Clojure语言八:Sequence

sequence定义

sequence不是一般的list，实际上实现了ISeq接口，ISeq接口定义如下：

package clojure.lang;

/**
 * A persistent, functional, sequence interface
 * <p/>
 * ISeqs are immutable values, i.e. neither first(), nor rest() changes
 * or invalidates the ISeq
 */
public interface ISeq extends IPersistentCollection, Sequential{

Object first();

ISeq next();

ISeq more();

ISeq cons(Object o);

}

sequence的操作

参考官方文档的说明：

The Seq interface
(first coll)
Returns the first item in the collection. Calls seq on its argument. If coll is nil, returns nil.
(rest coll)
Returns a sequence of the items after the first. Calls seq on its argument. If there are no more items, returns a logical sequence for which seq returns nil.
(cons item seq)
Returns a new seq where item is the first element and seq is the rest.

下面的操作定义了一个sequence, 然后返回第一个元素。

user=> (first (map #(print-str %) [1 2 3]))
"1"

返回除了第一元素之外的其余元素组成的列表：

user=> (rest (map #(print-str %) [1 2 3]))
("2" "3")

注意，#(print-str %) 函数将元素转变成了字符串。

lazy sequence

只有当sequence真正被使用的时候，才会被真正填充元素。这叫做lazy sequence。前面的(map实际上返回的就是lazy sequence。只有当调用first的时候才获得包含真正数据的sequence。

script和REPL的区别

不过在REPL会话环境下，lazy sequence是无效的，因为REPL会强制填充sequence，并返回。只是为了方便你看到。

xml-seq

现在回到前面一篇使用的xml

http://blog.csdn.net/sheismylife/article/details/8444290

xml-seq可以返回lazy sequence

user=> (use 'clojure.xml)
nil
user=> (parse "/home/chenshu/a.xml")
{:tag :service, :attrs nil, :content [{:tag :mongodb, :attrs nil, :content [{:tag :uri, :attrs nil, :content ["localhost"]}]} {:tag :socket, :attrs nil, :content [{:tag :port_number, :attrs nil, :content ["7777"]} {:tag :login_timeout, :attrs nil, :content ["200"]} {:tag :check_timeout, :attrs nil, :content ["200"]}]}]}
user=> (xml-seq (parse "/home/chenshu/a.xml"))
({:tag :service, :attrs nil, :content [{:tag :mongodb, :attrs nil, :content [{:tag :uri, :attrs nil, :content ["localhost"]}]} {:tag :socket, :attrs nil, :content [{:tag :port_number, :attrs nil, :content ["7777"]} {:tag :login_timeout, :attrs nil, :content ["200"]} {:tag :check_timeout, :attrs nil, :content ["200"]}]}]} {:tag :mongodb, :attrs nil, :content [{:tag :uri, :attrs nil, :content ["localhost"]}]} {:tag :uri, :attrs nil, :content ["localhost"]} "localhost" {:tag :socket, :attrs nil, :content [{:tag :port_number, :attrs nil, :content ["7777"]} {:tag :login_timeout, :attrs nil, :content ["200"]} {:tag :check_timeout, :attrs nil, :content ["200"]}]} {:tag :port_number, :attrs nil, :content ["7777"]} "7777" {:tag :login_timeout, :attrs nil, :content ["200"]} "200" {:tag :check_timeout, :attrs nil, :content ["200"]} "200")
user=> 

格式化一下，再看：

(
 {:tag :service, :attrs nil, :content [{:tag :mongodb, :attrs nil, :content [{:tag :uri, :attrs nil, :content ["localhost"]}]} 
                                       {:tag :socket, :attrs nil, :content [{:tag :port_number, :attrs nil, :content ["7777"]} 
                                                                            {:tag :login_timeout, :attrs nil, :content ["200"]} 
                                                                            {:tag :check_timeout, :attrs nil, :content ["200"]}]}]
  }
 {:tag :mongodb, :attrs nil, :content [{:tag :uri, :attrs nil, :content ["localhost"]}]} 
 {:tag :uri, :attrs nil, :content ["localhost"]} 
 "localhost" 
 {:tag :socket, :attrs nil, :content [{:tag :port_number, :attrs nil, :content ["7777"]} 
                                      {:tag :login_timeout, :attrs nil, :content ["200"]} 
                                      {:tag :check_timeout, :attrs nil, :content ["200"]}]} 
 {:tag :port_number, :attrs nil, :content ["7777"]} 
 "7777" 
 {:tag :login_timeout, :attrs nil, :content ["200"]} 
 "200" 
 {:tag :check_timeout, :attrs nil, :content ["200"]} 
 "200"
 )

可以看到xml-seq返回的结果第一部分就是parse返回的结果，之后就是每个xml元素单独一个list, 分别是mongodb, mongodb.uri, socket, socket.port_number, socket.login_timeout和socket.check_timeout.