help with define-syntax

help with define-syntax

[Viktor Pavlenko]

Hello,

(warning: this is a newbie question, please be patient:)

I'm experimenting with syntax rules and can't figure out how to
achieve something that seems reasonable.

I want to define a syntax to combine each of supplied procedures with
`and' (well my ultimate goal is more ambitious but first things first)
while adding some extra argument to them, and it works fine if done
like this:

--------------------------------------------------------------8<
(use-syntax (ice-9 syncase))

(define (f1 a b c)
  (lambda (ls)
    (format #t "in proc made by f1: ~A/~A~%" (list a b c) ls)
    #t))

(define (f2 a b)
  (lambda (ls)
    (format #t "in proc made by f2: ~A/~A~%" (list a b) ls)
    #t))

(define (f3 a)
  (lambda (ls)
    (format #t "in proc made by f3: ~A/~A~%" (list a) ls)
    #t))

(define-syntax *flt*
  (syntax-rules()
    ((_ (a) (and (f1 a1 ...) (f2 a2 ...) ...))
     (lambda (ls)
       (display "in xxx") (newline)
       (and ((f1 a a1 ...) ls) ((f2 a a2 ...) ls) ...)))))

(define pr
  (*flt* ("arg-a")
	 (and (f3) (f1 "arg-f1-1" "arg-f1-2") (f3) (f2 "arg-f2-1"))))

(pr '(1 2 3))
(pr '(11 22 33))
--------------------------------------------------------------8<

Now, I want to avoid calling f1, f2, ... each time the resulting
procedure is called but rather make a closure with `let', something
like

--------------------------------------------------------------8<
(define-syntax *flt*
  (syntax-rules()
    ((_ (a) (and (f1 a1 ...) (f2 a2 ...) ...))
     (let ((f1-pr (f1 a a1 ...)) (f2-pr (f2 a a2 ...)) ...)
       (lambda (ls)
	 (display "in xxx") (newline)
	 (and (f1-pr ls) (f2-pr ls) ...))))))
--------------------------------------------------------------8<

but it won't work: I get an "extra ellipsis" error in the last `and'
statement, and if I comment it out, `let' will complain about
"duplicate bound variable".

A hint how to do it properly would be much appreciated.

TIA

-- 
Viktor


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Re: help with define-syntax

[Lynn Winebarger]

Viktor Pavlenko wrote:
 > Hello,
 >
 > (warning: this is a newbie question, please be patient:)
 >
 > I'm experimenting with syntax rules and can't figure out how to
 > achieve something that seems reasonable.
 >
 > I want to define a syntax to combine each of supplied procedures with
 > `and' (well my ultimate goal is more ambitious but first things first)
 > while adding some extra argument to them, and it works fine if done
 > like this:
 >   [...]
 > (define-syntax *flt*
 >   (syntax-rules()
 >     ((_ (a) (and (f1 a1 ...) (f2 a2 ...) ...))
     Just to make sure, you know the above "and" is a variable for whatever
actually appears there, so the "and" below won't necessarily be 'and
after expansion.
      It seems like unnecessary confusion.  You should probably just match
(_ (a) (a1 ...) (a2 ...)) [because the same thing goes for "f1" and "f2"].

 >      (lambda (ls)
 >        (display "in xxx") (newline)
 >        (and ((f1 a a1 ...) ls) ((f2 a a2 ...) ls) ...)))))
 > [...]
 > --------------------------------------------------------------8<
 >
 > Now, I want to avoid calling f1, f2, ... each time the resulting
 > procedure is called but rather make a closure with `let', something
 > like
 >
 > --------------------------------------------------------------8<
 > (define-syntax *flt*
 >   (syntax-rules()
 >     ((_ (a) (and (f1 a1 ...) (f2 a2 ...) ...))
 >      (let ((f1-pr (f1 a a1 ...)) (f2-pr (f2 a a2 ...)) ...)
 >        (lambda (ls)
 > 	 (display "in xxx") (newline)
 > 	 (and (f1-pr ls) (f2-pr ls) ...))))))
 > --------------------------------------------------------------8<
 >
 > but it won't work: I get an "extra ellipsis" error in the last `and'
 > statement, and if I comment it out, `let' will complain about
 > "duplicate bound variable".

     The extra ellipsis error is because neither f2-pr nor ls are bound in the
pattern.  They're bound by the let, _after_ expansion.
     The duplicate bound variable doesn't have anything to do with the extra ellipsis,
except that not having the extra ellipsis allows expansion of *flt* to occur.
     The reason for the duplicate bound variable is still that f2-pr is not a syntax variable,
so it gets literally reproduced in your template.  I.e.
(*flt* (foo) (bar (k1 x1 x2) (k2 y11 y12 y13) (k3 y21 y22))

will expand to

(let ((f1-pr (k1 x1 x2))
        (f2-pr (k2 y11 y12 y13))
        (f2-pr (k3 y21 y22)))
     [etc])

Lynn





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