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
