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<title>示例: 編碼S表達式 | Go语言圣经</title>
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<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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<meta name="description" content="">
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<meta name="generator" content="GitBook 2.5.2">
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<body>
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<div class="book" data-level="12.4" data-chapter-title="示例: 編碼S表達式" data-filepath="ch12/ch12-04.md" data-basepath=".." data-revision="Thu Dec 31 2015 16:18:40 GMT+0800 (中国标准时间)">
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<div class="book"
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data-level="12.4"
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data-chapter-title="示例: 編碼S表達式"
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data-filepath="ch12/ch12-04.md"
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data-basepath=".."
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data-revision="Sat Jan 02 2016 16:00:23 GMT+0800 (中国标准时间)"
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data-innerlanguage="">
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<div class="book-summary">
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@@ -2024,7 +2030,128 @@
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<section class="normal" id="section-">
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<h2 id="124-示例-編碼s表達式">12.4. 示例: 編碼S表達式</h2>
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<p>TODO</p>
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<p>Display是一個用於顯示結構化數據的調試工具，但是它併不能將任意的Go語言對象編碼爲通用消息然後用於進程間通信。</p>
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<p>正如我們在4.5節中中看到的，Go語言的標準庫支持了包括JSON、XML和ASN.1等多種編碼格式。還有另一種依然被廣泛使用的格式是S表達式格式，采用類似Lisp語言的語法。但是和其他編碼格式不同的是，Go語言自帶的標準庫併不支持S表達式，主要是因爲它沒有一個公認的標準規范。</p>
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<p>在本節中，我們將定義一個包用於將Go語言的對象編碼爲S表達式格式，它支持以下結構：</p>
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<pre><code>42 integer
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"hello" string (with Go-style quotation)
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foo symbol (an unquoted name)
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(1 2 3) list (zero or more items enclosed in parentheses)
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</code></pre><p>布爾型習慣上使用t符號表示true，空列表或nil符號表示false，但是爲了簡單起見，我們暫時忽略布爾類型。同時忽略的還有chan管道和函數，因爲通過反射併無法知道它們的確切狀態。我們忽略的還浮點數、複數和interface。支持它們是練習12.3的任務。</p>
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<p>我們將Go語言的類型編碼爲S表達式的方法如下。整數和字符串以自然的方式編碼。Nil值編碼爲nil符號。數組和slice被編碼爲一個列表。</p>
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<p>結構體被編碼爲成員對象的列表，每個成員對象對應一個個僅有兩個元素的子列表，其中子列表的第一個元素是成員的名字，子列表的第二個元素是成員的值。Map被編碼爲鍵值對的列表。傳統上，S表達式使用點狀符號列表(key . value)結構來表示key/value對，而不是用一個含雙元素的列表，不過爲了簡單我們忽略了點狀符號列表。</p>
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<p>編碼是由一個encode遞歸函數完成，如下所示。它的結構本質上和前面的Display函數類似：</p>
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<pre><code class="lang-Go">gopl.io/ch12/sexpr
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<span class="hljs-keyword">func</span> encode(buf *bytes.Buffer, v reflect.Value) error {
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<span class="hljs-keyword">switch</span> v.Kind() {
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<span class="hljs-keyword">case</span> reflect.Invalid:
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buf.WriteString(<span class="hljs-string">"nil"</span>)
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<span class="hljs-keyword">case</span> reflect.Int, reflect.Int8, reflect.Int16,
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reflect.Int32, reflect.Int64:
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fmt.Fprintf(buf, <span class="hljs-string">"%d"</span>, v.Int())
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<span class="hljs-keyword">case</span> reflect.Uint, reflect.Uint8, reflect.Uint16,
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reflect.Uint32, reflect.Uint64, reflect.Uintptr:
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fmt.Fprintf(buf, <span class="hljs-string">"%d"</span>, v.Uint())
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<span class="hljs-keyword">case</span> reflect.String:
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fmt.Fprintf(buf, <span class="hljs-string">"%q"</span>, v.String())
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<span class="hljs-keyword">case</span> reflect.Ptr:
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<span class="hljs-keyword">return</span> encode(buf, v.Elem())
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<span class="hljs-keyword">case</span> reflect.Array, reflect.Slice: <span class="hljs-comment">// (value ...)</span>
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buf.WriteByte(<span class="hljs-string">'('</span>)
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<span class="hljs-keyword">for</span> i := <span class="hljs-number">0</span>; i < v.Len(); i++ {
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<span class="hljs-keyword">if</span> i > <span class="hljs-number">0</span> {
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buf.WriteByte(<span class="hljs-string">' '</span>)
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}
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<span class="hljs-keyword">if</span> err := encode(buf, v.Index(i)); err != <span class="hljs-constant">nil</span> {
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<span class="hljs-keyword">return</span> err
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}
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}
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buf.WriteByte(<span class="hljs-string">')'</span>)
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<span class="hljs-keyword">case</span> reflect.Struct: <span class="hljs-comment">// ((name value) ...)</span>
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buf.WriteByte(<span class="hljs-string">'('</span>)
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<span class="hljs-keyword">for</span> i := <span class="hljs-number">0</span>; i < v.NumField(); i++ {
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<span class="hljs-keyword">if</span> i > <span class="hljs-number">0</span> {
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buf.WriteByte(<span class="hljs-string">' '</span>)
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}
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fmt.Fprintf(buf, <span class="hljs-string">"(%s "</span>, v.Type().Field(i).Name)
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<span class="hljs-keyword">if</span> err := encode(buf, v.Field(i)); err != <span class="hljs-constant">nil</span> {
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<span class="hljs-keyword">return</span> err
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}
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buf.WriteByte(<span class="hljs-string">')'</span>)
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}
