Clojure Under a Microscope(1)： Clojure 如何理解代码（上）

$ java -cp clojure.jar  clojure.main -e "(println (+ 2 2))"
4

( p r i n t l n   ( +   2   2 ) )

(def r
  (-> "(println (+ 2 2))"
      (java.io.StringReader.)
      (clojure.lang.LineNumberingPushbackReader.)))

(.read r)            ; => 40  '('
(.read r)            ; => 112 'p'
(.read r)            ; => 114 'r'
(.unread r 114)      ; 『吐』回 'r' 
(.read r)            ; => 114 'r'   
(.read r)            ; => 105 'i' 
(.getLineNumber r)   ;  获取行号，从 1 开始
(.getColumnNumber r) ;  获取列号，从 0 开始
......

(int \()  ; => 40
(int \你) ; => 20320

user=> (select-keys (meta #'clojure.core/+) [:column :line :file])

{:column 1, :line 965, :file "clojure/core.clj"}

user=> (read-string  "(+ 1 2")
RuntimeException EOF while reading  clojure.lang.Util.runtimeException (Util.java:221)

(def a 1)
(def b 2)
(println (+ 1 2))

(def b 2)
(println (+ a b))

 Unable to resolve symbol: a in this context

for(; ;){
           //读取到一个 List，返回。
          if(pendingForms instanceof List && !((List)pendingForms).isEmpty())
              return ((List)pendingForms).remove(0);

          //读一个字符
          int ch = read1(r);

          //跳过空白，注意，逗号也被认为是空白
          while(isWhitespace(ch))
              ch = read1(r);

          //读到末尾
          if(ch == -1)
              {
              if(eofIsError)
                  throw Util.runtimeException("EOF while reading");
              return eofValue;
              }

          //读到设定的返回字符，提前返回。
          if(returnOn != null && (returnOn.charValue() == ch)) {
              return returnOnValue;
          }

          //可能是数字
          if(Character.isDigit(ch))
              {
              Object n = readNumber(r, (char) ch);
              return n;
              }

          //根据字符，查找 reader 表，走入更具体的解析
          IFn macroFn = getMacro(ch);
          if(macroFn != null)
              {
              Object ret = macroFn.invoke(r, (char) ch, opts, pendingForms);
              //no op macros return the reader
              if(ret == r)
                  continue;
              return ret;
              }
          //如果是正负符号，进一步判断可能是数字
          if(ch == '+' || ch == '-')
              {
              //再读一个字符
              int ch2 = read1(r);
              //如果是数字
              if(Character.isDigit(ch2))
                  {
                  //先回退 ch2 ，继续调用 readNumber 读出数字。
                  unread(r, ch2);
                  Object n = readNumber(r, (char) ch);
                  return n;
                  }
              //不是数字，回退 ch2
              unread(r, ch2);
              }
          //读取 token，并解析
          String token = readToken(r, (char) ch);
          return interpretToken(token);
          }
}

   static IFn[] macros = new IFn[256]; //特殊宏字符到 Reader 函数的映射
  macros['"'] = new StringReader();  // 双引号开头的使用字符串Reader
  macros[';'] = new CommentReader();  // 注释
  macros['\''] = new WrappingReader(QUOTE); // quote 
  macros['@'] = new WrappingReader(DEREF);// deref符号 @
  macros['^'] = new MetaReader();   //元数据
  macros['`'] = new SyntaxQuoteReader(); // syntax quote
  macros['~'] = new UnquoteReader();   // unquote
  macros['('] = new ListReader();      //list 
  macros[')'] = new UnmatchedDelimiterReader();  //括号不匹配
  macros['['] = new VectorReader();   //vector
  macros[']'] = new UnmatchedDelimiterReader();  // 中括号不匹配
  macros['{'] = new MapReader();     // map
  macros['}'] = new UnmatchedDelimiterReader();  // 大括号不匹配
  macros['\\'] = new CharacterReader();   //字符，如\a
  macros['%'] = new ArgReader();   // 匿名函数便捷记法里的参数，如%, %1
  macros['#'] = new DispatchReader();  // #下面将提到的 dispatch macro
  
  static private IFn getMacro(int ch){
    if(ch < macros.length)
        return macros[ch];
    return null;
   }

List list = readDelimitedList(')', r, true);
IObj s = (IObj) PersistentList.create(list);
return s;

