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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: String processing
Date: Thu, 19 Sep 2024 06:23:59 -0000 (UTC)
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B. Pym wrote:

> 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)

Another way.

(generator-count (=>> string-trim (string-prefix? "(")) read-line)

Given:

(define-syntax ->>
  (syntax-rules ()
    [(_ x)  x]
    [(_ x (y ...) z ...)
     (->> (y ... x) z ...)]
    [(_ x y z ...)
     (->> (y x) z ...)]))

;; currying
(define-syntax =>>
  (syntax-rules ()
    [(_ func ...)
     (lambda (x) (->> x func ...))]))