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    JSON::XS - JSON serialising/deserialising, done correctly and fast

     use JSON::XS;

     # exported functions, they croak on error
     # and expect/generate UTF-8

     $utf8_encoded_json_text = to_json $perl_hash_or_arrayref;
     $perl_hash_or_arrayref  = from_json $utf8_encoded_json_text;

     # OO-interface

     $coder = JSON::XS->new->ascii->pretty->allow_nonref;
     $pretty_printed_unencoded = $coder->encode ($perl_scalar);
     $perl_scalar = $coder->decode ($unicode_json_text);

    This module converts Perl data structures to JSON and vice versa. Its
    primary goal is to be *correct* and its secondary goal is to be *fast*.
    To reach the latter goal it was written in C.

    As this is the n-th-something JSON module on CPAN, what was the reason
    to write yet another JSON module? While it seems there are many JSON
    modules, none of them correctly handle all corner cases, and in most
    cases their maintainers are unresponsive, gone missing, or not listening
    to bug reports for other reasons.

    See COMPARISON, below, for a comparison to some other JSON modules.

    See MAPPING, below, on how JSON::XS maps perl values to JSON values and
    vice versa.

    * correct unicode handling
        This module knows how to handle Unicode, and even documents how and
        when it does so.

    * round-trip integrity
        When you serialise a perl data structure using only datatypes
        supported by JSON, the deserialised data structure is identical on
        the Perl level. (e.g. the string "2.0" doesn't suddenly become "2"
        just because it looks like a number).

    * strict checking of JSON correctness
        There is no guessing, no generating of illegal JSON texts by
        default, and only JSON is accepted as input by default (the latter
        is a security feature).

    * fast
        Compared to other JSON modules, this module compares favourably in
        terms of speed, too.

    * simple to use
        This module has both a simple functional interface as well as an OO

    * reasonably versatile output formats
        You can choose between the most compact guarenteed single-line
        format possible (nice for simple line-based protocols), a pure-ascii
        format (for when your transport is not 8-bit clean, still supports
        the whole unicode range), or a pretty-printed format (for when you
        want to read that stuff). Or you can combine those features in
        whatever way you like.

    The following convinience methods are provided by this module. They are
    exported by default:

    $json_text = to_json $perl_scalar
        Converts the given Perl data structure (a simple scalar or a
        reference to a hash or array) to a UTF-8 encoded, binary string
        (that is, the string contains octets only). Croaks on error.

        This function call is functionally identical to:

           $json_text = JSON::XS->new->utf8->encode ($perl_scalar)

        except being faster.

    $perl_scalar = from_json $json_text
        The opposite of "to_json": expects an UTF-8 (binary) string and
        tries to parse that as an UTF-8 encoded JSON text, returning the
        resulting simple scalar or reference. Croaks on error.

        This function call is functionally identical to:

           $perl_scalar = JSON::XS->new->utf8->decode ($json_text)

        except being faster.

    $is_boolean = JSON::XS::is_bool $scalar
        Returns true if the passed scalar represents either JSON::XS::true
        or JSON::XS::false, two constants that act like 1 and 0,
        respectively and are used to represent JSON "true" and "false"
        values in Perl.

        See MAPPING, below, for more information on how JSON values are
        mapped to Perl.

    The object oriented interface lets you configure your own encoding or
    decoding style, within the limits of supported formats.

    $json = new JSON::XS
        Creates a new JSON::XS object that can be used to de/encode JSON
        strings. All boolean flags described below are by default

        The mutators for flags all return the JSON object again and thus
        calls can be chained:

           my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
           => {"a": [1, 2]}

    $json = $json->ascii ([$enable])
        If $enable is true (or missing), then the "encode" method will not
        generate characters outside the code range 0..127 (which is ASCII).
        Any unicode characters outside that range will be escaped using
        either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
        escape sequence, as per RFC4627. The resulting encoded JSON text can
        be treated as a native unicode string, an ascii-encoded,
        latin1-encoded or UTF-8 encoded string, or any other superset of

        If $enable is false, then the "encode" method will not escape
        Unicode characters unless required by the JSON syntax or other
        flags. This results in a faster and more compact format.

        The main use for this flag is to produce JSON texts that can be
        transmitted over a 7-bit channel, as the encoded JSON texts will not
        contain any 8 bit characters.

