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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第 2 章
一如我們所預期，沒有『一種真正的方式』能解釋程式語言的語法要怎麼對應到潛在的
意義。實際上有好幾種不同的方式，可以具體討論程式的意義，所有的方式都在形式、
抽象、表現、實際效率之間有不同的折衷。我們將在後續幾節檢視幾種主要的形式之
道，並且討論它們彼此的關係。
操作語意學
思考程式意義最實際的方式就是
它能做什麼
。當我們執行程式時，我們期待會發生什
麼？程式語言裡的不同建構物在執行階段的行為會是如何？將它們接在一起而形成更大
的程式，又會有什麼效果？
這就是
操作語意
（
operational semantic
）的基礎，一種擷取程式語言意義的方式，而
這種方式是由程式如何在某種裝置執行的規則所定義。其中的裝置通常就是
抽象機
（
abstract machine
）：虛構、理想化的電腦，為了解釋語言的程式將會如何執行的特定目
的而設計。為了精巧的擷取到它們執行階段的行為，不同類型的程式語言通常需要不同
設計的抽象機。
賦予了操作語意，就能相當嚴格且精確面對語言裡特定建構物的目的。不同於英文撰寫
的語言規格，那種規格可能隱含了模糊，並且忽略重要的邊緣案例；為了讓人信服語言
行為的傳達效果，形式操作規格必須明確且不能模糊。
小步語意
那麼，我們要如何設計抽象機，並以它來指定程式語言的操作語意？方式之一是想像有
種能直接在其語法操作估算程式的機器，以較少的步驟重複
化簡
（
reduce
）這部機器，
不論結果的意義，使其每一步驟都讓程式更接近它的最終結果。
這些小步驟化簡類似我們在學校學的代數運算式計算？舉例來說，若要計算 ...
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