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Date: Wed, 08 Jan 1997 21:29:43 GMTServer: NCSA/1.4.2Content-type: text/html<html><head><title>Bad Programming</title></head><body><h1>Programming Practices to Avoid</h1><hr><b>Executive Summary:</b> Dave Grove will do bad things to you if any of your turned-in homework has any of the following features<hr><DL><DT> <h3>Unmarked global variables</h3><DD> Global variables must be clearly "declared" at the top of a file, and must have names that begin and end with asterisks.Also there must be a comment explaining why this variables should/must be global.<DT> <h3>Mixing parameter passing styles</h3><DD> Lisp allows positional, keyword, <tt>&REST</tt>, <tt>&OPTIONAL</tt>, and various combinations of options for passing parameters to a function. You should use positional function parameters unless you want/need to sometimes omit a parameter, in which case use (all) keyword parameters. Do not mix parameterpassing conventions within a function. Do not use <tt>&REST</tt>unless you have an extremely good reason. Do not use <tt>&OPTIONAL</tt> at all.<DT> <h3>Nested defuns</h3><DD> Lisp allows you to define one function inside another. For example,<pre>(defun main-function (args) (let ((local-variable (process-args args))) (defun helper-function (x) -- some function of local-variable --) -- do some more processing --))</pre>This creates a <i>global</i> binding for <tt>helper-function</tt>and therefore is a case of a global side effect that is not well documented. Never put a <tt>DEFUN</tt> inside a <tt>DEFUN</tt>.The <tt>LAMBDA</tt> macro allows you to create local, unnamed functions"on the fly."<DT> <h3> "Declaring before initializing" </h3><DD> Those of us accustomed to programming in C or PASCAL are used tothe idea of declaring our variables at the beginning of a function,then initializint them later on. So for example we might write:<pre>;;; UGLY UGLY UGLY UGLY UGLY(defun count-symbols (nested-list) (let ((the-car NIL) ;; "Declarations" only (the-cdr NIL) ;; (symbols-in-car NIL) ;; (symbols-in-cdr NIL)) ;; (cond ((null nested-list) 0) (T (setf symbols-in-car 0) (setf the-car (car nested-list)) (if (symbolp the-car) (setf symbols-in-car 1)) (if (listp the-car) (setf symbols-in-car (count-symbols the-car))) (setf symbols-in-cdr (count-symbols (cdr nested-list))) (+ symbols-in-car symbols-in-cdr)))))</pre>This is <i>really bad</i> Lisp style because there is no need to "pre-declare" then set these variables. <p>Here's a better way to write it: we let the different clauses of the CONDseparate the cases for us. In many cases doing that carefully allows fewer conditionals and local variables.<pre>(defun count-symbols (nested-list) (cond ((null nested-list) 0) (T (let ((the-car (car nested-list)) (symbols-in-rest (count-symbols (cdr nested-list)))) (cond ((symbolp the-car) (+ 1 symbols-in-rest)) ((listp the-car) (+ (count-symbols the-car) symbols-in-rest)) (T symbols-in-rest))))))</pre>Here's another version that uses a conditional assignment to a variable.It's open to debate whether this is good style or not. It's the most purely "Lisp-like" of the three, but I personally find it a little hard to read. <pre>(defun count-symbols (nested-list) (cond ((null nested-list) 0) (T (let* ((the-car (car nested-list)) (symbols-in-car (cond ((symbolp the-car) 1) ((listp the-car) (count-symbols the-car)) (T 0)))) (+ symbols-in-car (count-symbols (cdr nested-list)))))))</pre><DT> <h3> Mixing Side-effects and Computation </h3><DD> Functions should be written either to compute a result, or toperform a side-effect (like I/O). You should avoid writing functionsthat both do substantial ammounts of computation to produce a resultand have interesting side-efftects.</dl><hr><address>cse341-webmaster@cs.washington.edu</i>(Last update: <!-- see man strftime for full formatting options--> 04/16/96 at 01PM)<i></address></body></html>⌨️ 快捷键说明
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