          JSON::XS->new->ascii (1)->encode ([chr 0x10401])
          => ["\ud801\udc01"]

    $json = $json->latin1 ([$enable])
        If $enable is true (or missing), then the "encode" method will
        encode the resulting JSON text as latin1 (or iso-8859-1), escaping
        any characters outside the code range 0..255. The resulting string
        can be treated as a latin1-encoded JSON text or a native unicode
        string. The "decode" method will not be affected in any way by this
        flag, as "decode" by default expects unicode, which is a strict
        superset of latin1.

        If $enable is false, then the "encode" method will not escape
        Unicode characters unless required by the JSON syntax or other

        The main use for this flag is efficiently encoding binary data as
        JSON text, as most octets will not be escaped, resulting in a
        smaller encoded size. The disadvantage is that the resulting JSON
        text is encoded in latin1 (and must correctly be treated as such
        when storing and transfering), a rare encoding for JSON. It is
        therefore most useful when you want to store data structures known
        to contain binary data efficiently in files or databases, not when
        talking to other JSON encoders/decoders.

          JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
          => ["\x{89}\\u0abc"]    # (perl syntax, U+abc escaped, U+89 not)

    $json = $json->utf8 ([$enable])
        If $enable is true (or missing), then the "encode" method will
        encode the JSON result into UTF-8, as required by many protocols,
        while the "decode" method expects to be handled an UTF-8-encoded
        string. Please note that UTF-8-encoded strings do not contain any
        characters outside the range 0..255, they are thus useful for
        bytewise/binary I/O. In future versions, enabling this option might
        enable autodetection of the UTF-16 and UTF-32 encoding families, as
        described in RFC4627.

        If $enable is false, then the "encode" method will return the JSON
        string as a (non-encoded) unicode string, while "decode" expects
        thus a unicode string. Any decoding or encoding (e.g. to UTF-8 or
        UTF-16) needs to be done yourself, e.g. using the Encode module.

        Example, output UTF-16BE-encoded JSON:

          use Encode;
          $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);

        Example, decode UTF-32LE-encoded JSON:

          use Encode;
          $object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);

    $json = $json->pretty ([$enable])
        This enables (or disables) all of the "indent", "space_before" and
        "space_after" (and in the future possibly more) flags in one call to
        generate the most readable (or most compact) form possible.

        Example, pretty-print some simple structure:

           my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
              "a" : [

    $json = $json->indent ([$enable])
        If $enable is true (or missing), then the "encode" method will use a
        multiline format as output, putting every array member or
        object/hash key-value pair into its own line, identing them

        If $enable is false, no newlines or indenting will be produced, and
        the resulting JSON text is guarenteed not to contain any "newlines".

        This setting has no effect when decoding JSON texts.

    $json = $json->space_before ([$enable])
        If $enable is true (or missing), then the "encode" method will add
        an extra optional space before the ":" separating keys from values
        in JSON objects.

        If $enable is false, then the "encode" method will not add any extra
        space at those places.

        This setting has no effect when decoding JSON texts. You will also
        most likely combine this setting with "space_after".

        Example, space_before enabled, space_after and indent disabled:

           {"key" :"value"}

    $json = $json->space_after ([$enable])
        If $enable is true (or missing), then the "encode" method will add
        an extra optional space after the ":" separating keys from values in
        JSON objects and extra whitespace after the "," separating key-value
        pairs and array members.

        If $enable is false, then the "encode" method will not add any extra
        space at those places.

        This setting has no effect when decoding JSON texts.

        Example, space_before and indent disabled, space_after enabled:

           {"key": "value"}

    $json = $json->relaxed ([$enable])
        If $enable is true (or missing), then "decode" will accept some
        extensions to normal JSON syntax (see below). "encode" will not be
        affected in anyway. *Be aware that this option makes you accept
        invalid JSON texts as if they were valid!*. I suggest only to use
        this option to parse application-specific files written by humans
        (configuration files, resource files etc.)

        If $enable is false (the default), then "decode" will only accept
        valid JSON texts.

