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The Art of Unix Programming
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Unix Programming - Basics of the Unix Philosophy - Rule of Extensibility: Design for the future, because it

Rule of Extensibility: Design for the future, because it will be here sooner than you think.

If it is unwise to trust other people's claims for “one true way”, it's even more foolish to believe them about your own designs. Never assume you have the final answer. Therefore, leave room for your data formats and code to grow; otherwise, you will often find that you are locked into unwise early choices because you cannot change them while maintaining backward compatibility.

When you design protocols or file formats, make them sufficiently self-describing to be extensible. Always, always either include a version number, or compose the format from self-contained, self-describing clauses in such a way that new clauses can be readily added and old ones dropped without confusing format-reading code. Unix experience tells us that the marginal extra overhead of making data layouts self-describing is paid back a thousandfold by the ability to evolve them forward without breaking things.

When you design code, organize it so future developers will be able to plug new functions into the architecture without having to scrap and rebuild the architecture. This rule is not a license to add features you don't yet need; it's advice to write your code so that adding features later when you do need them is easy. Make the joints flexible, and put “If you ever need to...” comments in your code. You owe this grace to people who will use and maintain your code after you.

You'll be there in the future too, maintaining code you may have half forgotten under the press of more recent projects. When you design for the future, the sanity you save may be your own.



[9] Pike's original adds “(See Brooks p. 102.)” here. The reference is to an early edition of The Mythical Man-Month [Brooks]; the quote is “Show me your flow charts and conceal your tables and I shall continue to be mystified, show me your tables and I won't usually need your flow charts; they'll be obvious”.

[10] Jonathan Postel was the first editor of the Internet RFC series of standards, and one of the principal architects of the Internet. A tribute page is maintained by the Postel Center for Experimental Networking.

[11] In full: “We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil”. Knuth himself attributes the remark to C.A.R.Hoare.


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The Art of Unix Programming
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