Erlang 新手：添加了 IF，变量不起作用？

Question

有谁擅长调试 Erlang 吗？我一生都无法弄清楚出了什么问题。无论我将 Fields 变量放在哪里，Erlang 都会说该行之前有一个错误...

编译消息：

./eventbus.erl:6: syntax error before: FieldPositions
./eventbus.erl:24: variable 'FieldPositions' is unbound
./eventbus.erl:28: Warning: variable 'Ref' is unused
./eventbus.erl:30: Warning: variable 'List' is unused
error

然后是代码本身。

-module(secret).
-export([listen/1, send/1]).

-define(TCP_OPTIONS, [binary, {packet, 0}, {active, false}, {reuseaddr, true}]).

FieldPositions = ["A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","AA","BB","CC","DD","EE","FF","GG","HH","II","JJ","KK","LL","MM","NN","OO","PP"].


listen(Port) ->
    {ok, LSocket} = gen_tcp:listen(Port, ?TCP_OPTIONS),
    accept(LSocket).

accept(LSocket) ->
    {ok, Socket} = gen_tcp:accept(LSocket),
    spawn(fun() -> loop(Socket) end),
    accept(LSocket).    

discrim(<<>>) ->
    ok;
discrim([]) ->
    ok;
discrim(Info) ->
    EventsList = string:tokens(Info,"|"),
    process_events(EventsList, FieldPositions).

process_events([],[]) ->
    ok;
process_events([],Ref) ->
    ok;
process_events(List,[]) ->
    ok;
process_events(List,Ref) ->
    [RHead|RTail] = Ref,
    [Head|Tail] = List,
    if
    Head == [] ->
        process_events(Tail, RTail);
    true ->
        io:format("message.bus ~s ~s",[RHead,Head]),
        process_events(Tail, RTail),
        ok
    end.

loop(Socket) ->
    case gen_tcp:recv(Socket, 0) of
        {ok, Data} ->
            %gen_tcp:send(Socket, Data),
            %io:format(Data),
            discrim(Data),
            io:format("~n"),
            loop(Socket);
        {error, closed} ->
            ok
    end.

Answer 1

您尝试在模块级别声明一个值 FieldPositions，这在 Erlang 中是不允许的。 （但是您可以在模块级别声明函数，或在函数内声明值。）

有许多替代方法可以实现您想要的效果：

• 使用 -define 宏，非常类似于TCP_OPTIONS 代码中的一个。

• 将 FieldPositions 的值内联到您想要使用它的位置。

• 编写一个常量返回函数，例如：field_positions() -> ["A","B",...,"PP"].

顺便说一句，根据我使用各种编译器和解释器的经验，我建议您不要过于字面地理解错误消息。特别是，我认为您应该将收到的错误视为“错误发生在 x 行周围的某处”，而不是字面上的“在 x 行之前”或“在 x 行处”。