        Currently accepted extensions are:

        * list items can have an end-comma
            JSON *separates* array elements and key-value pairs with commas.
            This can be annoying if you write JSON texts manually and want
            to be able to quickly append elements, so this extension accepts
            comma at the end of such items not just between them:

                  2, <- this comma not normally allowed
                  "k1": "v1",
                  "k2": "v2", <- this comma not normally allowed

        * shell-style '#'-comments
            Whenever JSON allows whitespace, shell-style comments are
            additionally allowed. They are terminated by the first
            carriage-return or line-feed character, after which more
            white-space and comments are allowed.

                 1, # this comment not allowed in JSON
                    # neither this one...

    $json = $json->canonical ([$enable])
        If $enable is true (or missing), then the "encode" method will
        output JSON objects by sorting their keys. This is adding a
        comparatively high overhead.

        If $enable is false, then the "encode" method will output key-value
        pairs in the order Perl stores them (which will likely change
        between runs of the same script).

        This option is useful if you want the same data structure to be
        encoded as the same JSON text (given the same overall settings). If
        it is disabled, the same hash migh be encoded differently even if
        contains the same data, as key-value pairs have no inherent ordering
        in Perl.

        This setting has no effect when decoding JSON texts.

    $json = $json->allow_nonref ([$enable])
        If $enable is true (or missing), then the "encode" method can
        convert a non-reference into its corresponding string, number or
        null JSON value, which is an extension to RFC4627. Likewise,
        "decode" will accept those JSON values instead of croaking.

        If $enable is false, then the "encode" method will croak if it isn't
        passed an arrayref or hashref, as JSON texts must either be an
        object or array. Likewise, "decode" will croak if given something
        that is not a JSON object or array.

        Example, encode a Perl scalar as JSON value with enabled
        "allow_nonref", resulting in an invalid JSON text:

           JSON::XS->new->allow_nonref->encode ("Hello, World!")
           => "Hello, World!"

    $json = $json->allow_blessed ([$enable])
        If $enable is true (or missing), then the "encode" method will not
        barf when it encounters a blessed reference. Instead, the value of
        the convert_blessed option will decide wether "null"
        ("convert_blessed" disabled or no "to_json" method found) or a
        representation of the object ("convert_blessed" enabled and
        "to_json" method found) is being encoded. Has no effect on "decode".

        If $enable is false (the default), then "encode" will throw an
        exception when it encounters a blessed object.

    $json = $json->convert_blessed ([$enable])
        If $enable is true (or missing), then "encode", upon encountering a
        blessed object, will check for the availability of the "TO_JSON"
        method on the object's class. If found, it will be called in scalar
        context and the resulting scalar will be encoded instead of the
        object. If no "TO_JSON" method is found, the value of
        "allow_blessed" will decide what to do.

        The "TO_JSON" method may safely call die if it wants. If "TO_JSON"
        returns other blessed objects, those will be handled in the same
        way. "TO_JSON" must take care of not causing an endless recursion
        cycle (== crash) in this case. The name of "TO_JSON" was chosen
        because other methods called by the Perl core (== not by the user of
        the object) are usually in upper case letters and to avoid
        collisions with the "to_json" function.

        This setting does not yet influence "decode" in any way, but in the
        future, global hooks might get installed that influence "decode" and
        are enabled by this setting.

        If $enable is false, then the "allow_blessed" setting will decide
        what to do when a blessed object is found.

    $json = $json->filter_json_object ([$coderef->($hashref)])
        When $coderef is specified, it will be called from "decode" each
        time it decodes a JSON object. The only argument is a reference to
        the newly-created hash. If the code references returns a single
        scalar (which need not be a reference), this value (i.e. a copy of
        that scalar to avoid aliasing) is inserted into the deserialised
        data structure. If it returns an empty list (NOTE: *not* "undef",
        which is a valid scalar), the original deserialised hash will be
        inserted. This setting can slow down decoding considerably.

        When $coderef is omitted or undefined, any existing callback will be
        removed and "decode" will not change the deserialised hash in any

        Example, convert all JSON objects into the integer 5:

           my $js = JSON::XS->new->filter_json_object (sub { 5 });
           # returns [5]
           $js->decode ('[{}]')
           # throw an exception because allow_nonref is not enabled
           # so a lone 5 is not allowed.
           $js->decode ('{"a":1, "b":2}');

    $json = $json->filter_json_single_key_object ($key [=>
        Works remotely similar to "filter_json_object", but is only called
        for JSON objects having a single key named $key.

