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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: tail recursion guidelines
Date: Thu, 12 Sep 2024 10:59:41 -0000 (UTC)
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Juho Snellman wrote:

> One useful detail that I didn't see mentioned in that chapter is the
> loop keyword "it". I find that it pops up often in code like
> 
>   (let ((x ...))
>     ...
>     (loop for e in list when (foo-bar e x) collect it))
> 
> , where foo-bar returns nil on failure and some sort of state object
> otherwise.

Gauche Scheme

(define (foo-bar x)  (and (positive? x) (sqrt x)))

(filter-map foo-bar '(-3 0 4 9))
  ===>
(2 3)


Here's an anaphoric "if" that provides "it".
(Adapted from an example in the Gauche documentation.)

(use util.match)

(define-syntax if@
  (er-macro-transformer
    (lambda (form rename id=?)
      (match form
        [(if@ test expr1 expr2)
         (quasirename rename
           `(let ((,'it ,test))
              (if ,'it ,expr1 ,expr2)))]))))

(if@ (find odd? '(2 3)) (print it) (print "false is " it))
3

(if@ (find odd? '(2 30)) (print it) (print "false is " it))
false is #f