From: Zelphir Kaltstahl <zelphirkaltstahl@posteo.de>
To: Guile User <guile-user@gnu.org>
Subject: Understanding `symbol??` macro from okmij.org
Date: Thu, 14 Mar 2024 00:18:59 +0000 [thread overview]
Message-ID: <9142ff20-0722-4fb7-b35a-7434e68d1777@posteo.de> (raw)
Hello Guile Users!
I am having trouble understanding how the `symbol??` macro from
https://okmij.org/ftp/Scheme/assert-syntax-rule.txt works.
Here is what I have so far:
~~~~start~~~~
;; (symbol?? FORM KT KF)
;; FORM is an arbitrary form or datum.
;; A symbol? predicate at the macro-expand time.
;; Expands in KT if FORM is a symbol (identifier), Otherwise, expands in KF.
;; KT: Continuation/Case in case of true.
;; KF: Continuation/Case in case of false.
(define-syntax symbol??
(syntax-rules ()
;; The check is done by first pattern matching against some other
;; forms, that are not a symbol.
((symbol?? (x . y) kt kf) kf) ; It's a pair, not a symbol
((symbol?? #(x ...) kt kf) kf) ; It's a vector, not a symbol
;; After those things are excluded, the thing might be a symbol.
((symbol?? maybe-symbol kt kf)
(let-syntax ((test
(syntax-rules ()
((test maybe-symbol t f) t)
((test x t f) f))))
(test abracadabra kt kf)))))
~~~~~end~~~~~
So I don't understand why `test` works or how it works to check, whether
`maybe-symbol` is a symbol or not. And I don't understand how `abracadabra`
helps with that, or why it helps, to pass something undefined.
Can someone help me understand, what is going on there?
Actually I am trying to understand the whole page, and I got there while looking
for a good `assert` macro ... rabbit hole and all that.
Best regards,
Zelphir
--
repositories:https://notabug.org/ZelphirKaltstahl
next reply other threads:[~2024-03-14 0:18 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-14 0:18 Zelphir Kaltstahl [this message]
2024-03-14 1:01 ` Understanding `symbol??` macro from okmij.org Jean Abou Samra
2024-03-14 1:03 ` Maxime Devos
2024-03-14 13:46 ` Zelphir Kaltstahl
2024-03-14 14:52 ` Olivier Dion
2024-03-14 15:57 ` Jean Abou Samra
2024-03-14 16:39 ` Olivier Dion
2024-03-14 18:01 ` Jean Abou Samra
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