        This $coderef is called before the one specified via
        "filter_json_object", if any. It gets passed the single value in the
        JSON object. If it returns a single value, it will be inserted into
        the data structure. If it returns nothing (not even "undef" but the
        empty list), the callback from "filter_json_object" will be called
        next, as if no single-key callback were specified.

        If $coderef is omitted or undefined, the corresponding callback will
        be disabled. There can only ever be one callback for a given key.

        As this callback gets called less often then the
        "filter_json_object" one, decoding speed will not usually suffer as
        much. Therefore, single-key objects make excellent targets to
        serialise Perl objects into, especially as single-key JSON objects
        are as close to the type-tagged value concept as JSON gets (its
        basically an ID/VALUE tuple). Of course, JSON does not support this
        in any way, so you need to make sure your data never looks like a
        serialised Perl hash.

        Typical names for the single object key are "__class_whatever__", or
        "$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even
        things like "__class_md5sum(classname)__", to reduce the risk of
        clashing with real hashes.

        Example, decode JSON objects of the form "{ "__widget__" => <id> }"
        into the corresponding $WIDGET{<id>} object:

           # return whatever is in $WIDGET{5}:
              ->filter_json_single_key_object (__widget__ => sub {
                    $WIDGET{ $_[0] }
              ->decode ('{"__widget__": 5')

           # this can be used with a TO_JSON method in some "widget" class
           # for serialisation to json:
           sub WidgetBase::TO_JSON {
              my ($self) = @_;

              unless ($self->{id}) {
                 $self->{id} =;
                 $WIDGET{$self->{id}} = $self;

              { __widget__ => $self->{id} }

    $json = $json->shrink ([$enable])
        Perl usually over-allocates memory a bit when allocating space for
        strings. This flag optionally resizes strings generated by either
        "encode" or "decode" to their minimum size possible. This can save
        memory when your JSON texts are either very very long or you have
        many short strings. It will also try to downgrade any strings to
        octet-form if possible: perl stores strings internally either in an
        encoding called UTF-X or in octet-form. The latter cannot store
        everything but uses less space in general (and some buggy Perl or C
        code might even rely on that internal representation being used).

        The actual definition of what shrink does might change in future
        versions, but it will always try to save space at the expense of

        If $enable is true (or missing), the string returned by "encode"
        will be shrunk-to-fit, while all strings generated by "decode" will
        also be shrunk-to-fit.

        If $enable is false, then the normal perl allocation algorithms are
        used. If you work with your data, then this is likely to be faster.

        In the future, this setting might control other things, such as
        converting strings that look like integers or floats into integers
        or floats internally (there is no difference on the Perl level),
        saving space.

    $json = $json->max_depth ([$maximum_nesting_depth])
        Sets the maximum nesting level (default 512) accepted while encoding
        or decoding. If the JSON text or Perl data structure has an equal or
        higher nesting level then this limit, then the encoder and decoder
        will stop and croak at that point.

        Nesting level is defined by number of hash- or arrayrefs that the
        encoder needs to traverse to reach a given point or the number of
        "{" or "[" characters without their matching closing parenthesis
        crossed to reach a given character in a string.

        Setting the maximum depth to one disallows any nesting, so that
        ensures that the object is only a single hash/object or array.

        The argument to "max_depth" will be rounded up to the next highest
        power of two. If no argument is given, the highest possible setting
        will be used, which is rarely useful.

        See SECURITY CONSIDERATIONS, below, for more info on why this is

    $json = $json->max_size ([$maximum_string_size])
        Set the maximum length a JSON text may have (in bytes) where
        decoding is being attempted. The default is 0, meaning no limit.
        When "decode" is called on a string longer then this number of
        characters it will not attempt to decode the string but throw an
        exception. This setting has no effect on "encode" (yet).

        The argument to "max_size" will be rounded up to the next highest
        power of two (so may be more than requested). If no argument is
        given, the limit check will be deactivated (same as when 0 is

        See SECURITY CONSIDERATIONS, below, for more info on why this is

    $json_text = $json->encode ($perl_scalar)
        Converts the given Perl data structure (a simple scalar or a
        reference to a hash or array) to its JSON representation. Simple
        scalars will be converted into JSON string or number sequences,
        while references to arrays become JSON arrays and references to
        hashes become JSON objects. Undefined Perl values (e.g. "undef")
        become JSON "null" values. Neither "true" nor "false" values will be

    $perl_scalar = $json->decode ($json_text)
        The opposite of "encode": expects a JSON text and tries to parse it,
        returning the resulting simple scalar or reference. Croaks on error.

