--- JSON-XS/XS.pm 2007/12/19 11:42:52 1.79 +++ JSON-XS/XS.pm 2008/03/19 03:17:38 1.86 @@ -1,3 +1,5 @@ +=encoding utf-8 + =head1 NAME JSON::XS - JSON serialising/deserialising, done correctly and fast @@ -60,15 +62,16 @@ =item * correct Unicode handling -This module knows how to handle Unicode, and even documents how and when -it does so. +This module knows how to handle Unicode, documents how and when it does +so, and even documents what "correct" means. =item * 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). +like a number). There minor I exceptions to this, read the MAPPING +section below to learn about those. =item * strict checking of JSON correctness @@ -78,17 +81,17 @@ =item * fast -Compared to other JSON modules, this module compares favourably in terms -of speed, too. +Compared to other JSON modules and other serialisers such as Storable, +this module usually compares favourably in terms of speed, too. =item * simple to use -This module has both a simple functional interface as well as an OO -interface. +This module has both a simple functional interface as well as an objetc +oriented interface interface. =item * reasonably versatile output formats -You can choose between the most compact guaranteed single-line format +You can choose between the most compact guaranteed-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 @@ -176,11 +179,11 @@ =item 2. Perl does I associate an encoding with your strings. -Unless you force it to, e.g. when matching it against a regex, or printing -the scalar to a file, in which case Perl either interprets your string as -locale-encoded text, octets/binary, or as Unicode, depending on various -settings. In no case is an encoding stored together with your data, it is -I that decides encoding, not any magical metadata. +... until you force it to, e.g. when matching it against a regex, or +printing the scalar to a file, in which case Perl either interprets your +string as locale-encoded text, octets/binary, or as Unicode, depending +on various settings. In no case is an encoding stored together with your +data, it is I that decides encoding, not any magical meta data. =item 3. The internal utf-8 flag has no meaning with regards to the encoding of your string. @@ -708,19 +711,19 @@ 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. +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. +precision. Otherwise it will preserve the number as a string value (in +which case you lose roundtripping ability, as the JSON number will be +re-encoded toa JSON string). 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. +precision (in which case you might lose perfect roundtripping ability, but +the JSON number will still be re-encoded as a JSON number). =item true, false @@ -776,16 +779,18 @@ =item 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. +Blessed objects are not directly representable in JSON. See the +C and C methods on various options on +how to deal with this: basically, you can choose between throwing an +exception, encoding the reference as if it weren't blessed, or provide +your own serialiser method. =item 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: +JSON C values, scalars that have last been used in a string context +before encoding as JSON strings, and anything else as number value: # dump as number encode_json [2] # yields [2] @@ -813,7 +818,106 @@ $x *= 1; # same thing, the choice is yours. You can not currently force the type in other, less obscure, ways. Tell me -if you need this capability. +if you need this capability (but don't forget to explain why its needed +:). + +=back + + +=head1 ENCODING/CODESET FLAG NOTES + +The interested reader might have seen a number of flags that signify +encodings or codesets - C, C and C. There seems to be +some confusion on what these do, so here is a short comparison: + +C controls wether the JSON text created by C (and expected +by C) is UTF-8 encoded or not, while C and C only +control wether C escapes character values outside their respective +codeset range. Neither of these flags conflict with each other, although +some combinations make less sense than others. + +Care has been taken to make all flags symmetrical with respect to +C and C, that is, texts encoded with any combination of +these flag values will be correctly decoded when the same flags are used +- in general, if you use different flag settings while encoding vs. when +decoding you likely have a bug somewhere. + +Below comes a verbose discussion of these flags. Note that a "codeset" is +simply an abstract set of character-codepoint pairs, while an encoding +takes those codepoint numbers and I them, in our case into +octets. Unicode is (among other things) a codeset, UTF-8 is an encoding, +and ISO-8859-1 (= latin 1) and ASCII are both codesets I encodings at +the same time, which can be confusing. + +=over 4 + +=item C flag disabled + +When C is disabled (the default), then C/C generate +and expect Unicode strings, that is, characters with high ordinal Unicode +values (> 255) will be encoded as such characters, and likewise such +characters are decoded as-is, no canges to them will be done, except +"(re-)interpreting" them as Unicode codepoints or Unicode characters, +respectively (to Perl, these are the same thing in strings unless you do +funny/weird/dumb stuff). + +This is useful when you want to do the encoding yourself (e.g. when you +want to have UTF-16 encoded JSON texts) or when some other layer does +the encoding for you (for example, when printing to a terminal using a +filehandle that transparently encodes to UTF-8 you certainly do NOT want +to UTF-8 encode your data first and have Perl encode it another time). + +=item C flag enabled + +If the C-flag is enabled, C/C will encode all +characters using the corresponding UTF-8 multi-byte sequence, and will +expect your input strings to be encoded as UTF-8, that is, no "character" +of the input string must have any