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Syntax-Case macro that selects the N-th element from a list
From: |
Dr. Arne Babenhauserheide |
Subject: |
Syntax-Case macro that selects the N-th element from a list |
Date: |
Mon, 05 Apr 2021 13:30:21 +0200 |
User-agent: |
mu4e 1.4.15; emacs 27.2 |
Hi,
In dryads-wake I need selection of the element in a list in a macro from
user-input. Currently I have multiple macros, and the correct one (which
strips the non-selected choices) is selected in a simple cond:
(define-syntax-rule (Choose resp . choices)
"Ask questions, apply consequences"
(cond
((equal? resp 1) ;; resp is user-input. It is a natural number.
(Respond1 choices))
((equal? resp 2)
(Respond2 choices))
((equal? resp 3)
(Respond3 choices))
(else
#f)))
For this however I have three syntax-case macros:
(define-syntax Respond1
(lambda (x)
(syntax-case x ()
((_ ((question consequences ...) choices ...))
#`(begin
(respond consequences ...)))
((_ (choices ...))
#`(begin #f)))))
(define-syntax Respond2
(lambda (x)
(syntax-case x ()
((_ (choice choices ...))
#`(begin
(Respond1 (choices ...))))
((_ (choices ...))
#`(begin #f)))))
(define-syntax Respond3
(lambda (x)
(syntax-case x ()
((_ (a b choices ...))
#`(Respond1 (choices ...)))
((_ (choices ...))
#`(begin #f)))))
I would like to get rid of those three definitions and replace them by
at most two (one that strips N initial list entries, and Respond1).
I cannot move to procedures, because I have code that must be executed
only during final processing, and when I evaluate any of the
consequences (as it happens with procedure-arguments), then the timing
of the code execution does not match anymore. So I must absolutely do
this in macros.
I’ve tried to get that working, but all my tries failed. Is there a way
and can you show it to me?
This is a minimal working example. The output should stay the same,
except for part 4, which needs this change to work (see at the bottom),
but I would like to:
- replace Respond2 and Respond3 by something recursive, so resp can have
arbitrary high values (not infinite: max the length of the options) and
- replace the cond-clause by a call to the recursive macro.
(define-syntax-rule (respond consequence consequence2 ...)
(begin
(write consequence)
(when (not (null? '(consequence2 ...)))
(write (car (cdr (car `(consequence2 ...))))))))
(define-syntax Respond1
(lambda (x)
(syntax-case x ()
((_ ((question consequences ...) choices ...))
#`(begin
(respond consequences ...)))
((_ (choices ...))
#`(begin #f)))))
(define-syntax Respond2
(lambda (x)
(syntax-case x ()
((_ (choice choices ...))
#`(begin
(Respond1 (choices ...))))
((_ (choices ...))
#`(begin #f)))))
(define-syntax Respond3
(lambda (x)
(syntax-case x ()
((_ (a b choices ...))
#`(Respond1 (choices ...)))
((_ (choices ...))
#`(begin #f)))))
(define-syntax-rule (Choose resp . choices)
"Ask questions, apply consequences"
(cond
((equal? resp 1)
(Respond1 choices))
((equal? resp 2)
(Respond2 choices))
((equal? resp 3)
(Respond3 choices))
(else
#f)))
(display "Choose 1: should be bar:")
(Choose 1 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar))
(newline)
(display "Choose 2: should be warhar:")
(Choose 2 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar))
(newline)
(display "Choose 3: should be mar:")
(Choose 3 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar))
(newline)
(display "Choose 4: should be tar:")
(Choose 4 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar))
(newline)
(display "Choose 5: should be #f:")
(Choose 5 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar))
(newline)
Best wishes,
Arne
--
Unpolitisch sein
heißt politisch sein
ohne es zu merken
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- Syntax-Case macro that selects the N-th element from a list,
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