        JSON numbers and strings become simple Perl scalars. JSON arrays
        become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
        becomes 1, "false" becomes 0 and "null" becomes "undef".

    ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
        This works like the "decode" method, but instead of raising an
        exception when there is trailing garbage after the first JSON
        object, it will silently stop parsing there and return the number of
        characters consumed so far.

        This is useful if your JSON texts are not delimited by an outer
        protocol (which is not the brightest thing to do in the first place)
        and you need to know where the JSON text ends.

           JSON::XS->new->decode_prefix ("[1] the tail")
           => ([], 3)

    This section describes how JSON::XS maps Perl values to JSON values and
    vice versa. These mappings are designed to "do the right thing" in most
    circumstances automatically, preserving round-tripping characteristics
    (what you put in comes out as something equivalent).

    For the more enlightened: note that in the following descriptions,
    lowercase *perl* refers to the Perl interpreter, while uppcercase *Perl*
    refers to the abstract Perl language itself.

        A JSON object becomes a reference to a hash in Perl. No ordering of
        object keys is preserved (JSON does not preserver object key
        ordering itself).

        A JSON array becomes a reference to an array in Perl.

        A JSON string becomes a string scalar in Perl - Unicode codepoints
        in JSON are represented by the same codepoints in the Perl string,
        so no manual decoding is necessary.

        A JSON number becomes either an integer, numeric (floating point) or
        string scalar in perl, depending on its range and any fractional
        parts. On the Perl level, there is no difference between those as
        Perl handles all the conversion details, but an integer may take
        slightly less memory and might represent more values exactly than
        (floating point) numbers.

        If the number consists of digits only, JSON::XS will try to
        represent it as an integer value. If that fails, it will try to
        represent it as a numeric (floating point) value if that is possible
        without loss of precision. Otherwise it will preserve the number as
        a string value.

        Numbers containing a fractional or exponential part will always be
        represented as numeric (floating point) values, possibly at a loss
        of precision.

        This might create round-tripping problems as numbers might become
        strings, but as Perl is typeless there is no other way to do it.

    true, false
        These JSON atoms become "JSON::XS::true" and "JSON::XS::false",
        respectively. They are overloaded to act almost exactly like the
        numbers 1 and 0. You can check wether a scalar is a JSON boolean by
        using the "JSON::XS::is_bool" function.

        A JSON null atom becomes "undef" in Perl.

    The mapping from Perl to JSON is slightly more difficult, as Perl is a
    truly typeless language, so we can only guess which JSON type is meant
    by a Perl value.

    hash references
        Perl hash references become JSON objects. As there is no inherent
        ordering in hash keys (or JSON objects), they will usually be
        encoded in a pseudo-random order that can change between runs of the
        same program but stays generally the same within a single run of a
        program. JSON::XS can optionally sort the hash keys (determined by
        the *canonical* flag), so the same datastructure will serialise to
        the same JSON text (given same settings and version of JSON::XS),
        but this incurs a runtime overhead and is only rarely useful, e.g.
        when you want to compare some JSON text against another for

    array references
        Perl array references become JSON arrays.

    other references
        Other unblessed references are generally not allowed and will cause
        an exception to be thrown, except for references to the integers 0
        and 1, which get turned into "false" and "true" atoms in JSON. You
        can also use "JSON::XS::false" and "JSON::XS::true" to improve

           to_json [\0,JSON::XS::true]      # yields [false,true]

    JSON::XS::true, JSON::XS::false
        These special values become JSON true and JSON false values,
        respectively. You cna alos use "\1" and "\0" directly if you want.

    blessed objects
        Blessed objects are not allowed. JSON::XS currently tries to encode
        their underlying representation (hash- or arrayref), but this
        behaviour might change in future versions.

    simple scalars
        Simple Perl scalars (any scalar that is not a reference) are the
        most difficult objects to encode: JSON::XS will encode undefined
        scalars as JSON null value, scalars that have last been used in a
        string context before encoding as JSON strings and anything else as
        number value:

