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Re: help with define-syntax
From: |
Lynn Winebarger |
Subject: |
Re: help with define-syntax |
Date: |
Thu, 29 Jan 2004 19:12:51 -0500 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.0.1) Gecko/20020830 |
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