Difficulty unpacking JSON tuple string

Question

I figured out how to use rebar. I'm trying to use jsx (jiffy doesn't work properly on Windows) to parse json that I obtained using the openexchangerates.org API, but I can't even figure out how to correctly utilize Erlang's extensive binary functionality in order to unpack the JSON tuple obtained. Using the following code snippet, I managed to get a tuple that has all the data I need:

-module(currency).
-export([start/0]).

start() -> 
    URL = "http://openexchangerates.org",
    Endpoint = "/api/latest.json?app_id=<myprivateid>",
    X = string:concat(URL, Endpoint),
    % io:format("~p~n",[X]).
    inets:start(),
    {ok, Req} = httpc:request(X),
    Req.

Here is the obtained response:

9> currency:start().
{{"HTTP/1.1",200,"OK"},
 [{"cache-control","public"},
  {"connection","close"},
  {"date","Fri, 15 Aug 2014 01:28:06 GMT"},
  {"etag","\"d9ad180d4af1caaedab6e622ec0a8a70\""},
  {"server","Apache"},
  {"content-length","4370"},
  {"content-type","application/json; charset=utf-8"},
  {"last-modified","Fri, 15 Aug 2014 01:00:56 GMT"},
  {"access-control-allow-origin","*"}],
 "{\n  \"disclaimer\": \"Exchange rates are provided for informational purposes only, and do not constitute financial advice of any kind. Although every attempt is made to ensure quality, NO guarantees are given whatsoever of accuracy, validity, availability, or fitness for any purpose - please use at your own risk. All usage is subject to your acceptance of the Terms and Conditions of Service, available at: https://openexchangerates.org/terms/\",\n  \"license\": \"Data sourced from various providers with public-facing APIs; copyright may apply; resale is prohibited; no warranties given of any kind. Bitcoin data provided by http://coindesk.com. All usage is subject to your acceptance of the License Agreement available at: https://openexchangerates.org/license/\",\n  \"timestamp\": 1408064456,\n  \"base\": \"USD\",\n  \"rates\": {\n    \"AED\": 3.673128,\n    \"AFN\": 56.479925,\n    \"ALL\": 104.147599,\n    \"AMD\": 413.859001,\n    \"ANG\": 1.789,\n    \"AOA\": 97.913074,\n    \"ARS\": 8.274908,\n    \"AUD\": 1.073302,\n    \"AWG\": 1.79005,\n    \"AZN\": 0.783933,\n    \"BAM\": 1.46437,\n    \"BBD\": 2,\n    \"BDT\": 77.478631,\n    \"BGN\": 1.464338,\n    \"BHD\": 0.377041,\n    \"BIF\": 1546.956667,\n    \"BMD\": 1,\n    \"BND\": 1.247024,\n    \"BOB\": 6.91391,\n    \"BRL\": 2.269422,\n    \"BSD\": 1,\n    \"BTC\": 0.0019571961,\n    \"BTN\": 60.843812,\n    \"BWP\": 8.833083,\n    \"BYR\": 10385.016667,\n    \"BZD\": 1.99597,\n    \"CAD\": 1.0906,\n    \"CDF\": 924.311667,\n    \"CHF\": 0.906799,\n    \"CLF\": 0.02399,\n    \"CLP\": 577.521099,\n    \"CNY\": 6.153677,\n    \"COP\": 1880.690016,\n    \"CRC\": 540.082202,\n    \"CUP\": 1.000688,\n    \"CVE\": 82.102201,\n    \"CZK\": 20.81766,\n    \"DJF\": 178.76812,\n    \"DKK\": 5.579046,\n    \"DOP\": 43.43789,\n    \"DZD\": 79.8973,\n    \"EEK\": 11.70595,\n    \"EGP\": 7.151305,\n    \"ERN\": 15.062575,\n    \"ETB\": 19.83205,\n    \"EUR\": 0.748385,\n    \"FJD\": 1.85028,\n    \"FKP\": 0.599315,\n    \"GBP\": 0.599315,\n    \"GEL\": 1.74167,\n    \"GGP\": 0.599315,\n    \"GHS\": 3.735499,\n    \"GIP\": 0.599315,\n    \"GMD\": 39.73668,\n    \"GNF\": 6995.309935,\n    \"GTQ\": 7.839405,\n    \"GYD\": 205.351249,\n    \"HKD\": 7.750863,\n    \"HNL\": 21.04854,\n    \"HRK\": 5.708371,\n    \"HTG\": 44.66625,\n    \"HUF\": 233.847801,\n    \"IDR\": 11682.083333,\n    \"ILS\": 3.471749,\n    \"IMP\": 0.599315,\n    \"INR\": 60.81923,\n    \"IQD\": 1178.211753,\n    \"IRR\": 26354,\n    \"ISK\": 115.976,\n    \"JEP\": 0.599315,\n    \"JMD\": 112.604801,\n    \"JOD\": 0.707578,\n    \"JPY\": 102.501401,\n    \"KES\": 88.106539,\n    \"KGS\": 51.96,\n    \"KHR\": 4056.578416,\n    \"KMF\": 368.149,\n    \"KPW\": 900,\n    \"KRW\": 1021.166657,\n    \"KWD\": 0.283537,\n    \"KYD\": 0.826373,\n    \"KZT\": 182.076001,\n    \"LAK\": 8049.834935,\n    \"LBP\": 1509.068333,\n    \"LKR\": 130.184301,\n    \"LRD\": 91.49085,\n    \"LSL\": 10.56165,\n    \"LTL\": 2.583284,\n    \"LVL\": 0.521303,\n    \"LYD\": 1.244127,\n    \"MAD\": 8.372529,\n    \"MDL\": 13.7178,\n    \"MGA\": 2495.605,\n    \"MKD\": 45.99967,\n    \"MMK\": 972.1784,\n    \"MNT\": 1884.666667,\n    \"MOP\": 7.986251,\n    \"MRO\": 292.0081,\n    \"MTL\": 0.683602,\n    \"MUR\": 30.61708,\n    \"MVR\": 15.37833,\n    \"MWK\": 392.9201,\n    \"MXN\": 13.07888,\n    \"MYR\": 3.175156,\n    \"MZN\": 30.3522,\n    \"NAD\": 10.56145,\n    \"NGN\": 162.303701,\n    \"NIO\": 26.07651,\n    \"NOK\": 6.157432,\n    \"NPR\": 97.66846,\n    \"NZD\": 1.179688,\n    \"OMR\": 0.38501,\n    \"PAB\": 1,\n    \"PEN\": 2.795018,\n    \"PGK\": 2.464545,\n    \"PHP\": 43.66429,\n    \"PKR\": 99.5662,\n    \"PLN\": 3.126223,\n    \"PYG\": 4272.421673,\n    \"QAR\": 3.641137,\n    \"RON\": 3.320192,\n    \"RSD\": 87.82784,\n    \"RUB\": 36.00216,\n    \"RWF\": 690.269,\n    \"SAR\": 3.750523,\n    \"SBD\": 7.269337,\n    \"SCR\": 12.40801,\n    \"SDG\": 5.699103,\n    \"SEK\": 6.86018,\n    \"SGD\": 1.246263,\n    \"SHP\": 0.599315,\n    \"SLL\": 4372.166667,\n    \"SOS\": 841.5678,\n    \"SRD\": 3.275,\n    \"STD\": 18316.816667,\n    \"SVC\": 8.745567,\n    \"SYP\": 150.751249,\n    \"SZL\": 10.56279,\n    \"THB\": 31.86192,\n    \"TJS\": 4.9856,\n    \"TMT\": 2.8501,\n    \"TND\": 1.719658,\n    \"TOP\": 1.8861,\n    \"TRY\": 2.15338,\n    \"TTD\": 6.343484,\n    \"TWD\": 30.00481,\n    \"TZS\": 1661.865,\n    \"UAH\": 13.02466,\n    \"UGX\": 2614.28,\n    \"USD\": 1,\n    \"UYU\": 23.70693,\n    \"UZS\": 2337.106637,\n    \"VEF\": 6.295009,\n    \"VND\": 21191.15,\n    \"VUV\": 94.6,\n    \"WST\": 2.301222,\n    \"XAF\": 491.286739,\n    \"XAG\": 0.05031657,\n    \"XAU\": 0.00076203,\n    \"XCD\": 2.70154,\n    \"XDR\": 0.654135,\n    \"XOF\": 491.394602,\n    \"XPF\": 89.414091,\n    \"YER\": 214.985901,\n    \"ZAR\": 10.55678,\n    \"ZMK\": 5253.075255,\n    \"ZMW\": 6.169833,\n    \"ZWL\": 322.355006\n  }\n}"}

