November 2017
Beginner to intermediate
344 pages
7h 2m
Chinese
Content preview from 深入理解運算原理｜從簡單的機器到無所不能的程式
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298
|
第 9 章
另一個問題（已在第 271 頁『通用系統可以永遠循環』探討過），是以十分強大
1
的語
言所編寫的程式，不需要產生結果而完全能夠永遠執行。這使得執行它們也無法可靠的
研究調查任意程式，因為有時候不可能提前告知程式是不是即將永遠執行（請見第 283
頁的『停機問題』），因此，任何試著執行候選程式的自動化核對器，都將面臨擁有答案
卻永遠一去不回的風險。
最後，即使程式確實提前擁有其可用的所有輸入資料，但無論什麼原因，它最終一定都
會終止，而非永遠循環，執行該程式觀察其結果可能所費不貲或並不方便。這可能需
要花費長時間才能完成，或者擁有不可逆的副作用（發送電子郵件、金融轉帳、發射導
彈），對於測試的目的來說，都是不適合或不需要。
這所有的原因使得不需實際執行程式就能發現程式的資訊非常有用。這麼做的方式之
一，是使用
抽象解譯
（
abstract interpretation
），這種分析技術會藉著執行程式的簡化版
本，再以執行結果來推論原本的特性。
抽象解譯
抽象解譯為我們提供了困難問題的處理方式，例如太大、太複雜的問題，或者是有太多
未知而無法直接處理的問題。抽象解譯的主要概念是使用
抽象
（
abstract
）模型，也就
是丟掉足夠的細節，使其容易控制管理的實際問題的模型（也許讓它更小、更簡單，或
者排除未知），但也保留足夠的細節，讓它的解決方案能與原本的問題有關。
為了讓這個模糊的想法更為具體，讓我們來看一個抽象解譯的簡單應用。
路線規劃
假設你正在某個不熟悉的國家旅遊，並且計劃開車到另一個城市。你打算走哪條路？直 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.