value > 255, as UTF-8 does not allow +that. + +The C flag therefore switches between two modes: disabled means you +will get a Unicode string in Perl, enabled means you get an UTF-8 encoded +octet/binary string in Perl. + +=item C or C flags enabled + +With C (or C) enabled, C will escape characters +with ordinal values > 255 (> 127 with C) and encode the remaining +characters as specified by the C flag. + +If C is disabled, then the result is also correctly encoded in those +character sets (as both are proper subsets of Unicode, meaning that a +Unicode string with all character values < 256 is the same thing as a +ISO-8859-1 string, and a Unicode string with all character values < 128 is +the same thing as an ASCII string in Perl). + +If C is enabled, you still get a correct UTF-8-encoded string, +regardless of these flags, just some more characters will be escaped using +C<\uXXXX> then before. + +Note that ISO-8859-1-I strings are not compatible with UTF-8 +encoding, while ASCII-encoded strings are. That is because the ISO-8859-1 +encoding is NOT a subset of UTF-8 (despite the ISO-8859-1 I being +a subset of Unicode), while ASCII is. + +Surprisingly, C will ignore these flags and so treat all input +values as governed by the C flag. If it is disabled, this allows you +to decode ISO-8859-1- and ASCII-encoded strings, as both strict subsets of +Unicode. If it is enabled, you can correctly decode UTF-8 encoded strings. + +So neither C nor C are incompatible with the C flag - +they only govern when the JSON output engine escapes a character or not. + +The main use for C is to relatively efficiently store binary data +as JSON, at the expense of breaking compatibility with most JSON decoders. + +The main use for C is to force the output to not contain characters +with values > 127, which means you can interpret the resulting string +as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about any character set and +8-bit-encoding, and still get the same data structure back. This is useful +when your channel for JSON transfer is not 8-bit clean or the encoding +might be mangled in between (e.g. in mail), and works because ASCII is a +proper subset of most 8-bit and multibyte encodings in use in the world. =back @@ -828,6 +932,17 @@ =over 4 +=item JSON 2.xx + +A marvellous piece of engineering, this module either uses JSON::XS +directly when available (so will be 100% compatible with it, including +speed), or it uses JSON::PP, which is basically JSON::XS translated to +Pure Perl, which should be 100% compatible with JSON::XS, just a bit +slower. + +You cannot really lose by using this module, especially as it tries very +hard to work even with ancient Perl versions, while JSON::XS does not. + =item JSON 1.07 Slow (but very portable, as it is written in pure Perl). @@ -907,9 +1022,10 @@ =head2 JSON and YAML -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. +You often hear that JSON is a 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 that works for +all cases. If you really must use JSON::XS to generate YAML, you should use this algorithm (subject to change in future versions): @@ -917,15 +1033,47 @@ 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 +This will I 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 noticeably shorter than the 1024 characters YAML allows. +lengths that JSON doesn't have and also has different and incompatible +unicode handling, so you should make sure that your hash keys are +noticeably shorter than the 1024 "stream characters" YAML allows and that +you do not have codepoints with values outside the Unicode BMP (basic +multilingual page). YAML also does not allow C<\/> sequences in strings +(which JSON::XS does not I generate). + +There might be other incompatibilities that I am not aware of (or the YAML +specification has been changed yet again - it does so quite often). 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 problems when you +least expect it. + +=over 4 -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 problems. +=item (*) + +This is spread actively by the YAML team, however. For many years now they +claim YAML were a superset of JSON, even when proven otherwise. + +Even the author of this manpage was at some point accused of providing +"incorrect" information, despite the evidence presented (claims ranged +from "your documentation contains inaccurate and negative statements about +YAML" (the only negative comment is this footnote, and it didn't exist +back then; the question on which claims were inaccurate was never answered +etc.) to "the YAML spec is not up-to-date" (the *real* and supposedly +JSON-compatible spec is apparently not currently publicly available) +to actual requests to replace this section by *incorrect* information, +suppressing information about the real problem). + +So whenever you are told that YAML was a superset of JSON, first check +wether it is really true (it might be when you check it, but it certainly +was not true when this was written). I would much prefer if the YAML team +would spent their time on actually making JSON compatibility a truth +(JSON, after all, has a very small and simple specification) instead of +trying to lobby/force people into reporting untruths. + +=back =head2 SPEED @@ -1017,9 +1165,13 @@ has a smaller stack, you should adjust this setting accordingly with the C 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... +Something else could bomb you, too, that I forgot to think of. In that +case, you get to keep the pieces. I am always open for hints, though... + +Also keep in mind that JSON::XS might leak contents of your Perl data +structures in its error messages, so when you serialise sensitive +information you might want to make sure that exceptions thrown by JSON::XS +will not end up in front of untrusted eyes. If you are using JSON::XS to return packets to consumption by JavaScript scripts in a browser you should have a look at