Erlang代码片段

gistfile1.txt

.列表操作

lists:foreach (fun(X) -> io:format("E=~p~n",[X]) end, [1,2,3]).

lists:duplicate (10, 16#f).  % [15,15,15,15,15,15,15,15,15,15]

"abc-123" -> "abc"

no_vsn(Name) -> lists:takewhile (fun($-)->false;(_)-> true end,Name).

"abc-123" -> "123"

vsn(Name) ->

    case lists:dropwhile (fun($-)->false;(_)->true end,Name) of

    [_Sep|Vsn] -> Vsn;

    _ -> "0"

    end.

取偶数

EvenN = lists:filter (fun (N) -> N rem 2 == 0 end, lists:seq (1,100))

折叠

lists:foldl (fun(F, Last) -> F(Last) end, foo(), [fun whee/1, fun bar/1])

Fun = fun
    (A = #auth{dir = [Dir]}, Acc) -> [{Dir, A}| Acc];
    (A, Acc) -> [A| Acc]
end,
Authdirs = lists:foldr(Fun, [], SC#sconf.authdirs),

将URL中的空格换成+
UW = lists:map (fun($ )->$+;(C)->C end,Words)

判断是否为空格

is_nb_space(X) ->
    lists:member (X, [$\s, $\t]).

>Data = [{"apple", "red"}, {"banana", "yellow"}, {"pear", "white"}].

>lists:keymember ("pear",1,Data).

  true

>lists:keydelete ("banana",1,Data).
  [{"apple","red"},{"pear","white"}]

>lists:keyreplace ("apple",1,Data,{"tomato", "red"}).
  [{"tomato","red"},{"banana","yellow"},{"pear","white"}]

> rd(user,{id,name}).   %% 用户记录

> lists:keysearch ("wang", #user.name, [#user{id=1,name="li"}, #user{id=2,name="wang"}]).

> {value,#user{id = 2,name = "wang"}}

case lists:any(fun(Str) -> lists:prefix(Str, "redColor") end, ["apple","red"]) of
   true -> {ok, Num, [H|T]};
   false -> false
end


.二进制操作

-define(BYTE, 8/unsigned-big-integer).

<<C:2/unit :?BYTE>> 意思就是取两个单位，每单位8bits。

unit的默认值取决于type，如果type是integer或者float，则为1，binary为8。

<<C:4/binary, _/binary>> 意思就是取4个单位，每单位8比特，总共4*8比特

1>A = <<1:32/little,1:8/little>>.   %低位在前
<<1,0,0,0,1>>
2> B = <<1:32/big,1:8/big>>.        %高位在前
  <<0,0,0,1,1>>
3> C = <<1:1,0:1>>.
<<2:2>>


> B = <<3.2:32/big-float>>.
> io:format("~w~n", [B]).
> <<64,76,204,205>>
> <<R:32/big-float>> = B.
> R.
> 3.200000047683716

<< PointID:32/unsigned-little-integer, Value:32/little-float>> = Bin.


对于utf相关类型，不能指定unit spec

utf8匹配1－4个bytes（参考RFC-2279）
utf16匹配2 或 4 个bytes (参考 RFC-2781）
utf32匹配4个 bytes


1> <<1024/utf8>>.
<<208,128>>
2> <<1024/utf16>>.
<<4,0>>
3> <<1024/utf32>>.
<<0,0,4,0>>
4> Bin = <<1024/utf8>>.
<<208,128>>
5> <<U/utf8>> = Bin.
<<208,128>>
6> U.
1024


.Binary Comprehensions

<< <<case X of $a -> $X; _ -> $Y end>> || <<X>> <= <<"abcaa">> >>.


返回： <<"XYYXX">>

<< <<X>> || <<X:1>>  <= <<255, 3:2>> >>.

返回： <<1,1,1,1,1,1,1,1,1,1>>

.位操作

    O2 = ((C1 band 16#03) bsl 4) bor (C2 bsr 4).

