Erlang 接收表达式究竟是如何工作的？

Question

• 为什么接收表达式有时称为选择性接收？
• 什么是“保存队列”？
• after 部分如何工作？

Answer 1

该过程涉及一个特殊的“保存队列”，当您第一次遇到接收表达式时，您可能会忽略它的存在。

可选地，表达式中可能有一个后段，会使过程稍微复杂化。

接收表达式最好用流程图来解释：

receive
  pattern1 -> expressions1;
  pattern2 -> expressions2;
  pattern3 -> expressions3
after
  Time -> expressionsTimeout
end

Answer 2

为什么接收表达式有时被称为选择性接收？

-module(my).
%-export([test/0, myand/2]).
-compile(export_all).
-include_lib("eunit/include/eunit.hrl").

start() ->
    spawn(my, go, []).

go() ->
    receive
        {xyz, X} ->
            io:format("I received X=~w~n", [X])
    end.

在 erlang shell 中：

1> c(my).
my.erl:3: Warning: export_all flag enabled - all functions will be exported
{ok,my}

2> Pid = my:start().                
<0.79.0>

3> Pid ! {hello, world}.
{hello,world}

4> Pid ! {xyz, 10}.     
I received X=10
{xyz,10}

注意发送的第一条消息没有输出，但发送的第二条消息有输出。接收是选择性的：它没有接收到所有消息，它只接收到与指定模式匹配的消息。

什么是“保存队列”？

-module(my).
%-export([test/0, myand/2]).
-compile(export_all).
-include_lib("eunit/include/eunit.hrl").

start() ->
    spawn(my, go, []).

go() ->
    receive
        {xyz, X} ->
            io:format("I received X=~w~n", [X])
    end,
    io:format("What happened to the message that didn't match?"),
    receive 
        Any ->
            io:format("It was saved rather than discarded.~n"),
            io:format("Here it is: ~w~n", [Any])
    end.

在 erlang shell 中：

1> c(my).               
my.erl:3: Warning: export_all flag enabled - all functions will be exported 
{ok,my}

2> Pid = my:start().    
<0.79.0>

3> Pid ! {hello, world}.
{hello,world}

4> Pid ! {xyz, 10}.     
I received X=10
What happened to the message that didn't match?{xyz,10}
It was saved rather than discarded.
Here it is: {hello,world}

after 部分如何工作？

-module(my).
%-export([test/0, myand/2]).
-compile(export_all).
-include_lib("eunit/include/eunit.hrl").

start() ->
    spawn(my, go, []).

go() ->
    receive
        {xyz, X} ->
            io:format("I received X=~w~n", [X])
    after 10000 ->
        io:format("I'm not going to wait all day for a match.  Bye.")
    end.

在 erlang shell 中：

1> c(my).           
my.erl:3: Warning: export_all flag enabled - all functions will be exported 
{ok,my}

2> Pid = my:start().
<0.79.0>

3> Pid ! {hello, world}.
{hello,world}
I'm not going to wait all day.  Bye.4> 

另一个例子：

-module(my).
%-export([test/0, myand/2]).
-compile(export_all).
-include_lib("eunit/include/eunit.hrl").

sleep(X) ->
    receive
    after X * 1000 ->
        io:format("I just slept for ~w seconds.~n", [X])
    end.

在 erlang shell 中：

1> c(my).
my.erl:3: Warning: export_all flag enabled - all functions will be exported 
{ok,my}

2> my:sleep(5).                   
I just slept for 5 seconds.
ok