//收集结果
ArrayList a = new ArrayList();

for(; ;)
  {
  int ch = read1(r);
  //忽略空白
  while(isWhitespace(ch))
      ch = read1(r);
  //非法终止
  if(ch == -1)
      {
      if(firstline < 0)
          throw Util.runtimeException("EOF while reading");
      else
          throw Util.runtimeException("EOF while reading, starting at line " + firstline);
      }
  //读到终止符号，也就是右括号)，停止
  if(ch == delim)
      break;
  //可能是macro fn
  IFn macroFn = getMacro(ch);
  if(macroFn != null)
      {
      Object mret = macroFn.invoke(r, (char) ch);
      //no op macros return the reader
      
      //macro fn 如果是no op，返回reader本身
      if(mret != r)
          //非no op,加入结果集合
          a.add(mret);
      }
  else
      {
      //非macro，回退ch
      unread(r, ch);
      //读取object并加入结果集合
      Object o = read(r, true, null, isRecursive);
      //同样，根据约定，如果返回是r，表示null
      if(o != r)
          a.add(o);
      }
  }
//返回收集的结果集合

return a;

user=> (meta (read-string "^:private (+ 2 2)"))
{:private true}

   //meta对象，可能是map，可能是symbol，也可能是字符串，例如(defn t [^"[B" bs] (String. bs))
   Object meta = read(r, true, null, true);
  //symbol 或者 字符串，就是简单的type hint tag
  if(meta instanceof Symbol || meta instanceof String)
      meta = RT.map(RT.TAG_KEY, meta);
  //如果是keyword，证明是布尔值的开关变量，如 ^:dynamic ^:private
  else if (meta instanceof Keyword)
      meta = RT.map(meta, RT.T);
  //如果连 map 都不是，那很抱歉，非法的meta数据
  else if(!(meta instanceof IPersistentMap))
      throw new IllegalArgumentException("Metadata must be Symbol,Keyword,String or Map");

  //读取要附加元数据的目标对象
  Object o = read(r, true, null, true);
  if(o instanceof IMeta)
      //如果可以附加，那么继续走下去
      {
      if(line != -1 && o instanceof ISeq)
          {
          //如果是ISeq，加入行号，列号
          meta = ((IPersistentMap) meta).assoc(RT.LINE_KEY, line).assoc(RT.COLUMN_KEY, column);
          }
      if(o instanceof IReference)
          {
          //如果是 ref，重设 meta
          ((IReference)o).resetMeta((IPersistentMap) meta);
          return o;
          }
      //增加 meta 到原有的 ometa
      Object ometa = RT.meta(o);
      for(ISeq s = RT.seq(meta); s != null; s = s.next()) {
      IMapEntry kv = (IMapEntry) s.first();
      ometa = RT.assoc(ometa, kv.getKey(), kv.getValue());
      }
      //关联到o
      return ((IObj) o).withMeta((IPersistentMap) ometa);
      }
  else
      //不可附加元素，抱歉，直接抛出异常
      throw new IllegalArgumentException("Metadata can only be applied to IMetas");

user=> ^:private 3
IllegalArgumentException Metadata can only be applied to IMetas  clojure.lang.LispReader$MetaReader.invoke (LispReader.java:820)

dispatchMacros['^'] = new MetaReader();  //元数据，老的形式 #^
dispatchMacros['\''] = new VarReader();   //读取var，#'a，所谓var-quote
dispatchMacros['"'] = new RegexReader();  //正则，#"[a-b]"
dispatchMacros['('] = new FnReader();    //匿名函数快速记法 #(println 3)
dispatchMacros['{'] = new SetReader();   // #{1} 集合
dispatchMacros['='] = new EvalReader();  // eval reader，支持 var 和 list的eval
dispatchMacros['!'] = new CommentReader();  //注释宏, #!开头的行将被忽略
dispatchMacros['<'] = new UnreadableReader();   // #< 不可读
dispatchMacros['_'] = new DiscardReader();   //#_ 丢弃

user=> (read-string "(read-string "(if 1 2 3 4)")")
(if 1 2 3 4)
user=> (if 1 2 3 4)
CompilerException java.lang.RuntimeException: Too many arguments to if, compiling:(NO_SOURCE_PATH:93:1)