. 文件操作

Destination = filename:join ([code:root_dir(), "include", no_vsn( New_lib )]),

file:make_dir (Destination),

lists:foreach( fun(File) ->

    FileName = lists:last(string:tokens(File,"/\\")),

    file:copy (File, filename:join([Destination,FileName]))

    end,

    filelib:wildcard (filename:join([Source,"*"])) ).

w_ctl_file(Sid, Port, Key) ->
case catch
   begin
     F = yaws:ctl_file(Sid),
     error_logger:info_msg("Ctlfile : ~s~n", [F]),
     file:write_file(F, io_lib:format("~w.", [{Port,Key}])),
    {ok, FI} = file:read_file_info(F),
    ok = file:write_file_info(F, FI#file_info{mode = 8#00600})
   end of
   {'EXIT', _} -> error;
   _ -> ok
end.

remove(Path, File) ->
    Desc = filename:join([Path,File]),
    case filelib:is_dir (Desc) of
    true ->
        case file:list_dir (Desc) of
        {ok,Sub} -> lists:foreach(fun(S) -> remove(Desc,S) end,Sub);
        {error,Reason} -> io:format("error: ~p~n",[Reason])
        end,
        file:del_dir (Desc);
    false ->
        file:delete (Desc)
    end.

file:write_file("test.txt", "12 13 14 15 16 17 18").

{ok, Bin } = file:read_file (File ),

Rules = string:tokens (erlang:binary_to_list (Bin), "\n").

case file:open(File, [write]) of
        {ok, FD} ->
            %io:put_chars(FD, Text), <-- ERROR
            ok = file:close(FD),

            file:write_file(File, unicode:characters_to_binary(Text));   <-- HACK

        {error, R} ->
            R1 = file:format_error(R),
            report("could not write file '~s': ~s.", [File, R1]),
            exit(error)

end.

或

file:open(File, [write, {encoding, utf8}]).

{ok, Fd} = file:open("test_data.dat", [binary, write]),
ok = file:write(Fd, lists:duplicate(100, "1")),
ok = file:close(Fd).
 

.日期,时间操作

{{Y,M,D},{H,M,S}} = calendar:local_time ().

calendar:day_of_the_week (2009,2,24).

next_day({Y, M, D}) ->
    Day1 = calendar:date_to_gregorian_days (Y, M, D),
    calendar:gregorian_days_to_date (Day1 + 1).

w3cdtf(GregSecs) ->

    Date = calendar:gregorian_seconds_to_datetime(GregSecs),
    {{Y, Mo, D},{H, Mi, S}} = Date,
    [UDate|_] = calendar:local_time_to_universal_time_dst(Date),
    {DiffD,{DiffH,DiffMi,_}}=calendar:time_difference(UDate,Date).

.字符串操作

count_chars(String, Char) ->
    length ([X || X <- String, X == Char]).

格式化字符串, 像C语言的sscanf()

Cmd = io_lib:format (\"cd ~s/include; rm ~s; ln -s ../lib/~s/include ~s\", [code:root_dir(), no_vsn( New_lib ), New_lib, no_vsn( New_lib )]),

os:cmd(Cmd);

PN = [erlang:list_to_integer(N) || N <- string:tokens ( vsn( Package_that_might_be_newer ), "." )],

packages_from_html(Html,Packages) ->
    case string:str (Html,"class=\"package\"") of
    0 -> Packages;
    Start ->
        Sub = string:sub_string(Html,Start),
        T1 = string:sub_string(Sub,string:chr(Sub,$>)+1),
        PName = string:sub_string (T1,1,string:chr(T1,$<)-1),
        T2 = string:sub_string(T1,string:str(T1,"<i>")+3),
        Descr = string:sub_string(T2,1,string:str(T2,"</i>")-1),
        packages_from_html(T2,[{PName,Descr}|Packages])
    end.

当使用Erlang程序与其它语言程序通讯时，可能需要把一个字符串转换成为Erlang的Term，可以这样实现
{ok, Tokens, _} = erl_scan:string(String),
{ok, Term} = erl_parse:parse_term(Tokens).
注意这里的String需要以句号结尾。

例如在erlang shell下：
> {ok, Tokens, _} = erl_scan:string("{1, {2}, [3]}.").
> {ok, {X, Y, Z}} = erl_parse:parse_term(Tokens).

