parsing json return from http in erlang - json

Question

I test with this code :
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.
the response from the http is : {"code":200,"permission":true,"fee_payer":0,"fee_payee":19}
But now I like to do the same think but the return of http in this case for example is :
{"CIN":"08321224","Name":21}
so I have just CIN and Name in this case
I try to change the previous
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.
but it displys :
test:get_fee("0001").
200
{error,http_error}

Answer 1

So I'll nitpick on the style here, because you will be far better off if you follow the semantical idea of Erlang:
get_fee(Num) ->
Url = lists:concat(["http://localhost/GTW/Operation.php","?ACCOUNT_NUM=", Num]),
inets:start(),
This is the wrong place to start inets. It should be started outside this function as you only need to
do this one.
{Flag, Response} = http:request(get, {Url, []}, [], []),
This part is better coded with a pattern match. The discrimination of Flag and Response can
be decoded directly with a simple match. Write,
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.
I would recommend against changing {error, nxdomain} to {error, dns_error} since nxdomain perfectly
codes this case in any case. Just pass the error tuple to the caller and have him handle it.
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}};
Introduce a new function like this to handle the inner parts of your code base. And hoist the matching to
top-level. This helps in the long run by decoupling your code into functions.
Do note that in a JSON structure, there is no order on an "object" so you can't assume that the structure is
{"code":200,"permission":true,"fee_payer":0,"fee_payee":19}
But a decode does not have to preserve this structure, according to JSON. So a valid decode may be:
[{"fee_payee", 19}, {"fee_payer", 0}, {"permission", true}, {"code", 200}]
This will fail to match in your code and you are setting yourself up for some nasty errors later on.
You want something along the lines of:
get_fee_payer(PL) -> proplists:get_value("fee_payer", PL).
Another thing which will be a problem with your programming style is that you are hiding error-information cases. In Erlang, you can often get away with only handling the "happy path" through the code and leave all the error handling out until you know what kind of errors are there in the code base. Then you can begin adding in error handling slowly. Defensive programming is not a thing you should be doing if you can avoid it.

Answer 2

You changed:
if ReturnCode =:= 200 ->
to:
if ReturnCode =:= "08321224" ->
However, that needs to stay the same in your version. 200 is the HTTP status code for "OK" - the first step here is to verify that the server actually processed the request and returned a positive reply. You'll find that number only in Body - that is what the if CIN =:= "08321224"-> part is for.