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buf.WriteByte(<span class="hljs-string">')'</span>)
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<span class="hljs-keyword">case</span> reflect.Map: <span class="hljs-comment">// ((key value) ...)</span>
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buf.WriteByte(<span class="hljs-string">'('</span>)
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<span class="hljs-keyword">for</span> i, key := <span class="hljs-keyword">range</span> v.MapKeys() {
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<span class="hljs-keyword">if</span> i > <span class="hljs-number">0</span> {
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buf.WriteByte(<span class="hljs-string">' '</span>)
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}
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buf.WriteByte(<span class="hljs-string">'('</span>)
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<span class="hljs-keyword">if</span> err := encode(buf, key); err != <span class="hljs-constant">nil</span> {
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<span class="hljs-keyword">return</span> err
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}
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buf.WriteByte(<span class="hljs-string">' '</span>)
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<span class="hljs-keyword">if</span> err := encode(buf, v.MapIndex(key)); err != <span class="hljs-constant">nil</span> {
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<span class="hljs-keyword">return</span> err
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}
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buf.WriteByte(<span class="hljs-string">')'</span>)
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}
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buf.WriteByte(<span class="hljs-string">')'</span>)
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<span class="hljs-keyword">default</span>: <span class="hljs-comment">// float, complex, bool, chan, func, interface</span>
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<span class="hljs-keyword">return</span> fmt.Errorf(<span class="hljs-string">"unsupported type: %s"</span>, v.Type())
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}
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<span class="hljs-keyword">return</span> <span class="hljs-constant">nil</span>
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}
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</code></pre>
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<p>Marshal函數是對encode的保證，以保持和encoding/...下其它包有着相似的API：</p>
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<pre><code class="lang-Go"><span class="hljs-comment">// Marshal encodes a Go value in S-expression form.</span>
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<span class="hljs-keyword">func</span> Marshal(v <span class="hljs-keyword">interface</span>{}) ([]<span class="hljs-typename">byte</span>, error) {
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<span class="hljs-keyword">var</span> buf bytes.Buffer
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<span class="hljs-keyword">if</span> err := encode(&buf, reflect.ValueOf(v)); err != <span class="hljs-constant">nil</span> {
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<span class="hljs-keyword">return</span> <span class="hljs-constant">nil</span>, err
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}
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<span class="hljs-keyword">return</span> buf.Bytes(), <span class="hljs-constant">nil</span>
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}
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</code></pre>
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<p>下面是Marshal對12.3節的strangelove變量編碼後的結果：</p>
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<pre><code>((Title "Dr. Strangelove") (Subtitle "How I Learned to Stop Worrying and Lo
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ve the Bomb") (Year 1964) (Actor (("Grp. Capt. Lionel Mandrake" "Peter Sell
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ers") ("Pres. Merkin Muffley" "Peter Sellers") ("Gen. Buck Turgidson" "Geor
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ge C. Scott") ("Brig. Gen. Jack D. Ripper" "Sterling Hayden") ("Maj. T.J. \
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"King\" Kong" "Slim Pickens") ("Dr. Strangelove" "Peter Sellers"))) (Oscars
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("Best Actor (Nomin.)" "Best Adapted Screenplay (Nomin.)" "Best Director (N
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omin.)" "Best Picture (Nomin.)")) (Sequel nil))
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</code></pre><p>整個輸出編碼爲一行中以減少輸出的大小，但是也很難閲讀。這里有一個對S表達式格式化的約定。編寫一個S表達式的格式化函數將作爲一個具有挑戰性的練習任務；不過 <a href="http://gopl.io" target="_blank">http://gopl.io</a> 也提供了一個簡單的版本。</p>
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<pre><code>((Title "Dr. Strangelove")
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(Subtitle "How I Learned to Stop Worrying and Love the Bomb")
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(Year 1964)
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(Actor (("Grp. Capt. Lionel Mandrake" "Peter Sellers")
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("Pres. Merkin Muffley" "Peter Sellers")
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("Gen. Buck Turgidson" "George C. Scott")
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("Brig. Gen. Jack D. Ripper" "Sterling Hayden")
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("Maj. T.J. \"King\" Kong" "Slim Pickens")
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("Dr. Strangelove" "Peter Sellers")))
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(Oscars ("Best Actor (Nomin.)"
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"Best Adapted Screenplay (Nomin.)"
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"Best Director (Nomin.)"
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"Best Picture (Nomin.)"))
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(Sequel nil))
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</code></pre><p>和fmt.Print、json.Marshal、Display函數類似，sexpr.Marshal函數處理帶環的數據結構也會陷入死循環。</p>
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<p>在12.6節中，我們將給出S表達式解碼器的實現步驟，但是在那之前，我們還需要先了解如果通過反射技術來更新程序的變量。</p>
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<p><strong>練習 12.3：</strong> 實現encode函數缺少的分支。將布爾類型編碼爲t和nil，浮點數編碼爲Go語言的格式，複數1+2i編碼爲#C(1.0 2.0)格式。接口編碼爲類型名和值對，例如("[]int" (1 2 3))，但是這個形式可能會造成歧義：reflect.Type.String方法對於不同的類型可能返迴相同的結果。</p>
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<p><strong>練習 12.4：</strong> 脩改encode函數，以上面的格式化形式輸出S表達式。</p>
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<p><strong>練習 12.5：</strong> 脩改encode函數，用JSON格式代替S表達式格式。然後使用標準庫提供的json.Unmarshal解碼器來驗證函數是正確的。</p>
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<p><strong>練習 12.6：</strong> 脩改encode，作爲一個優化，忽略對是零值對象的編碼。</p>
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<p><strong>練習 12.7：</strong> 創建一個基於流式的API，用於S表達式的解碼，和json.Decoder(§4.5)函數功能類似。</p>
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</section>
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