什麼是好設計，打從開始 coding 以來就是一個不斷追求的大哉問，從
SOLID 看到 Design Patterns，每一個原則看起來好有道理，每一個設計模式都在等著你的套用。

但是最近幾年的工作讓我有不一樣的心得，而不再費心追求「完美」的設計。同一個問題，每個人都會想出不一樣的解法，都說自己的解比較好。於是這些設計原則，常就淪為爭論的工具，當我覺得你的解法做太多事情時，搬出YAGNI; 當我覺得你的解法太複雜時，搬出KISS; 當覺得一個類別太肥大的時候，就搬出 SRP，反正都可以找到支持的論點就對了。很多時候其實不管哪一種方案，都是可以接受的可行方案，於是後來我就不想去爭個你死我活、斤斤計較當下的細節設計了。好設計應該是演進(grow)出來的，我首先在乎的反而是有沒有留下可用的測試和清楚好讀的程式，這樣才可以留下(後人)改進的機會。

回到正題，Kent Beck 在 eXtreme Programming 中，對於什麼是 Simple Design，留下了四個 Xp Simplicity Rules，我個人蠻喜歡的，因為很簡單又基本：

1. Pass All Tests 通過全部測試：有測試才能夠讓軟體不斷的演進(grow)下去，不然你不是被 regression bugs 淹死，就是只能不計成本把code砍掉重練。如何寫好測試也是個大哉問，xUnit Test Patterns: Refactoring Test Code一書是其中的bible，嫌厚的話，可以先看看Refactoring Test Code這一篇paper，都是在講重構你的測試程式。當然，事情總是過猶不及，老闆付錢給你是為了得到 code，不是 tests。測試只是幫助我們寫好code的工具，追求100%完整的好測試並不是目的。

2. Reveals Intent (Self-Documenting Code) 程式能夠表達出意圖：其中最重要而且基本的，就是要有好的程式命名，無論是類別、方法、變數等等。專講 Readable 的書有一本可以推薦 The Art of Readable Code。我這裡還推薦一個我最喜歡的技巧 Composed Method (搞笑談軟工也有一篇介紹)，這招也是 Kent Beck 大師提出的，將細節的實作抽取成一個小方法，讓整個方法裡的操作都是類似的粒度，小小一招，大有作用。

3. No Duplication (DRY) 不重複：”Every piece of knowledge must have a single, unambiguous, authoritative representation within a system” 完整定義請複誦三次。有時候重複並不是這麼明顯，例如作用重複的程式，但命名不同，或是命名重複，但程式作用不一樣。這裡推薦 David Chelimsky 的影片 Maintaining Balance While Reducing Duplication (slides)和 Maintaining Balance while Reducing Duplication: Part II，可以增進對 DRY 的理解，DRY 不只是在講不要重複程式碼而已。

4. Has no superfluous parts (Minimizes the number of classes and methods) 不多餘：這一條也就是 YAGNI，不要實作多餘的功能。我個人不怎麼喜愛這個原則。不是不贊同，不需要的功能花時間去寫當然是浪費大家時間，而是這個原則太容易例外了。有時候會寫下多餘的程式來增加功能或擴充性，那是因為你擁有的經驗、直覺跟當時的情境告訴你應該這麼做，這可以在不久的將來就派上用場，過於強調 YAGNI 反而一板一眼教條主義了。光單看”減少類別跟方法的數量”跟 SRP 和 Composed Method 就有所衝突了，後兩者都會增加類別和方法的數量。

重要性 1 > 2 > 3 > 4，也有版本講 1 > 3 > 2 > 4。

參考資料

• Xp Simplicity Rules
• Agile in a Flash: Simple Design
• The 4 rules of simple design
• Clean Code: Four Simple Design Rules – Obligatory Read

每次看技術書籍的時候，特別喜歡翻過一遍作者放在章節開頭的名言佳句(如果有的話)，通常都蠻有意思的。所以在自己的書裡也想這樣做，這是目前收集的名言佳句。不過要做到每則名言都剛好呼應該章內容真是困難啊。

1. Life’s too short to build something nobody wants – Ash Maurya, Running Lean 作者
2. Give someone a program, you frustrate them for a day; teach them how to program, you frustrate them for a lifetime. – David Leinweber
3. There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies and the other way is to make it so complicated that there are no obvious deficiencies. — C.A.R. Hoare
4. Actually, I’m trying to make Ruby natural, not simple. Ruby is simple in appearance, but is very complex inside, just like our human body. – Matz, Ruby 發明人
5. Much of the essence of building a program is in fact the debugging of the specification. – Fred Brooks, The Mythical Man-Month 作者
6. The first 90% of the code accounts for the first 90% of the development time. The remaining 10% of the code accounts for the other 90% of the development time. – Tom Cargill
7. Complication is what happens when you try to solve a problem you don’t understand – Andy Boothe
8. Weeks of programming can save you hours of planning. – Unknown
9. Controlling complexity is the essence of computer programming. — Brian Kernighan
10. All problems in computer science can be solved by another level of indirection(abstraction) – David Wheeler
    …except for the problem of too many layers of indirection. – Kevlin Henney’s corollary
11. Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning. — Rick Cook
12. Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. — Brian W. Kernighan
13. I’m not a great programmer; I’m just a good programmer with great habits. ― Kent Beck
14. Most of you are familiar with the virtues of a programmer. There are three, of course: laziness, impatience, and hubris. – Larry Wall, Perl 程式語言發明人
15. Any fool can write code that a computer can understand. Good programmers write code that humans can understand. – Martin Fowler
16. Measuring programming progress by lines of code is like measuring aircraft building progress by weight. – Bill Gates
17. It’s not a bug – it’s an undocumented feature. – Unknown
18. The most depressing thing about life as a programmer, I think, is if you’re faced with a chunk of code that either someone else wrote or, worse still, you wrote yourself but you no longer dare to modify. That’s depressing. – Peyton Jones
19. It works on my machine! – 數以萬計的程式設計師
20. Talk is cheap. Show me the code. – Linus Torvalds
21. I thought of objects being like biological cells and/or individual computers on a network, only able to communicate with messages — Alan Kay, Smalltalk 發明人
22. When you choose a language, you’re choosing more than a set of technical trade-offs—you’re choosing a community. -Joshua Bloch
23. Quality, Speed or Cheap. Pick two. – Unknown
24. Developer testing isn’t primarily about verifying code. It’s about making great code. If you can’t test something, it might be your testing skills failing you but it’s probably your code code’s design. Testable code is almost always better code. – Chad Fowler
25. We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. – Donald Knuth
26. No code is faster than no code. – Merb core tenet
27. If you’re the smartest person in the room, you’re in the wrong room. – Unknown
28. How long would it take your organization to deploy a change (to production) that involves just one single line of code? Do you do this on a repeatable, reliable basis? – Mary Poppendieck
29. Nine people can’t make a baby in a month.  — Fred Brooks, The Mythical Man-Month作者
30. Good code is its own best documentation. As you’re about to add a comment, ask yourself, ‘How can I improve the code so that this comment isn’t needed?’ Improve the code and then document it to make it even clearer. – Steve McConnell, Code Complete 作者
31. A person does not really understand something until after teaching it to a computer. – Donald Knuth

有特別喜歡哪句嗎? 或是你自己有喜愛的名言，歡迎告訴我。