erlang中的一个List作为参数的实验记录

书承上篇，写道了http_server模块中的httpd_sup.erl中start_link函数和它的回调函数init；代码如下：

%%%=========================================================================
%%%  API
%%%=========================================================================
start_link(HttpdServices) ->
    supervisor:start_link({local, ?MODULE}, ?MODULE, [HttpdServices]).

start_link(HttpdServices, stand_alone) ->
    supervisor:start_link(?MODULE, [HttpdServices]).

..........

%%%=========================================================================
%%%  Supervisor callback
%%%=========================================================================
init([HttpdServices]) ->
    RestartStrategy = one_for_one,
    MaxR = 10,
    MaxT = 3600,
    Children = child_specs(HttpdServices, []),
    {ok, {{RestartStrategy, MaxR, MaxT}, Children}}.

有个疑问，为什么这里HttpdServices本来就是一个List，为什么init函数定义时要使用方括号把HttpdServices包起来呢？

做个实验，测试代码如下：

  1 -module(test_list).
  2
  3 -export([init/1, init/6, func/2]).
  4
  5
  6 init([Li]) ->
  7     io:format("List maybe as : ~p~n", [Li]).
  8
  9 init(L1, L2, L3, L4, L5, L6) ->
 10     io:format("fucker, item in List is : ~w~n", [L1]),
 11     io:format("fucker, item in List is : ~w~n", [L2]),
 12     io:format("fucker, item in List is : ~w~n", [L3]),
 13     io:format("fucker, item in List is : ~w~n", [L4]),
 14     io:format("fucker, item in List is : ~w~n", [L5]),
 15     io:format("fucker, item in List is : ~w~n", [L6]).
 16
 17 func(Li, Tp) ->
 18     io:format("List maybe as : ~p~n", [Li]),
 19     io:format("Tuple maybe as : ~p~n", [Tp]).
 20

实验过程，记录如下：

31> L1.
[1,2,3,555,6,7,8]
32> L2.
[1,3,44,56,7,666]

34> T1 = {1,11}.
{1,11}
35> T2 = {2,22}.
{2,22}
36> T3 = {3,33}.
{3,33}
37> T4 = {4,44}.
{4,44}
38> T5 = {5,55}.
{5,55}
39> T6 = {6,66}.
{6,66}
40> L3 = [T1, T2, T3, T4, T5, T6].

43> L3.
[{1,11},{2,22},{3,33},{4,44},{5,55},{6,66}]
44>

48> test_list:init(L2).
** exception error: no function clause matching
test_list:init([1,3,44,56,7,666]) (test_list.erl, line 6)
49>

54> test_list:init(L3).
** exception error: no function clause matching test_list:init([{1,11},
                                                                {2,22},
                                                                {3,33},
                                                                {4,44},
                                                                {5,55},
                                                                {6,66}]) (test_list.erl, line 6)

59> test_list:init(T1, T2, T3, T4, T5, T6).
fucker, item in List is : {1,11}
fucker, item in List is : {2,22}
fucker, item in List is : {3,33}
fucker, item in List is : {4,44}
fucker, item in List is : {5,55}
fucker, item in List is : {6,66}
ok
60> test_list:init(1,2,3,45,6,7).
fucker, item in List is : 1
fucker, item in List is : 2
fucker, item in List is : 3
fucker, item in List is : 45
fucker, item in List is : 6
fucker, item in List is : 7
ok
61>

61> test_list:init(L1,2,3,45,6,7).
fucker, item in List is : [1,2,3,555,6,7,8]
fucker, item in List is : 2
fucker, item in List is : 3
fucker, item in List is : 45
fucker, item in List is : 6
fucker, item in List is : 7
ok
62> test_list:init(L1,T2,3,45,6,7).
fucker, item in List is : [1,2,3,555,6,7,8]
fucker, item in List is : {2, 22}
fucker, item in List is : 3
fucker, item in List is : 45
fucker, item in List is : 6
fucker, item in List is : 7
ok
63>

对于init/6函数，L2和L3都有6个元素，似乎跟init/6能匹配，但是结果很明显：不能！

List和其他元素，组成的6个，可以匹配init/6；

List、Tuple和其他元素，组成的6个，也可以匹配init/6；

但是唯独6个元素的List，不能匹配init/6。

64> test_list:init(L1).
** exception error: no function clause matching
                    test_list:init([1,2,3,555,6,7,8]) (test_list.erl, line 6)
65> test_list:init(L2).
** exception error: no function clause matching
                    test_list:init([1,3,44,56,7,666]) (test_list.erl, line 6)
66> test_list:init([L1,L2]).
** exception error: no function clause matching
                    test_list:init([[1,2,3,555,6,7,8],[1,3,44,56,7,666]]) (test_list.erl, line 6)
67> test_list:init([[L1,L2]]).
List maybe as : [[1,2,3,555,6,7,8],[1,3,44,56,7,666]]
ok
68>
69> test_list:init([L2]).
List maybe as : [1,3,44,56,7,666]
ok

72> test_list:init([L3]).
List maybe as : [{1,11},{2,22},{3,33},{4,44},{5,55},{6,66}]
ok

对于init/1函数，L1和L2作为一个List，似乎跟init/1能匹配，但是结果很明显：还是不能！

[L1, L2]算是一个List结构，也不能匹配！

只有[L1]， [L2]， [[L1, L2]]，都可以匹配！

所以似乎可以得到这个结论：当只有一个List结构作为参数时，必须使用方括号包起来；

init([HttpdServices])这样使用方括号把List结构包住，才能处理HttpdServices作为一个长度不定的List的情况。