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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: String processing
Date: Wed, 18 Sep 2024 23:13:11 -0000 (UTC)
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Pierre Mai wrote:

> > We've had this discussion before.  Perl was designed to make file scanning
> > and pattern matching easy.  The programs we've been designing in this
> > thread are precisely the kind of application that Perl is intended for.
> > I'm well-enough versed in Lisp to know that the equivalent of:
> >
> > while (<>) { # Loop over input lines
> >   counter++ if /^\s*\(/; # if first non-white character is open-paren, count i
> t
> > }
> >
> > would be much more verbose without being significantly more expressive.
>                                            -------------
> 
> If it weren't for the regexp, which needs a comment of 10 words to
> explain what it does, and which is easy to get wrong (either comment,
> or regexp that is), I could believe that statement, but so I have to
> humbly disagree ;)
> 
> Anyways, this is Common Lisp:
> 
> (loop for line = (read-line *standard-input* nil nil)
>       while line
>       count (starts-with (left-trim-whitespace line) "("))
> 
> This uses two trivial string functions which are probably part of
> every working CL user[1].  With an extensible LOOP facility, this could
> even be clarified further...
.....
> Footnotes:
> [1]  Here are some very simple, inefficient sample implementations:
> (defun starts-with (string substring)
>   "Detect whether the `string' starts with `substring'."
>   (eql 0 (search substring string)))
> 
> (defun left-trim-whitespace (string &optional (ws-bag '(#\Space #\Tab)))
>   "Trims any whitespace characters (i.e. characters in `ws-bag') from
> the left side of `string'."
>   (string-left-trim ws-bag string))

Gauche Scheme

(use gauche.generator)

Using a regular expression:

(generator-count #/^\s*[(]/ read-line)

Without using a regular expression:

(use srfi-13)  ;; string-trim string-prefix?

(generator-count (^s (string-prefix? "(" (string-trim s))) read-line)