From 08c9bcb3fed37d9483258bd97aaebc47b54b3fa3 Mon Sep 17 00:00:00 2001 From: EuAndreh Date: Sun, 17 Nov 2024 19:49:05 -0300 Subject: rm -rf v2/ --- v2/src/content/pt/hea/condicional-ternario-lisp.md | 65 ---------------------- 1 file changed, 65 deletions(-) delete mode 100644 v2/src/content/pt/hea/condicional-ternario-lisp.md (limited to 'v2/src/content/pt/hea/condicional-ternario-lisp.md') diff --git a/v2/src/content/pt/hea/condicional-ternario-lisp.md b/v2/src/content/pt/hea/condicional-ternario-lisp.md deleted file mode 100644 index 6971827..0000000 --- a/v2/src/content/pt/hea/condicional-ternario-lisp.md +++ /dev/null @@ -1,65 +0,0 @@ ---- - -title: Three-way conditional for number signs on Lisp - -date: 2021-04-24 3 - -update: 2021-08-14 - -categories: lisp scheme common-lisp - ---- - -A useful macro from Paul Graham's [On Lisp][on-lisp] book: - -```lisp -(defmacro nif (expr pos zero neg) - (let ((g (gensym))) - `(let ((,g ,expr)) - (cond ((plusp ,g) ,pos) - ((zerop ,g) ,zero) - (t ,neg))))) -``` - -After I looked at this macro, I started seeing opportunities to using it in -many places, and yet I didn't see anyone else using it. - -The latest example I can think of is section 1.3.3 of [Structure and -Interpretation of Computer Programs][sicp], which I was reading recently: - -```scheme -(define (search f neg-point pos-point) - (let ((midpoint (average neg-point pos-point))) - (if (close-enough? neg-point post-point) - midpoint - (let ((test-value (f midpoint))) - (cond ((positive? test-value) - (search f neg-point midpoint)) - ((negative? test-value) - (search f midpoint pos-point)) - (else midpoint)))))) -``` - -Not that the book should introduce such macro this early, but I couldn't -avoid feeling bothered by not using the `nif` macro, which could even remove -the need for the intermediate `test-value` variable: - -```scheme -(define (search f neg-point pos-point) - (let ((midpoint (average neg-point pos-point))) - (if (close-enough? neg-point post-point) - midpoint - (nif (f midpoint) - (search f neg-point midpoint) - (midpoint) - (search f midpoint pos-point))))) -``` - -It also avoids `cond`'s extra clunky parentheses for grouping, which is -unnecessary but built-in. - -As a macro, I personally feel it tilts the balance towards expressivenes -despite its extra cognitive load toll. - -[on-lisp]: http://www.paulgraham.com/onlisptext.html [sicp]: -https://mitpress.mit.edu/sites/default/files/sicp/index.html -- cgit v1.2.3