I don't understand why this code oesn't work:

X = "Arthur".
B = <<X>>.

JSX allows a lot of parsing functionality but only if I have a binary as my representation of JSON, and this JSON I'm getting from the currency API is a string in a tuple... I'm a bit lost as to where to start to research. Unpacking a tuple using pattern matching is supposedly quite simple (I've done some Prolog programming and I can see that erlang has similar behavior) but is there a another, better, Erlang-appropriate way to grab the "rates" part of the JSON I'm receiving as a response?

Thank you! I'm working on a cool web app to learn erlang and this is a good first step. I have three Erlang books and I'm reading through them diligently but the problem is that I want as much practical exposure as early on as possible. I love this language but I want to get a solid grounding as fast as possible.

Thank you!

Answer 1

I figured out my problem.

So first of all, I wasn't thinking of how simple it is to simply count how many elements there are in the tuple. That being said, I realized there were only three.

So the line I needed was

{A,B,C} = Req.

After that, I only wanted to look at the last one (C, the JSON payload), which was a string. I found out through another source (not to disregard what you told me, Kay!) that you need to use the list functions, since strings and just lists of integers within an ASCII range (I think), in this case list_to_binary.

Once I used this line: D = list_to_binary(C), and subsequently E = jsx:decode(D), I got this output:

[{<<"disclaimer">>,
  <<"Exchange rates are provided for infor
 attempt is made to ensure quality, NO gua
se - please use at your own risk. All usag
s://openexchangerates.org/terms/">>},
 {<<"license">>,
  <<"Data sourced from various providers w
any kind. Bitcoin data provided by http://
t: https://openexchangerates.org/license/"
 {<<"timestamp">>,1408068012},
 {<<"base">>,<<"USD">>},
 {<<"rates">>,
  [{<<"AED">>,3.673128},
   {<<"AFN">>,56.479925},
   {<<"ALL">>,104.1526},
   {<<"AMD">>,413.859001},
   {<<"ANG">>,1.789},
   {<<"AOA">>,97.913949},
   {<<"ARS">>,8.274608},
   {<<"AUD">>,1.073236},
   {<<"AWG">>,1.79005},
   {<<"AZN">>,0.783933},
   {<<"BAM">>,1.46437},
   {<<"BBD">>,2},
   {<<"BDT">>,77.478631},
   {<<"BGN">>,1.464358},
   {<<"BHD">>,0.377041},
   {<<"BIF">>,1546.956667},
   {<<"BMD">>,1},
   {<<"BND">>,1.246774},
   {<<"BOB">>,6.91391},
   {<<"BRL">>,2.269462},
   {<<"BSD">>,1},
   {<<"BTC">>,0.0019393375},
   {<<"BTN">>,60.843812},
   {<<"BWP">>,8.833083},
   {<<"BYR">>,10385.016667},
   {<<"BZD">>,1.99597},
   {<<"CAD">>,1.090486},
   {<<"CDF">>,924.311667},
   {<<"CHF">>,0.906833},
   {<<"CLF">>,0.02399},
   {<<"CLP">>,577.521099},
   {<<"CNY">>,6.151637},
   {<<"COP">>,1880.690016},
   {<<"CRC">>,540.082202},
   {<<"CUP">>,1.000688},
   {<<"CVE">>,82.049699},
   {<<"CZK">>,20.818},
   {<<"DJF">>,179.084119},
   {<<"DKK">>,5.579049},
   {<<"DOP">>,43.43789},
   {<<"DZD">>,79.8641},
   {<<"EEK">>,11.7064},
   {<<"EGP">>,7.150475},
   {<<"ERN">>,15.062575},
   {<<"ETB">>,19.83205},
   {<<"EUR">>,0.748419},
   {<<"FJD">>,1.850441},
   {<<"FKP">>,0.599402},
   {<<"GBP">>,0.599402},
   {<<"GEL">>,1.74167},
   {<<"GGP">>,0.599402},
   {<<"GHS">>,3.735499},
   {<<"GIP">>,0.599402},
   {<<"GMD">>,39.73668},
   {<<"GNF">>,6995.309935},
   {<<"GTQ">>,7.839405},
   {<<"GYD">>,205.351249},
   {<<"HKD">>,7.750754},
   {<<"HNL">>,21.04854},
   {<<"HRK">>,5.708511},
   {<<"HTG">>,44.66625},
   {<<"HUF">>,233.8448},
   {<<"IDR">>,11685.75},
   {<<"ILS">>,3.471469},
   {<<"IMP">>,0.599402},
   {<<"INR">>,60.82523},
   {<<"IQD">>,1178.211753},
   {<<"IRR">>,26355.666667},
   {<<"ISK">>,115.96},
   {<<"JEP">>,0.599402},
   {<<"JMD">>,112.604801},
   {<<"JOD">>,0.707778},
   {<<"JPY">>,102.495401},
   {<<"KES">>,88.107639},
   {<<"KGS">>,51.991},
   {<<"KHR">>,4056.578416},
   {<<"KMF">>,368.142141},
   {<<"KPW">>,900},
   {<<"KRW">>,1021.353328},
   {<<"KWD">>,0.283537},
   {<<"KYD">>,0.826373},
   {<<"KZT">>,182.076001},
   {<<"LAK">>,8049.834935},
   {<<"LBP">>,1509.068333},
   {<<"LKR">>,130.184301},
   {<<"LRD">>,91.49085},
   {<<"LSL">>,10.56165},
   {<<"LTL">>,2.583364},
   {<<"LVL">>,0.521328},
   {<<"LYD">>,1.244147},
   {<<"MAD">>,8.372619},
   {<<"MDL">>,13.7178},
   {<<"MGA">>,2495.605},
   {<<"MKD">>,46.00037},
   {<<"MMK">>,972.1784},
   {<<"MNT">>,1885},
   {<<"MOP">>,7.986291},
   {<<"MRO">>,292.0081},
   {<<"MTL">>,0.683738},
   {<<"MUR">>,30.61748},
   {<<"MVR">>,15.37833},
   {<<"MWK">>,392.9201},
   {<<"MXN">>,13.07883},
   {<<"MYR">>,3.175406},
   {<<"MZN">>,30.3272},
   {<<"NAD">>,10.56145},
   {<<"NGN">>,162.303701},
   {<<"NIO">>,26.07651},
   {<<"NOK">>,6.156902},
   {<<"NPR">>,97.66846},
   {<<"NZD">>,1.179692},
   {<<"OMR">>,0.38501},
   {<<"PAB">>,1},
   {<<"PEN">>,2.795018},
   {<<"PGK">>,2.464545},
   {<<"PHP">>,43.68439},
   {<<"PKR">>,99.5642},
   {<<"PLN">>,3.126203},
   {<<"PYG">>,4272.421673},
   {<<"QAR">>,3.641297},
   {<<"RON">>,3.319212},
   {<<"RSD">>,87.8205},
   {<<"RUB">>,36.00206},
   {<<"RWF">>,690.088},
   {<<"SAR">>,3.750583},
   {<<"SBD">>,7.258136},
   {<<"SCR">>,12.40829},
   {<<"SDG">>,5.697837},
   {<<"SEK">>,6.857347},
   {<<"SGD">>,1.246447},
   {<<"SHP">>,0.599402},
   {<<"SLL">>,4360},
   {<<"SOS">>,841.5678},
   {<<"SRD">>,3.275},
   {<<"STD">>,18322.733333},
   {<<"SVC">>,8.745567},
   {<<"SYP">>,150.793749},
   {<<"SZL">>,10.56279},
   {<<"THB">>,31.87122},
   {<<"TJS">>,4.985575},
   {<<"TMT">>,2.8501},
   {<<"TND">>,1.719698},
   {<<"TOP">>,1.889033},
   {<<"TRY">>,2.15342},
   {<<"TTD">>,6.343484},
   {<<"TWD">>,29.99281},
   {<<"TZS">>,1661.865},
   {<<"UAH">>,13.02466},
   {<<"UGX">>,2614.28},
   {<<"USD">>,1},
   {<<"UYU">>,23.70693},
   {<<"UZS">>,2337.773304},
   {<<"VEF">>,6.295009},
   {<<"VND">>,21191.15},
   {<<"VUV">>,94.4875},
   {<<"WST">>,2.301222},
   {<<"XAF">>,491.283058},
   {<<"XAG">>,0.05031404},
   {<<"XAU">>,7.6211e-4},
   {<<"XCD">>,2.70154},
   {<<"XDR">>,0.654135},
   {<<"XOF">>,491.394602},
   {<<"XPF">>,89.416091},
   {<<"YER">>,214.985901},
   {<<"ZAR">>,10.55649},
   {<<"ZMK">>,5252.024745},
   {<<"ZMW">>,6.169833},
   {<<"ZWL">>,322.355006}]}]
ok

