如果您在文件中定义了函数接收/2,则意味着您具有以下内容:
-module(ifelse).
-export([receiving/0,receiving/2]).
receiving() ->
some_code.
receiving(A,B) ->
other_code.
你可以用
Pid = spawn(ifelse,receiving,[1,2])。
顺便说一句,在erlang中写if语句是不常见的,原因是如果一个case不匹配任何条件,代码就会崩溃。
5> F=fun(X) -> if (X rem 2) == 0 -> X+1 end end.
#Fun<erl_eval.6.82930912>
6> F(4).
5
7> F(5).
** exception error: no true branch found when evaluating an if expression
8>
如果你想避免这种情况,你必须有一个默认的守卫(然后它看起来像一个案例)。
8> F1=fun(X) -> if (X rem 2) == 0 -> X+1;
8> true -> X end end.
#Fun<erl_eval.6.82930912>
9> F1(4).
5
10> F1(5).
11>
编写函数的常用方法更像是这样:
receiving() ->
receive
{Pid,_A,_A} when is_pid(Pid) ->
% use the pattern matching to verify that the 2 elements are equal
% and a guard test to check that the first element is a pid.
% Note that in this case it is a strict equals. I use _A because the code doesn't
% care of the value itself
io:format(" B equals A ~n"),
Pid ! "True";
{Pid,_,_} when is_pid(Pid) ->
% use pattern maching to verify the that message is a tupple of 3 elements
% and a guard test to check that the first element is a pid.
% For the 2 last elements I use _, so the data is not bound to any variable,
% only the structure is tested
io:format(" B does not equal A ~n"),
Pid ! "False";
_ -> {error, wrong_value}
end.
我在 shell 中对此进行了测试:
14> F = fun() ->
14> receive
14> {Pid,_A,_A} when is_pid(Pid) ->
14> io:format(" B equals A ~n"),
14> Pid ! "True";
14> {Pid,_,_} when is_pid(Pid) ->
14> io:format(" B does not equal A ~n"),
14> Pid ! "False";
14> _ -> {error, wrong_value}
14> end
14> end.
#Fun<erl_eval.20.82930912>
15> Pid = spawn(F).
<0.58.0>
16> Pid ! {self(),1,2}.
B does not equal A
{<0.51.0>,1,2}
17> % the returm value of "proc ! Mess" is Mess. It is what we get on the console on previous line
17> flush(). % use flush() to get the messages received by the shell
Shell got "False"
ok
18> Pid ! {self(),test,test}. % the process Pid is terminated now. when we send a message to it,
18> % it is simply "lost".
{<0.51.0>,test,test}
19> % it is necessary though to use a new variable Pid1 and spawn a new process
19> % (see rvirding message and user601836 answer)
19> Pid1 = spawn(F).
<0.63.0>
20> Pid1 ! {self(),test,test}.
B equals A
{<0.51.0>,test,test}
21> flush().
Shell got "True"
ok
22> Pid2 = spawn(F).
<0.68.0>
23> Pid2 ! {hello,test,test}.
{hello,test,test}
24> flush().
ok
25> % of course there is no message sent back, no io:format to print something on the console,
25> % the returned value of the function in the error case is "lost".
25> % if you want to have a permanent process you must have a recursive loop,
25> % calling receiving() were needed.