.端口操作

1>Port = open_port({spawn, "cat"}, [stream]).
#Port<0.522>
2> port_command(Port, "Hello World\n").
true
3> flush().
Shell got {#Port<0.522>,{data,{eol,"Hello World\n"}}}
ok
4> port_close(Port).
true


spawn_link(fun() -> tbz2(YPid, Dir) end),

tbz2(YPid, Dir) ->
   process_flag(trap_exit, true), 
   P = open_port({spawn, "tar cj ."},[{cd, Dir},use_stdio, binary, exit_status]),
   stream_loop(YPid, P).

stream_loop(YPid, P) ->
receive
   {P, {data, Data}} ->
     yaws_api:stream_chunk_deliver_blocking(YPid, Data),
     stream_loop(YPid, P);
   {P, {exit_status, _}} ->
     yaws_api:stream_chunk_end(YPid);
   {'EXIT', YPid, Status} ->
    exit(Status);
   Else ->
    error_logger:error_msg("Could not deliver zip file: ~p\n", [Else])
end.

getuid() ->
      case os:type() of
          {win32, _} -> {ok, "0"};
          _ ->
              load_setuid_drv(),
              P = open_port({spawn, "setuid_drv g"},[]),
              receive
                  {P, {data, "ok " ++ IntList}} ->  {ok, IntList}
              end
      end.

load_setuid_drv() ->
      Path = case yaws_generated:is_local_install() of
          true -> filename:dirname(code:which(?MODULE)) ++ "/../priv/lib";
          false ->
              PrivDir = code:priv_dir(yaws),
              filename:join(PrivDir,"lib")
      end,
      case erl_ddll:load_driver(Path, "setuid_drv") of
          ok -> ok;
         {error, Reason} ->
              error_logger:format("Failed to load setuid_drv (from ~p) : ~p",[Path, erl_ddll:format_error(Reason)]),
              exit(normal)
     end.


例外处理


call(URL, Options, Payload) ->
try
   {ok, CallPayloadDeep} = encode_call_payload(Payload)，
   CallPayload = lists:flatten(CallPayloadDeep)，
   {ok, Response} = http:request(post, {URL,
       [{"Content-length",length(CallPayload)}],
       "application/x-www-form-urlencoded",CallPayload},Options, [])，

   RespBody= if (size(Response) == 2) or (size(Response) == 3) ->
                element(size(Response), Response)
             end,
   decode_call_payload(RespBody)
catch error:Err -> error_logger:error_report([
                         {'json_rpc:call', error},{error, Err},
                         {stack, erlang:get_stacktrace()}]),
                   {error,Err}

catch _:_ -> ignore
end .

validate(A, B) ->
try
   mnesia:transaction((fun my_funky_mnesia_test/2)(A,B))
catch
   exit:{aborted, all_valid} -> ok;
   exit:{aborted, Error} -> {error, Error}
end.

-spec my_funky_mnesia_test(some_t(), another_t()) -> no_return().

my_funky_mnesia_test(A, B) ->
   some_test_db_operations(A, B),
   mnesia:abort(all_valid).


嵌套记录


-record(name, {first = "Robert", last = "Ericsson"}). 
-record(person, {name = #name{}, phone}). 
demo() -> 
   P = #person{name= #name{first="Robert",last="Virding"}, phone=123}, 
   First = (P#person.name)#name.first.  
动态运行代码


1> FunStr = "fun (A) -> A+B end.".
2> {ok, Tokens, _} = erl_scan:string(FunStr).
3> {ok, [Form]} = erl_parse:parse_exprs(Tokens).
4> Bindings = erl_eval:add_binding('B', 2, erl_eval:new_bindings()).
5> {value, Fun, _} = erl_eval:expr(Form, Bindings).
6> Fun(1).


remote_load_code(Module, Node) ->
    {_, Binary, Filename} = code:get_object_code(Module),
    rpc:call(Node, code, load_binary, [Module, Filename, Binary])


进程关系


Start the supervisor:

1> {ok, Sup} = supervisor:start_link(ignore_sup, []).
{ok,<0.43.0>}

It has no children:

2> supervisor:which_children(Sup).
[]

Starting the child:

3> supervisor:start_child(Sup, []).