So this is closer to what I want, but how do I extract a specific currency easily? Like ZWL, for example.

Answer 2

get_currencies() ->    
    URL = "http://openexchangerates.org",
    Endpoint = "/api/latest.json?app_id=<myprivateid>",
    X = string:concat(URL, Endpoint),
    % io:format("~p~n",[X]).
    inets:start(),
    {ok, {_,_,R}} = httpc:request(X),
    E = jsx:decode(lists_to_binary(R)),
    Base = proplists:get_value(<<"base">>,E),
    Sec = proplists:get_value(<<"timestamp">>,E),
    {Days,Time} = calendar:seconds_to_daystime(Sec),
    Date = calendar:gregorian_days_to_date(Days+719528),
    Currencies = proplists:get_value(<<"rates">>,E),
    fun(C) -> V = proplists:get_value(C,Currencies),
                  {change,Date,Time,Base,C,V}
    end.

and somewhere in your code:

GC = get_currencies(), %% you can store GC in an ets, a server state...
%% but don't forget to refresh it :o)

and use it later

{change,D,T,B,C,V} = GC(<<"ZWL">>),
%% should return {change,{2014,8,15},{2,0,12},<<"USD">>,<<"ZWL">>,322.355006}

[edit]

When I use an external application such as jsx (using rebar itself), I use also rebar and its dependency mechanism to create my own application, in my opinion it is the most convenient way. (In other cases, I use also rebar :o)

Then I build my application using the OTP behaviors (application, supervisor, gen_server...). It is a lot of modules to write, but some of them are very very short (application and supervisors) and they facilitate the application structure (see What is OTP if you are not familiar with this).

In your case, my first idea is to have a gen server that build and store the GC anonymous function in its state, each time it get a cast message such as update_currencies, and provide the answer each time it get a call message such as {get_change,Cur} (and maybe refresh GC if it is undefined or out dated).

You will also have to decide where the errors will be managed - it may be nowhere: if the gen_server does nothing else but answer to this currency query: if something wrong appears it will crash and be restarted by its supervisor - because this code has many interfaces with the out world and so subject to numerous failures (no Internet access, structure of answer change from site, bad currency request from user...)