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The Art of Unix Programming
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Unix Programming - Basics of the Unix Philosophy - Rule of Robustness: Robustness is the child oftransparency and simplicity.

Rule of Robustness: Robustness is the child oftransparency and simplicity.

Software is said to be robust when it performs well under unexpected conditions which stress the designer's assumptions, as well as under normal conditions.

Most software is fragile and buggy because most programs are too complicated for a human brain to understand all at once. When you can't reason correctly about the guts of a program, you can't be sure it's correct, and you can't fix it if it's broken.

It follows that the way to make robust programs is to make their internals easy for human beings to reason about. There are two main ways to do that: transparency and simplicity.

For robustness, designing in tolerance for unusual or extremely bulky inputs is also important. Bearing in mind the Rule of Composition helps; input generated by other programs is notorious for stress-testing software (e.g.,the original Unix C compiler reportedly needed small upgrades to cope well with Yacc output). The forms involved often seem useless to humans. For example, accepting empty lists/strings/etc., even in places where a human would seldom or never supply an empty string, avoids having to special-case such situations when generating the input mechanically.

-- Henry Spencer

One very important tactic for being robust under odd inputs is to avoid having special cases in your code. Bugs often lurk in the code for handling special cases, and in the interactions among parts of the code intended to handle different special cases.

We observed above that software is transparent when you can look at it and immediately see what is going on. It is simple when what is going on is uncomplicated enough for a human brain to reason about all the potential cases without strain. The more your programs have both of these qualities, the more robust they will be.

Modularity (simple parts, clean interfaces) is a way to organize programs to make them simpler. There are other ways to fight for simplicity. Here's another one.


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The Art of Unix Programming
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