=ERROR REPORT==== 13-Apr-2011::18:59:01 ===
Ignoring: <0.46.0>
{ok,undefined}

The child specification is installed:

4> supervisor:which_children(Sup).
[{undefined,undefined,worker,[ignore_server]}]

But the child process reported above is not alive:

5> is_process_alive(list_to_pid("<0.46.0>")).
false


进制转换


integer_to_hex(I) ->
      case catch erlang:integer_to_list(I, 16) of
          {'EXIT', _} ->
               old_integer_to_hex(I);
          Int ->
               Int
      end.


old_integer_to_hex(I) when I<10 ->
      integer_to_list(I);
old_integer_to_hex(I) when I<16 ->
      [I-10+$A];
old_integer_to_hex(I) when I>=16 ->
      N = trunc(I/16),
      old_integer_to_hex(N) ++ old_integer_to_hex(I rem 16).


%% hex_to_integer

hex_to_integer(Hex) ->
      erlang:list_to_integer(Hex, 16).

%% string_to_hex

string_to_hex(String) ->
      HEXC = fun (D) when D > 9 -> $a + D - 10;
                 (D) -> $0 + D
             end,
      lists:foldr(fun (E, Acc) ->
              [HEXC(E div 16),HEXC(E rem 16)|Acc] end, [], String).


%% hex_to_string

hex_to_string(Hex) ->
      DEHEX = fun (H) when H >= $a -> H - $a + 10;
                  (H) when H >= $A -> H - $A + 10;
                  (H) -> H - $0
              end,
  {String, _} =lists:foldr(fun

           (E, {Acc, nolow}) -> {Acc, DEHEX(E)};
           (E, {Acc, LO}) -> {[DEHEX(E)*16+LO|Acc], nolow}
      end, {[], nolow}, Hex),
      String.


数据结构


stdlib中包含大量的数据结构如lists，array，dict，gb_sets，gb_trees，ets，dets等


.gb_trees
1>X = gb_trees:from_orddict(orddict:from_list([{trace,false},{limit,-1},{timeout,-1}])).
{3,{timeout,-1,{limit,-1,nil,nil},{trace,false,nil,nil}}}

2>gb_trees:update(limit,true,X).
{3,{timeout,-1,{limit,true,nil,nil},{trace,false,nil,nil}}}


.ets/dets

E = ets:new (my_code, [public, set]),
ets:match_delete  (E,'_'),

ets:insert (E, {num_files, 0}),
ets:insert(E, {num_bytes, 0}).

%% T = ets:new(?MODULE, [{write_concurrency, true}]).

{ok,?MODULE}=dets:open_file(?MODULE,[{type,set},{file,"fcnum.dets"}]),
dets:insert(?MODULE, {1, [1,2,3,4,5,6,7]}),

dets:insert(?MODULE, {1, [7,6,5,4,3,2,1]}),
dets:close (?MODULE).

>Table = ets:new(foobar, [set, public]).
>ets:tab2list (Table).
[]
> ets:insert(Table, {1}).
true
> ets:insert(Table, {2}).
true
> ets:tab2list(Table).
[{1},{2}]


1> rd(data, {id, nick}).
data
2> rd(wrapper, {id, data}).
wrapper

3> L = [#wrapper{id=1,data=#data{id=1}}, #wrapper{id=2,data=#data{id=2,nick=n}}].
[#wrapper{id = 1,data = #data{id = 1,nick = undefined}},
#wrapper{id = 2,data = #data{id = 2,nick = n}}]

4> qlc:e(qlc:q([W || #wrapper{data = #data{nick = N}} = W <- L, N =/= undefined])).
[#wrapper{id = 2,data = #data{id = 2,nick = n}}]