4.3.2. Erlang技巧¶

%仅适用于erl shell的命令
rp(xxx()).    % 显示全部内容
f().          %清除所有变量
f(Pid).       %清除变量Pid
c(useless).   %编译useless.erl

% atom相关
1> atom.
atom
2> atoms_rule.
atoms_rule
3> atoms_rule@erlang.
atoms_rule@erlang
4> 'Atoms can be cheated!'.
'Atoms can be cheated!'
5> atom = 'atom'.
atom

exit/1
exit(Pid, Reason).
exit(Pid, kill).
process_flag(trap_exit, true).


SpawnFun = spawn(fun functionname:fun/1).
spawn(Mod, Func, Args).

%% 实例说明:
do_it() ->
    process_flag(trap_exit, true),  % 指定当前进程为系统进程
    Pid = erlang:spawn_link(timer, sleep, [9999999]), % 很长时间执行不完
    exit(Pid, kill),    % 杀掉进程
    receive
        {'EXIT', Pid, killed} ->  % 会收到此种进程
            ok
    end.

try-catch语法结构内无法构成尾递归

判断是否字符串:

-spec is_string(String :: any()) -> true | false.
is_string(String) when is_list(String) ->
    Fun = fun(E) ->
        E >= 0 andalso E =< 255
          end,
    lists:all(Fun, String);
is_string(_String) ->
    ?FALSE.

一堆二进制转化成可读串:

A = <<123,34,73,78,70,79,34,58,123,34,78,73,67,75,78,65,77,69,34,58,34,
          227,128,130,83,117,110,102,108,111,119,101,114,91,229,164,170,
          233,152,179,93,34,125,125>>.
io:format("~ts",[A]).

小工具:

etop可以给进程排个名(memory, runtime, msg_q);
pman显示所有进程列表,并能trace进程的函数调用到文件中;
dbg进行trace,pattern trace更是方便的一塌糊涂;
tv可以查看ets,mnesia表;
percept可以绘制出进程并发统计图;
dialyzer对代码进行静态分析;
fprof进行性能分析. 这些你都用了么?

汉字:

unicode:characters_to_binary("谢谢谢谢谢谢谢谢谢谢谢").

// 打印汉字
erl> io:format("~ts~n", ["汉字"]).
汉字
ok

// 消息有为汉字时json出错
S = list_to_binary(xmerl_ucs:to_utf8("灯泡"))

操作符:

op  Description
==  等于
/=  不等于
=<  小于或等于
< 小于
>=  大于或等于
> 大于
=:= 恒等于
=/= 恒不等于
/ 浮点除法  number
div   整数除法  integer
bnot  一元 not 位运算  integer
rem 整数求余  integer
% 整数2进制运算符
band  与运算 integer
bor 或运算 integer
bxor  异或运算  integer
bsl 左移运算  integer
bsr 右移运算  integer
rem
bnot
% 2进制运算符
not   非
and   与
or    或
xor   异或
% 短路2进制运算符
orelse
andalso

% 实例:
2#00100 = 2#00010 bsl 1.
2#00001 = 2#00010 bsr 1.
2#10101 = 2#10001 bor 2#00101.

位运算:

8> <<R:8, Rest/binary>> = Pixels.
% 几种方式:
Value
Value:Size
Value/TypeSpecifierList
Value:Size/TypeSpecifierList

Type: integer | float | binary | bytes | bitstring | bits | utf8 | utf16 | utf32
Signedness: signed | unsigned
Endianness: big | little | native
Unit: unit:Integer

10> <<X1/unsigned>> =  <<-44>>.
<<"Ô">>
11> X1.
212
12> <<X2/signed>> =  <<-44>>.
<<"Ô">>
13> X2.
-44
14> <<X2/integer-signed-little>> =  <<-44>>.
<<"Ô">>
15> X2.
-44
16> <<N:8/unit:1>> = <<72>>.
<<"H">>
17> N.
72
18> <<N/integer>> = <<72>>.
<<"H">>
19> <<Y:4/little-unit:8>> = <<72,0,0,0>>.
<<72,0,0,0>>
20> Y.
72
21> <<Ver:5/binary, _/binary>> = <<"/v1.0/fdafk/fdsaf">>.
<<"/v1.0/fdafk/fdsaf">>
22> Ver.
<<"/v1.0">>

匿名函数递归:

F = fun(This, [], Total) -> Total;
     (This, [H|T], Total) -> This(This, T, H+Total) end.

实例:删除simple_one_for_one的子:
Fun =
  fun
    (This, List, 10) ->
      io:format("done~n");
    (This, [Pid | List], Num) ->
      io:format("kill pid:~p~n", [Pid]),
      exit(Pid, kill),
      This(This, List, Num+1),
      ok
  end.
Fun(Fun, [Pid || {_, Pid, _, _} <- supervisor:which_children(demo_lager_sup)], 0).

源码分析方法¶

反编译文件的源码:

Beam = "./_build/default/lib/octopus/ebin/fatscale_servant.beam".
{ok, {_, [{abstract_code, {_,Abs}}]}} =  beam_lib:chunks(
                Beam, [abstract_code]).
io:fwrite("~s~n", [erl_prettypr:format(erl_syntax:form_list(Abs))]).
or
file:write_file("abc.erl", erl_prettypr:format(erl_syntax:form_list(Abs))).