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)niluser=> (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.