Perplexing JSON decoding issue.

Miyako has asked for the wisdom of the Perl Monks concerning the following question:

Hello, I am having a slight issue in Perl 5.2. I've used libjson-perl for a few years and written many things which used it to decode Json from websites. I am now in the process of writing a bot for the chat app Discord which uses a Json api, much like other places I've written bots for previously. I know my code is right, or at least should be, yet I am getting the "not a hash reference error." Here is the output I am getting in my terminal.

$VAR1 = [
          {
            'mentions' => [],
            'author' => {
                          'avatar' => '035a793a88596c440cb0434f272d8f3
+c',
                          'discriminator' => '4559',
                          'id' => '159495601208164353',
                          'username' => 'MiyakoProductions'
                        },
            'embeds' => [],
            'channel_id' => '193239098226376705',
            'tts' => bless( do{\(my $o = 0)}, 'JSON::XS::Boolean' ),
            'mention_roles' => [],
            'mention_everyone' => $VAR1->[0]{'tts'},
            'content' => 'so i wrote a simple script to pull each one 
+automatically',
            'id' => '233412710220562432',
            'type' => 0,
            'pinned' => $VAR1->[0]{'tts'},
            'edited_timestamp' => undef,
            'timestamp' => '2016-10-06T02:18:46.715000+00:00',
            'attachments' => []
          }
        ];

Not a HASH reference at discord.pl line 70.
[download]

My code itself is:

sub getMessages($) {
    my $apiUrl = $baseUrl . "channels/" . $_[0] . "/messages?limit=1";
    print "$apiUrl\n";
    my $discordClient = WWW::Mechanize->new( agent => 'DiscordBot (Lib
+Discord-Perl5 1)' );
    $discordClient->add_header('Authorization' => "$authToken");
    my $clientConnected = eval { $discordClient->get($apiUrl, 'Content
+-Type' => 'application/json'); };
    if (! $clientConnected ) { errorCon(); }
    else { # Set values when connection is successful
        our $apiContent = $discordClient->content;
        #print Dumper($apiContent) . "\n";
        my @apiJson = decode_json( $apiContent );
        print "Pretty Response:\n" . Dumper( @apiJson ) . "\n";
        my $msgID = $apiJson[0]->{'id'};
        my $msgAuthor = $apiJson[0]->{'author'}->{'username'};
        my $msgContent = $apiJson[0]->{'content'};
        
        if ($msgID > $lastMsgID) {
            print "<$msgAuthor> $msgContent\n";
            $lastMsgID = $msgID;
        }
    }
}

sub errorCon {
    print "Error: Unable to connect to Discord!\n";
}
[download]

The response is generally 50 messages long, however I limit it to 1 message for ease of parsing and debugging. Therefore, I am treating the response first as an array, and yes I have tried to foreach through it. After treating it as an array I do the normal syntax to retrieve the value of a key. Is there something I am missing?

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Re: Perplexing JSON decoding issue. by haukex (Archbishop) on Oct 06, 2016 at 09:24 UTC
Hi Miyako, I'll take a guess: If your JSON string looks like "`[ ... ]`", then `decode_json` should return an array reference, not an array as you seem to want with `my @apiJson = decode_json( $apiContent );`. Instead, you'll have to dereference that array reference with something like `my $apiJson = decode_json( $apiContent ); my $msgID = $apiJson->[0]->{'id'};` Hope this helps, -- Hauke D	[reply] [d/l] [select]
Re: Perplexing JSON decoding issue. by choroba (Cardinal) on Oct 06, 2016 at 09:23 UTC
Hallo Miyako, welcome to the Monastery! Your sample code doesn't have enough lines to die at line 70. Is Dumper the one found in Data::Dumper? If so, you should give it a reference to the variable you want it to print - otherwise, it prints the elements of the array, which means `$apiJson[0]` is an array reference, and `$apiJson[0][0]` is the hash reference you want to use. Or, don't use an array at all: `my $apiJson = decode_json( $apiContent ); print "Pretty Response:\n", Dumper( $apiJson ); my $msgID = $apiJson->[0]{id};` [download] ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l] [select]
Re: Perplexing JSON decoding issue. by Miyako (Initiate) on Oct 06, 2016 at 17:27 UTC
@choroba, it is the Dumper from Data::Dumper; I use it often when working with Json to see the entire response formatted nicely. Helps a lot to see what variables and arrays are listed. @haukex, that worked to fix it perfectly. Thanks for that. I'd have torn my hair out all day trying to figure out why it wasn't working when every other time I used Json it worked. I also noticed that it worked as $apiJson[0]->[0]->{'id'}; Not entirely sure what makes that different but I am using the way you wrote it as it is shorter by a few characters for the same result. My terminal now shows: `msgId: 233638292636041218 <MiyakoProductions> test3`	[reply] [d/l]
Re^2: Perplexing JSON decoding issue. by haukex (Archbishop) on Oct 06, 2016 at 17:58 UTC
Hi Miyako, I also noticed that it worked as $apiJson[0]->[0]->{'id'}; Not entirely sure what makes that different Compare these two: `my @apiJson = decode_json( $apiContent ); my $msgID = $apiJson[0]->[0]{id}; # or # my $apiJson = decode_json( $apiContent ); my $msgID = $apiJson->[0]{id};` [download] In the first, you're taking the return value of `decode_json`, which I believe is just a single value, an arrayref or hashref, storing it in an array `@apiJson` which now just has a single element, and accessing that element with `$apiJson[0]`. In the second version, you're taking that single return value and storing it in the scalar `$apiJson` directly. (As far as I can tell `decode_json` behaves the same in list and array context.) Hope this helps, -- Hauke D	[reply] [d/l] [select]