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#
msgid ""
msgstr ""
msgid "title: Three-way conditional for number signs"
msgstr ""
msgid "date: 2021-04-24 3"
msgstr ""
msgid "layout: post"
msgstr ""
msgid "lang: en"
msgstr ""
msgid "ref: three-way-conditional-for-number-signs"
msgstr ""
msgid ""
"A useful macro from Paul Graham's [On "
"Lisp](http://www.paulgraham.com/onlisptext.html) book:"
msgstr ""
msgid ""
"(defmacro nif (expr pos zero neg)\n"
" (let ((g (gensym)))\n"
" `(let ((,g ,expr))\n"
" (cond ((plusp ,g) ,pos)\n"
" ((zerop ,g) ,zero)\n"
" (t ,neg)))))\n"
msgstr ""
msgid ""
"After I looked at this macro, I started seeing opportunities to using it in "
"many places, and yet I didn't see anyonelse using it."
msgstr ""
msgid ""
"The latest example I can think of is section 1.3.3 of [Structure and "
"Interpretation of Computer "
"Programs](https://mitpress.mit.edu/sites/default/files/sicp/index.html), "
"which I was reading recently:"
msgstr ""
msgid ""
"(define (search f neg-point pos-point)\n"
" (let ((midpoint (average neg-point pos-point)))\n"
" (if (close-enough? neg-point post-point)\n"
" midpoint\n"
" (let ((test-value (f midpoint)))\n"
" (cond ((positive? test-value)\n"
" (search f neg-point midpoint))\n"
" ((negative? test-value)\n"
" (search f midpoint pos-point))\n"
" (else midpoint))))))\n"
msgstr ""
msgid ""
"Not that the book should introduce such macro this early, but I couldn't "
"avoid feeling bothered by not using a `nif` macro, which could even remove "
"the need for the intermediate `test-value` variable:"
msgstr ""
msgid ""
"(define (search f neg-point pos-point)\n"
" (let ((midpoint (average neg-point pos-point)))\n"
" (if (close-enough? neg-point post-point)\n"
" midpoint\n"
" (nif (f midpoint)\n"
" (search f neg-point midpoint)\n"
" (midpoint)\n"
" (search f midpoint pos-point)))))\n"
msgstr ""
msgid ""
"It also avoids `cond`'s extra clunky parentheses for grouping, which is "
"unnecessary but built-in."
msgstr ""
msgid ""
"As a macro, I personally feel it tilts the balance towards expressivenes "
"despite its extra cognitive load toll."
msgstr ""
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