Hi,
I'm trying to program a macro which defines different selectors for mutable
pairs.
I know in Racket it's possible to use only mcar and mcdr, but mcdar etc. could
be useful for me.
I would like to use macro this way:
(with-mcxrs
(let ((a (mcons (mcons 1 2) (mcons (mcons 3 4) '()))))
(list (mcaar a)
(mcdar a)
(mcaadr a)
(mcdadr a))))
Mcxrs will be defined in macro's body and then whole expresion would be
evaluated - now with bindings in macro's environment so no error should appear.
I think procedure seek and macro defines3 are ok (when using separately they
work fine) but in a macro "with-mcxrs" an error appears: mcaar undefined.
Where's the problem? I can't figure it out. I would be glad if anyone could
give me some advice.
And could "defines3" be out of macro "with-mcxrs"? Or should it be inside -
like in my code?
Thanks a lot.
Code:
first I go though whole body and I store found mcxrs in list, which is called
"selectors" in let loop. And then call another macro "defines3" which should
return procedure - for example: (lambda (pair) (mcar (mcdr (mcar pair)))). And
this procedure should be defined as a value of particular mcaar or mcdar etc.
(define-macro with-mcxrs
(lambda body
`(define-macro defines3
(lambda (chars)
(let ((loop (gensym)))
`(lambda (pair)
(let ,loop ((chsez ,chars))
(if (null? chsez)
pair
(if (equal? (car chsez) #\a)
(mcar (,loop (cdr chsez)))
(mcdr (,loop (cdr chsez)))) ))))))
(define seek2
(let ((pom '() ))
(lambda (l)
(cond ((null? l) pom )
((list? (car l)) (begin(seek2 (car l))
(seek2 (cdr l))) )
(else
(if (not (symbol? (car l)))
(seek2 (cdr l))
(let* ((test (symbol->string(car l)))
(len (string-length test)))
(if (> len 4)
(if (or (equal? (substring test 0 3) "mca")
(equal? (substring test 0 3) "mcd"))
(begin
(set! pom (cons (car l) pom) )
(seek2 (cdr l)))
(seek2 (cdr l)))
(seek2 (cdr l)) ))))))))
(let ((loop (gensym)))
`(let ,loop ((selectors (,seek2 (quote ,@body)) )) ;;list of selectors
(cond ((null? selectors) (,@body))
(else
(define (car selectors)
(defines3 (string->list
(substring (car l) 2
(- (string-length (car l))1)) )))
(,loop (cdr selectors))) ))) ))
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