Subject: (ice-9 match) does not allow distinguishing between () and #nil

From: Maxime Devos <[email-masked]>
Date: Sun, 09 May 2021 18:42:40 +0200

Hi guilers,

I've found the following surprising behaviour:

(use-modules (ice-9 match))
(match (identity #nil) (() 'scheme-eol) (#nil 'elisp-eol))
--> scheme-eol, expected elisp-eol

(match '() (#nil 'elisp-eol) (() 'elisp-eol))
--> elisp-eol, expected scheme-eol

Treating () and #nil as equivalent makes sense, but should be
documented.

My suspicion, currently untested: the following code in
ice-9/match.upstream.scm ...

(define-syntax match-two
  (syntax-rules (_ ___ ..1 *** quote quasiquote ? $ = and or not set! get!)
    ((match-two v () g+s (sk ...) fk i)
     (if (null? v) (sk ... i) fk))
    [..]

should be:

(define-syntax match-two
  (syntax-rules (_ ___ ..1 *** quote quasiquote ? $ = and or not set! get!)
    ((match-two v () g+s (sk ...) fk i)
     (if (eq? v '()) (sk ... i) fk))
    ((match-two v #nil g+s
(sk ...) fk i)
     (if (eq? v #nil) (sk ... i) fk))
    [...]

And the following might need similar adjustment:

    ((match-two v (p) g+s sk fk i)
     (if (and (pair? v) (null? (cdr v)))
         (let ((w (car v)))
           (match-one w p ((car v) (set-car! v)) sk fk i))
         fk))

Greetings,
Maxime.