 1.9, merged it with the changes I had
 > made, and tried the result on the old test cases.  I noticed
 > that the old test sequence started with the definition of
 > |swap!| and then went on to
 > 
 >   ;; This macro may also be expressed as:
 >   
 >   (define-syntax swap!
 >     (lambda (form)
 >       (let ((a (cadr  form))
 >             (b (caddr form)))
 >         `(,(syntax let) ((,(syntax temp) ,a))
 >           (,(syntax set!) ,a ,b)
 >           (,(syntax set!) ,b ,(syntax temp))))))
 >
 > This second form has been removed from the new test sequence.
 > Indeed, it does not seem to work anymore.  Is it broken on
 > purpose, or have I got something screwed up again?
 
 No, it is broken on purpose.  The model changed a little from the 
 previous version.  Now, for increased safety, identifiers can only 
 capture each other if they were introduced in a single evaluation of 
 a SYNTAX or QUASISYNTAX expression.  So the two instances of TEMP above are 
 actually different and the expanded code will fail. 
 
 One could still make QUASISYNTAX macro-expressible by 
 introducing a form, maybe called WITH-SINGLE-RENAMING-FUNCTION
 (the opposite to WITH-FRESH-RENAMING-SCOPE of the previous version)
 that overrides this default in the obvious way, so that one would write  
 
    (define-syntax swap!
      (lambda (form)
        (let ((a (cadr  form))
              (b (caddr form)))
          (with-single-renaming-function    
           `(,(syntax let) ((,(syntax temp) ,a))
             (,(syntax set!) ,a ,b)
             (,(syntax set!) ,b ,(syntax temp)))))))
 
 I considered addign such a form, but then realized that it is the same as 
 the trivial combination (quasisyntax (unquote ...)), i.e.,
 
    (define-syntax swap!
      (lambda (form)
        (let ((a (cadr  form))
              (b (caddr form)))
          (quasisyntax   
           ,`(,(syntax let) ((,(syntax temp) ,a))
              (,(syntax set!) ,a ,b)
              (,(syntax set!) ,b ,(syntax temp))))))))
 
 so one might as well introduce quasisyntax from the beginning and
 avoid polluting the namespace with an extra form that would never be
 used except for implementing quasisyntax.  (An advantage of the current
 choice is that with-fresh-renaming-scope is no longer necessary and has 
 been dropped).  
  
 > KW > The appendix of R4RS says that it has been suggested (it doesn't
 > KW > say by whom) that #'<datum> and #`<datum> would be felicitous
 > KW > abbreviations for (syntax <datum>) and (quasisyntax <datum>).
 > KW > Could this be added to the SRFI?
 > >  
 > >  I will add that as a recommendation in the next revision.
 > 
 > Did that happen?  I don't see it.
 
 Sorry - it slipped under the radar.  I'll put it in the following version.
 

 > >    * I am specifying an improvement in the semantics of syntax-case
 > >      for better hygiene.
 > 
 > That's good (I think).  So when you are discussing that improvement
 > you certainly need to put what you are discussing into the examples.
 
 
 Actually, in the new version the hygiene modification is now 
 orthogonal to syntax-case, so this particular reason falls away.  
 
 
 > >    * A proposal that does not come with some form of pattern matching 
 > >      won't be regarded as usable by many Schemers.  
 > 
 > They will certainly demand that you _can_ implement pattern matching
 > in your system, but if your system does no more than provide an
 > implementation of syntax-case then they are likely to stay with
 > their own implementation.  In order to understand and love
 > the proposal it would be good to have as many examples as
 > possible of the new stuff.
 
 Agreed.  I have replaced some examples
 
 
 > >    * For R5RS compatibility, we need to provide syntax-rules.  
 > 
 > Again, we need to be able to implement it, but we don't need
 > to explain it, so it's better if the examples stand on their own.
 > Show that you can implement syntax-case and syntax-rules, but
 > then show off what you can do without them.
 > 
 > By the way, the changes I have made to your code are to
 > make a version of simple-macros.scm with the modules
 > ripped out.  This is not because I think modules are a bad
 > idea, I am just trying to understand macros by boiling
 > the implementation down to its bare core.  So I need test
 > cases that use only the core constructs.
 > 
 > KW > The link to Andrew Wright's pattern matcher
 > KW >  http://download.plt-scheme.org/.../match.ss
 > KW > at the end of kffd-match-tut.htm seems to be broken.
 > >  
 > >  Thank you - I removed it.  
 > 
 > Is it still available anywhere else?  I would like to know
 > what you had to change to adapt the run-time pattern matcher to
 > work with macros.  Is there any reason that there could not
 > be one matcher for both purposes?  That should be another
 > SRFI maybe.
 > 
 > -- 
 >      -- Keith Wright
 > 
 > Programmer in Chief, Free Computer Shop
 >  ---  Food, Shelter, Source code.  ---
 >