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----
-
-title: Three-way conditional for number signs on Lisp
-
-date: 2021-04-24 3
-
-update: 2021-08-14
-
-layout: post
-
-lang: en
-
-ref: three-way-conditional-for-number-signs-on-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