           # dump as number
           to_json [2]                      # yields [2]
           to_json [-3.0e17]                # yields [-3e+17]
           my $value = 5; to_json [$value]  # yields [5]

           # used as string, so dump as string
           print $value;
           to_json [$value]                 # yields ["5"]

           # undef becomes null
           to_json [undef]                  # yields [null]

        You can force the type to be a string by stringifying it:

           my $x = 3.1; # some variable containing a number
           "$x";        # stringified
           $x .= "";    # another, more awkward way to stringify
           print $x;    # perl does it for you, too, quite often

        You can force the type to be a number by numifying it:

           my $x = "3"; # some variable containing a string
           $x += 0;     # numify it, ensuring it will be dumped as a number
           $x *= 1;     # same thing, the choise is yours.

        You can not currently output JSON booleans or force the type in
        other, less obscure, ways. Tell me if you need this capability.

    As already mentioned, this module was created because none of the
    existing JSON modules could be made to work correctly. First I will
    describe the problems (or pleasures) I encountered with various existing
    JSON modules, followed by some benchmark values. JSON::XS was designed
    not to suffer from any of these problems or limitations.

    JSON 1.07
        Slow (but very portable, as it is written in pure Perl).

        Undocumented/buggy Unicode handling (how JSON handles unicode values
        is undocumented. One can get far by feeding it unicode strings and
        doing en-/decoding oneself, but unicode escapes are not working

        No roundtripping (strings get clobbered if they look like numbers,
        e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that
        will decode into the number 2.

    JSON::PC 0.01
        Very fast.

        Undocumented/buggy Unicode handling.

        No roundtripping.

        Has problems handling many Perl values (e.g. regex results and other
        magic values will make it croak).

        Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}"
        which is not a valid JSON text.

        Unmaintained (maintainer unresponsive for many months, bugs are not
        getting fixed).

    JSON::Syck 0.21
        Very buggy (often crashes).

        Very inflexible (no human-readable format supported, format pretty
        much undocumented. I need at least a format for easy reading by
        humans and a single-line compact format for use in a protocol, and
        preferably a way to generate ASCII-only JSON texts).

        Completely broken (and confusingly documented) Unicode handling
        (unicode escapes are not working properly, you need to set
        ImplicitUnicode to *different* values on en- and decoding to get
        symmetric behaviour).

        No roundtripping (simple cases work, but this depends on wether the
        scalar value was used in a numeric context or not).

        Dumping hashes may skip hash values depending on iterator state.

        Unmaintained (maintainer unresponsive for many months, bugs are not
        getting fixed).

        Does not check input for validity (i.e. will accept non-JSON input
        and return "something" instead of raising an exception. This is a
        security issue: imagine two banks transfering money between each
        other using JSON. One bank might parse a given non-JSON request and
        deduct money, while the other might reject the transaction with a
        syntax error. While a good protocol will at least recover, that is
        extra unnecessary work and the transaction will still not succeed).

    JSON::DWIW 0.04
        Very fast. Very natural. Very nice.

        Undocumented unicode handling (but the best of the pack. Unicode
        escapes still don't get parsed properly).

        Very inflexible.

        No roundtripping.

        Does not generate valid JSON texts (key strings are often unquoted,
        empty keys result in nothing being output)

        Does not check input for validity.

    You often hear that JSON is a subset (or a close subset) of YAML. This
    is, however, a mass hysteria and very far from the truth. In general,
    there is no way to configure JSON::XS to output a data structure as
    valid YAML.

    If you really must use JSON::XS to generate YAML, you should use this
    algorithm (subject to change in future versions):

       my $to_yaml = JSON::XS->new->utf8->space_after (1);
       my $yaml = $to_yaml->encode ($ref) . "\n";

    This will usually generate JSON texts that also parse as valid YAML.
    Please note that YAML has hardcoded limits on (simple) object key
    lengths that JSON doesn't have, so you should make sure that your hash
    keys are noticably shorter than the 1024 characters YAML allows.

