April 2014
Beginner to intermediate
830 pages
15h 21m
Chinese
Content preview from Mathematica 錦囊妙計
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第二章
而這個做法失敗的原因則在於
state
只有在
Block
仍有效時才有定義，因為
Block
指
令 採 用 動 態 變 數 範 圍（dynamic scoping）， 但 閉 包 需 要 的 是 詞 法 變 數 範 圍（lexical
scoping），你可能記得
Module
指令採用的便是詞法變數範圍，說不定使用
Module
指令
即可成功。
看起來似乎成功了，但缺點在於如果直接執行
counter
時，會出現下列結果。
[Page-070]
閉包中的
state
變數以
state$<n>
模擬，這是因為
Module
指令會在同名稱的全域變數已
存在的情況下，自動建立新變數，但新變數並沒有保護措施，使用者可以隨時隨地改變
它，這個缺陷讓使用
Module
指令的實作變得沒這麼優雅，而且此缺陷在 JavaScript 與
Lisp 語言中完全不會出現。
因此在本文的程式碼中使用了另一個策略，利用
HoldAll
屬性建立一個容器作為閉包的
詞法環境。因為變數與函數以未執行的形式儲存，所以不必擔心是否有全域變數使用了
相同的變數名稱。當閉包需要進行求值時，
evaluate
函數會使用
Block
環境建立區域變
數或函數，接著便將這些區域定義和已存數值的變數及函數進行連接，並呼叫適當的區
域函數。
實際上在 Mathematica 中實作閉包有何應用價值呢？計數器這種應用似乎太過於簡單，
但即便是計數器應用也能展現出一些重要的功能；首先，如果我們用全域變數做為計數
器，很可能會不小心存取到它，以閉包方式將計數器封裝，可限制其存
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