From: Zelphir Kaltstahl <zelphirkaltstahl@posteo.de>
To: Guile User <guile-user@gnu.org>
Subject: Macro for replacing a placeholder in an expression
Date: Wed, 27 Jul 2022 23:57:43 +0000 [thread overview]
Message-ID: <80083c86-19b0-7537-be70-f763bd5b390f@posteo.de> (raw)
Hello Guile Users,
I am trying to write a macro, which replaces all placeholders (in this case <?>)
with an identifier in an arbitrarily structured expression (arbitrary nesting of
expressions). I have the following code now:
~~~~
(define-syntax replace-result-placeholder
(syntax-rules (<?> replace-result-placeholder)
"Iterate through the parts of an expression, search for the
placeholder and replace the placeholder with the
result-identifier."
;; Transform trivial cases, base cases.
[(_ result-identifier <?>)
result-identifier]
[(_ result-identifier (<?>))
(result-identifier)]
[(_ result-identifier (op))
(op)]
;; If there already is such a list of transformed args
;; and there are still arguments not transformed.
[(_ res-id-outer
(op arg
args* ...
(list
;; Must match a compound expression here, to
;; avoid matching of other lists, like lists of
;; arguments in a lambda expression or
;; similar. Here we must only match a list of
;; arguments, which are yet to be transformed.
(replace-result-placeholder res-id-inner arg-to-transform)
other-args* ...)))
(replace-result-placeholder
res-id-outer
(op args* ...
(list (replace-result-placeholder res-id-outer arg-to-transform)
other-args* ...
(replace-result-placeholder res-id-inner arg))))]
;; If there already is such a list of transformed args
;; and there are no arguments not yet transformed.
[(_ res-id-outer
(op (list
(replace-result-placeholder res-id-inner arg-to-transform)
other-args* ...)))
((replace-result-placeholder res-id-outer op)
(replace-result-placeholder res-id-inner arg-to-transform)
other-args* ...)]
;; Create list of transformed args, if it does not yet
;; exist.
[(_ result-identifier (op arg args* ...))
(replace-result-placeholder
result-identifier
(op args* ...
(list
(replace-result-placeholder result-identifier arg))))]
;; Must place this trivial case last, to avoid
;; accidental matching of compound expressions.
[(_ result-identifier op)
op]
;; Catch all.
[(_ other* ...)
(syntax-error "unrecognized form in macro call:"
(quote
(replace-result-placeholder other* ...)))]
))
~~~~
This already seems to work mostly:
~~~~
scheme@(guile-user)> (define res 3)
scheme@(guile-user)> (replace-result-placeholder res <?>)
$18 = 3
scheme@(guile-user)> (replace-result-placeholder res (+ 1 <?>))
$19 = 4
scheme@(guile-user)> (replace-result-placeholder res (+ 1 (- 5 <?>)))
$20 = 3
scheme@(guile-user)> (replace-result-placeholder res (+ 1 (* <?> 2) (- 5 <?>)))
$21 = 9
scheme@(guile-user)>
~~~~
I was already happy, because everything seemed to work. However, when it comes
to replacing things inside lambda expressions, things seem to not work correctly:
~~~~
scheme@(guile-user)> (replace-result-placeholder res (lambda (a) (+ a <?>)))
While compiling expression:
Syntax error:
unknown file:965:0: lambda: invalid argument list in subform ((a)) of (replace-result-placeholder res (a))
~~~~
I think (but not 100% sure), that the ((a)) comes from the case:
~~~~
...
[(_ result-identifier (op))
(op)]
...
~~~~
(In this case it is not an "operation", but nevermind the name in that pattern.)
I do not understand, why it outputs something which is wrapped twice in
parentheses. I do not understand where that second pair of parentheses comes from.
I tried creating a simpler macro, for the specific case of a lambda expression,
to maybe find a solution this way. But it has got the same problem. At least it
now serves to reproduce the problem in a simpler example:
~~~~
scheme@(guile-user)> (define-syntax test
(syntax-rules (lambda)
[(_ (op args body* ...))
((test op) (test args) (test body* ...))]
[(_ thing1 thing2 things* ...)
((test thing1) (test thing2 things* ...))]
[(_ (thing))
(thing)]
[(_ thing)
thing]))
scheme@(guile-user)> (test (lambda (a) (+ a 1)))
While compiling expression:
Syntax error:
unknown file:798:0: lambda: invalid argument list in subform ((a)) of (test (a))
~~~~
There seems to be something about a template like (one-thing) that I do not
understand or something completely different is going on.
What am I doing wrong?
Best regards,
Zelphir
--
repositories: https://notabug.org/ZelphirKaltstahl
next reply other threads:[~2022-07-27 23:57 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-27 23:57 Zelphir Kaltstahl [this message]
2022-07-28 0:55 ` Macro for replacing a placeholder in an expression Maxime Devos
2022-07-28 10:23 ` Zelphir Kaltstahl
2022-07-28 10:28 ` Maxime Devos
2022-07-30 15:42 ` Zelphir Kaltstahl
2022-07-30 20:44 ` Maxime Devos
2022-07-30 21:10 ` Maxime Devos
2022-07-30 21:13 ` Maxime Devos
2022-08-05 9:42 ` Linus Björnstam
2022-08-06 14:28 ` Zelphir Kaltstahl
2022-07-28 1:04 ` Maxime Devos
2022-07-28 8:39 ` Zelphir Kaltstahl
2022-07-28 9:48 ` Maxime Devos
2022-07-28 11:26 ` Zelphir Kaltstahl
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