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 6 章
程式設計進階技巧
構建程式卻完全沒有 proc 耗費更多心力，但是我們已經看到，只要不在意用點創造
力，就可以完成真正的工作。我們很快的討論幾種在這個最小環境編寫程式碼的其他技
巧。
無限串流
使用程式碼表示資料有一些有趣的優點。我們以 proc 為基礎的串列不一定是靜態：串
列只是在我們將它傳給
FIRST
和
REST
之際，執行正確運算的程式碼，因此我們很容易就
能實作忙著計算它們內容的串列，也就是所謂的
串流
（
stream
）。實際上，串流甚至沒
有理由必須是有限，因為在消耗它們的時候，計算只能產生串列內容，所以它可以持續
不斷產生新的值。
舉例來說，以下實作了無限串流的零：
ZEROS = Z[-> f { UNSHIFT[f][ZERO] }]
這是『無欺騙手段』版本的
ZEROS = UNSHIFT [ZEROS] [ZERO]
，是根據自身
的資料結構定義。作為程式開發人員，根據自身定義遞迴函式的想法，通
常會讓我們覺得滿意，但是根據自身定義的資料結構看起來可能很奇怪，
而且不尋常；它們在這種情況都完全一樣，而且 Z 組合器會讓兩者完全
合乎邏輯。
我們可以在主控台看到
ZEROS
的行為就像是串列，儘管好像可以無限延長：
>> to_integer(FIRST[ZEROS])
=> 0
>> to_integer(FIRST[REST[ZEROS]])
=> 0
>> to_integer(FIRST[REST[REST[REST[REST[REST[ZEROS]]]]]])
=> 0
雖然會將此串流轉換成 ...
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