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Coding style used in Smalltalk/X classes

Introduction

This document describes the coding style used in Smalltalk/X's class library. The author is aware of the fact, that coding style is a very personal matter and should not be enforced by dictators.
However, it is useful to follow some rules, to enable other programmers an easier entry into the system.
Experienced Smalltalk programmers may want to skip this document.

If you have any suggestions or additions on this theme, let me know about it.

Class documentation

In Smalltalk/X, every class contains a method category called "documentation" in its class protocol. You will find at least two methods named version and documentation there.
The version methods comment consists of a single version line - this is created automatically by the source code control system (RCS or SCCS). In case of error reports, this string should be used to identify the exact version of the class.

The documentation methods comment describes the class, its uses and (if of public interrest) its instance and classvariables.

In some classes, you will find an examples method. This will consist of a comment giving typical uses (ready to select & doIt).

These methods consist of comments only; they are not meant to be executed. (actually, if evaluated, they will return the receiver; since empty methods are semantically equivalent to a '^ self' method).

The SystemBrowser automatically shows the documentation text (found either in documentation or in the class comment) whenever a class is selected. Thus, to be nice to other people browsing through the system, you should add a short description of what your class is about in these documentation methods.

Dont worry about memory usage when creating documentation methods - simple methods which return self (as empty methods do) all share a common piece of code, so there will NOT be thousands of empty methods filling up your memory. (to be exact: there is some little overhead per method created by the method object itself - not by the methods code. However, for production code, all documentation methods can be easily removed. Also, stc provides a command line argument, to skip all methods in the documentation category; to allow building more compact class libraries.)

If you dont like the above, use the classes comment string, which also does not eat up memory (it used to before release 2.10.3 of Smalltalk/X).

BTW: from the authors experience, you should not delay documentation too much. Write them down as soon as possible - otherwise you may not find the time to do so later - or you may simply forget to do it. Also, keep in mind that it may take more time to add those comments later, since you may have to reflect about what is going on. From our experience, the later the documentation is written in a project, the higher is its cost.

Method documentation

Every method should contain (at least) two comments: Example (from Collections enumeration protocol):
    select:aBlock
	"return a new collection with all elements from the receiver, for which
	 the argument aBlock evaluates to true"

	|newCollection|

	newCollection := self species new.
	self do:[:each |
	    (aBlock value:each) ifTrue:[newCollection add:each].
	].
	^ newCollection

	"
	 #(1 2 3 4) select:[:e | e odd]   
	 (1 to:10) select:[:e | e even]     
	"

Variable and method naming

Of course, you should give your variables and methods descriptive names. You should do so in any programming language. In smalltalk, a common trick is to encode the expected type of a variable in the name (which you dont have to in static typed languages). For example, a name like originPoint makes it totally clear, what the argument is about.

By convention, global variables and class variables should start with an upper case character - other variables and selectors by a lower case chracter.

Global Variables

Think twice before using globals. Beside increasing code complexity (by introducing side effects), use of globals may lead to conflicts if packages from different programming teams are merged and both use the same global name. Although the browser offers search functions for uses of globals, you have to manually edit (and think about) the code in this case. Avoid this by banning globals from your code.

In many situations, a global can be eliminated by by passing additional method arguments (which may even be an advantage later, offering more possibilities for reuse of a method).
Almost all globals can easily be replaced by a private classVariable instead and access be provided to other parts via class methods.

Code comments

You wont need too many comments in your methods, if the code is clean and straight forward. Dont add comments just for the comment. For example, a comment like:
	sum := sum + 1.         "add one to sum"
is stupid and filling your methods with this kind of "information" actually makes your code less readable.
(you may wonder why this is mentioned here; we have seen departments where code quality was measured by counting comments, which ended in people doing above rubbish - only to make the codecheckers happy)

However, if you use special tricks or uncommon constructs, you should add a comment describing what is going on - for yourself and for others.

Code indentation

The question of how code should be indented is a very subjective and the discussion often even a religious one. For that reason, we will not give any recommendations here. Instead, the two most commonly used styles are described in short here.
Take the one that you (and your friends) find to be the easiest to read.

Readability is usually better if you do not have to scroll when looking at a methods code. Therefore, methods should be short. On the other hand, dont break up a method into many short methods just for this; if a methods functionality requires more than a page of code, so be it.

Many other styles are possible, however, whichever you choose, follow these rules:

Currently, Smalltalk/X's codeViews do not support automatic indentation of program text. This will be added in a later release. However, it is still open, which styles will be supported by this autoindent function.


Copyright © Claus Gittinger Development & Consulting, all rights reserved

(cg@ssw.de)