解析json从erlang中的http返回

Question

我用这段代码测试:

get_fee(Transaction,SourceNumber,Amount, Currency) ->
Url = lists:concat(["http://localhost/test.php","?transaction=", Transaction, "&saccount=", SourceNumber,Amount,"&currency=",Currency]),
        inets:start(),
        {Flag, Response} = http:request(get, {Url, []}, [], []),
        case Flag of
            ok ->
                { { _, ReturnCode, _ }, _, Body } = Response,

                if ReturnCode =:= 200 ->
                        {ok,{_,[{_,Code},{_,Permission},{_,Payer},{_,Payee}]}} = json:decode_string(Body),
                            case Permission of true ->
                                if Code =:= 200 ->
                                    {ok,{Code, Payer, Payee}};
                                Code =:= 204 ->
                                    {nok,{Code, not_found}};
                                true ->
                                    {nok,{Code, parameter_error}}
                                end;
                            false ->
                                {nok,{Code, parameter_error}}
                            end;
                    true->
                        {error, http_error}
                end;
            error ->
                case Response of
                    nxdomain -> {error, dns_error};
                    _ -> {error, network_error}

                end
        end.

http的响应是: {"code":200,"permission":true,"fee_payer":0,"fee_payee":19}

但是现在我喜欢做同样的想法但是在这种情况下http的返回例如是:

{"CIN":"08321224","Name":21}

所以我在这种情况下只有CIN和Name

我试着改变以前的

get_fee(Num) ->
    Url = lists:concat(["http://localhost/GTW/Operation.php","?ACCOUNT_NUM=", Num]),




            inets:start(),
             {Flag, Response} = http:request(get, {Url, []}, [], []),
            case Flag of
                ok ->
                    { { _, ReturnCode, _ }, _, Body } = Response,
                    %% for debug
                   io:format("~p~n",[ReturnCode]),
                    if ReturnCode =:= "08321224" ->
                            {ok,{_,[{_,CIN},{_,Name}]}} = json:decode_string(Body),
                                case Name of 21 ->
                                                 io:format(CIN),
                                                 io:format(Name),
                                    if CIN =:= "08321224"->
                                        {ok,{CIN, Name}};
                                    CIN =:= 204 ->
                                        {nok,{CIN, not_found}};
                                    true ->
                                        {nok,{CIN, parameter_error}}
                                    end;
                                false ->
                                    {nok,{CIN, parameter_error}}
                                end;
                        true->
                            {error, http_error}
                    end;
                error ->
                    case Response of
                        nxdomain -> {error, dns_error};
                        _ -> {error, network_error}
                    %% for debug
                    %%io:format("pass2~n ~p~n",[Response]),
                    end
  end.

但它显示:

test:get_fee("0001").
200
{error,http_error}

Answer 1

所以我会挑选这里的风格,因为如果你遵循Erlang的语义,你会好得多:

get_fee(Num) ->
    Url = lists:concat(["http://localhost/GTW/Operation.php","?ACCOUNT_NUM=", Num]),
    inets:start(),

这是一个错误的起点inets.它应该在此功能之外启动,因为您只需要执行此操作.

{Flag, Response} = http:request(get, {Url, []}, [], []),

这部分用模式匹配更好地编码.辨别Flag和Response可以通过简单的匹配直接解码.写,

case http:request(get, {Url, []}, [], []) of
  {ok, {{_, 200, _}, _, Body}} ->
      {ok, R} = json:decode_string(Body),
      get_fee_decode_(get_cin(R), get_name(R));
  {error, Reason} -> {error, Reason}
end.

我建议不要改变{error, nxdomain},{error, dns_error}因为nxdomain在任何情况下都能完美地编码这个案例.只需将错误元组传递给调用者并让他处理它.

get_fee_decode_("08321224" = CIN, 21 = Name) -> {ok, {CIN, Name}};
get_fee_decode_("204" = CIN, 21) -> {nok, {CIN, not_found}};
get_fee_decode_(CIN, _Name) -> {nok, {CIN, parameter_error}};

引入一个这样的新函数来处理代码库的内部部分.并将匹配提升到顶级.从长远来看,这有助于将代码分离为函数.

请注意,在JSON结构中,"对象"上没有顺序,因此您不能假设结构是

{"code":200,"permission":true,"fee_payer":0,"fee_payee":19}

但根据JSON,解码不必保留这种结构.所以有效的解码可能是:

[{"fee_payee", 19}, {"fee_payer", 0}, {"permission", true}, {"code", 200}]

这将无法与您的代码匹配,您将在以后为自己设置一些令人讨厌的错误.

你想要的东西是:

get_fee_payer(PL) -> proplists:get_value("fee_payer", PL).

另一个与您的编程风格有关的问题是您隐藏了错误信息案例.在Erlang中,您通常可以通过代码处理"快乐路径"并将所有错误处理保留下来,直到您知道代码库中存在哪种错误为止.然后,您可以开始慢慢添加错误处理.如果可以避免,防御性编程不是你应该做的事情.