    There might be other incompatibilities that I am not aware of. In
    general you should not try to generate YAML with a JSON generator or
    vice versa, or try to parse JSON with a YAML parser or vice versa:
    chances are high that you will run into severe interoperability

    It seems that JSON::XS is surprisingly fast, as shown in the following
    tables. They have been generated with the help of the "eg/bench" program
    in the JSON::XS distribution, to make it easy to compare on your own

    First comes a comparison between various modules using a very short
    single-line JSON string:

       {"method": "handleMessage", "params": ["user1", "we were just talking"], \
       "id": null, "array":[1,11,234,-5,1e5,1e7, true,  false]}

    It shows the number of encodes/decodes per second (JSON::XS uses the
    functional interface, while JSON::XS/2 uses the OO interface with
    pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink).
    Higher is better:

       Storable   |  15779.925 |  14169.946 |
       module     |     encode |     decode |
       JSON       |   4990.842 |   4088.813 |
       JSON::DWIW |  51653.990 |  71575.154 |
       JSON::PC   |  65948.176 |  74631.744 |
       JSON::PP   |   8931.652 |   3817.168 |
       JSON::Syck |  24877.248 |  27776.848 |
       JSON::XS   | 388361.481 | 227951.304 |
       JSON::XS/2 | 227951.304 | 218453.333 |
       JSON::XS/3 | 338250.323 | 218453.333 |
       Storable   |  16500.016 | 135300.129 |

    That is, JSON::XS is about five times faster than JSON::DWIW on
    encoding, about three times faster on decoding, and over fourty times
    faster than JSON, even with pretty-printing and key sorting. It also
    compares favourably to Storable for small amounts of data.

    Using a longer test string (roughly 18KB, generated from Yahoo! Locals
    search API (

       module     |     encode |     decode |
       JSON       |     55.260 |     34.971 |
       JSON::DWIW |    825.228 |   1082.513 |
       JSON::PC   |   3571.444 |   2394.829 |
       JSON::PP   |    210.987 |     32.574 |
       JSON::Syck |    552.551 |    787.544 |
       JSON::XS   |   5780.463 |   4854.519 |
       JSON::XS/2 |   3869.998 |   4798.975 |
       JSON::XS/3 |   5862.880 |   4798.975 |
       Storable   |   4445.002 |   5235.027 |

    Again, JSON::XS leads by far (except for Storable which non-surprisingly
    decodes faster).

    On large strings containing lots of high unicode characters, some
    modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
    result will be broken due to missing (or wrong) unicode handling. Others
    refuse to decode or encode properly, so it was impossible to prepare a
    fair comparison table for that case.

    When you are using JSON in a protocol, talking to untrusted potentially
    hostile creatures requires relatively few measures.

    First of all, your JSON decoder should be secure, that is, should not
    have any buffer overflows. Obviously, this module should ensure that and
    I am trying hard on making that true, but you never know.

    Second, you need to avoid resource-starving attacks. That means you
    should limit the size of JSON texts you accept, or make sure then when
    your resources run out, thats just fine (e.g. by using a separate
    process that can crash safely). The size of a JSON text in octets or
    characters is usually a good indication of the size of the resources
    required to decode it into a Perl structure. While JSON::XS can check
    the size of the JSON text, it might be too late when you already have it
    in memory, so you might want to check the size before you accept the

    Third, JSON::XS recurses using the C stack when decoding objects and
    arrays. The C stack is a limited resource: for instance, on my amd64
    machine with 8MB of stack size I can decode around 180k nested arrays
    but only 14k nested JSON objects (due to perl itself recursing deeply on
    croak to free the temporary). If that is exceeded, the program crashes.
    to be conservative, the default nesting limit is set to 512. If your
    process has a smaller stack, you should adjust this setting accordingly
    with the "max_depth" method.

    And last but least, something else could bomb you that I forgot to think
    of. In that case, you get to keep the pieces. I am always open for
    hints, though...

    If you are using JSON::XS to return packets to consumption by javascript
    scripts in a browser you should have a look at
    <> to see wether
    you are vulnerable to some common attack vectors (which really are
    browser design bugs, but it is still you who will have to deal with it,
    as major browser developers care only for features, not about doing
    security right).

    While the goal of this module is to be correct, that unfortunately does
    not mean its bug-free, only that I think its design is bug-free. It is
    still relatively early in its development. If you keep reporting bugs
    they will be fixed swiftly, though.

     Marc Lehmann <>

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