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jsx/README.md
alisdair sullivan 04bd9dd827 interstitial README
2013-11-03 23:19:26 +00:00

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# jsx (v2.0) #
an erlang application for consuming, producing and manipulating [json][json].
inspired by [yajl][yajl]
jsx is built via [rebar][rebar] and continuous integration testing provided courtesy [travis][travis]
current status: [![Build Status](https://secure.travis-ci.org/talentdeficit/jsx.png?branch=develop)](http://travis-ci.org/talentdeficit/jsx)
jsx is released under the terms of the [MIT][MIT] license
copyright 2010-2013 alisdair sullivan
## index ##
* [quickstart](#quickstart)
* [description](#description)
- [json <-> erlang mapping](#json---erlang-mapping)
- [incomplete input](#incomplete-input)
* [data types](#data-types)
- [`json_term()`](#json_term)
- [`json_text()`](#json_text)
- [`event()`](#event)
- [`token()`](#token)
- [`option()`](#option)
* [exports](#exports)
- [`encoder/3`, `decoder/3` & `parser/3`](#encoder3-decoder3--parser3)
- [`decode/1,2`](#decode12)
- [`encode/1,2`](#encode12)
- [`format/1,2`](#format12)
- [`minify/1`](#minify1)
- [`prettify/1`](#prettify1)
- [`is_json/1,2`](#is_json12)
- [`is_term/1,2`](#is_term12)
* [callback exports](#callback_exports)
- [`Module:init/1`](#moduleinit1)
- [`Module:handle_event/2`](#modulehandle_event2)
* [acknowledgements](#acknowledgements)
## quickstart ##
#### to build the library and run tests ####
```bash
$ rebar compile
$ rebar eunit
```
or, to build using hipe
```bash
$ rebar -C hipe.cfg compile
$ rebar -C hipe.cfg eunit
```
#### to convert a utf8 binary containing a json string into an erlang term ####
```erlang
1> jsx:decode(<<"{\"library\": \"jsx\", \"awesome\": true}">>).
[{<<"library">>,<<"jsx">>},{<<"awesome">>,true}]
2> jsx:decode(<<"[\"a\",\"list\",\"of\",\"words\"]">>).
[<<"a">>, <<"list">>, <<"of">>, <<"words">>]
```
#### to convert an erlang term into a utf8 binary containing a json string ####
```erlang
1> jsx:encode([{<<"library">>,<<"jsx">>},{<<"awesome">>,true}]).
<<"{\"library\": \"jsx\", \"awesome\": true}">>
2> jsx:encode([<<"a">>, <<"list">>, <<"of">>, <<"words">>]).
<<"[\"a\",\"list\",\"of\",\"words\"]">>
```
#### to check if a binary or a term is valid json ####
```erlang
1> jsx:is_json(<<"[\"this is json\"]">>).
true
2> jsx:is_json("[\"this is not\"]").
false
3> jsx:is_term([<<"this is a term">>]).
true
4> jsx:is_term([this, is, not]).
false
```
#### to minify some json ####
```erlang
1> jsx:minify(<<"{
\"a list\": [
1,
2,
3
]
}">>).
<<"{\"a list\":[1,2,3]}">>
```
#### to prettify some json ####
```erlang
1> jsx:prettify(<<"{\"a list\":[1,2,3]}">>).
<<"{
\"a list\": [
1,
2,
3
]
}">>
```
## description ##
jsx is an erlang application for consuming, producing and manipulating
[json][json]
jsx follows the json [spec][rfc4627] as closely as possible with allowances for
real world usage
jsx is pragmatic. the json spec allows extensions so jsx extends the spec in a
number of ways. see the section on `strict` in [options](#option) below though
there's not supposed to be any comments in json but when did comments ever hurt
anyone? json has no official comments but this parser allows c/c++ style comments.
anywhere whitespace is allowed you can insert comments (both `// ...` and `/* ... */`)
all jsx decoder input should be `utf8` encoded binaries. sometimes you get binaries
that are almost but not quite valid utf8 whether due to improper escaping or poor
encoding. jsx replaces invalid codepoints and poorly formed sequences with the
unicode replacement character (`u+FFFD`)
json only allows keys and strings to be delimited by double quotes (`u+0022`) but
javascript allows them to be delimited by single quotes (`u+0027`) as well. jsx
follows javascript in this. strings that start with single quotes can contain double
quotes but must end with single quotes and must escape any single quotes they contain
json and jsx only recognize escape sequences as outlined in the json spec. it just
ignores bad escape sequences
### json &lt;-> erlang mapping ###
**json** | **erlang**
--------------------------------|--------------------------------
`number` | `integer()` and `float()`
`string` | `binary()`
`true`, `false` and `null` | `true`, `false` and `null`
`array` | `[]` and `[JSON]`
`object` | `[{}]` and `[{binary() OR atom(), JSON}]`
* numbers
javascript and thus json represent all numeric values with floats. as
this is woefully insufficient for many uses, **jsx**, just like erlang,
supports bigints. whenever possible, this library will interpret json
numbers that look like integers as integers. other numbers will be converted
to erlang's floating point type, which is nearly but not quite iee754.
negative zero is not representable in erlang (zero is unsigned in erlang and
`0` is equivalent to `-0`) and will be interpreted as regular zero. numbers
not representable are beyond the concern of this implementation, and will
result in parsing errors
when converting from erlang to json, numbers are represented with their
shortest representation that will round trip without loss of precision. this
means that some floats may be superficially dissimilar (although
functionally equivalent). for example, `1.0000000000000001` will be
represented by `1.0`
* strings
the json [spec][rfc4627] is frustratingly vague on the exact details of json
strings. json must be unicode, but no encoding is specified. javascript
explicitly allows strings containing codepoints explicitly disallowed by
unicode. json allows implementations to set limits on the content of
strings. other implementations attempt to resolve this in various ways. this
implementation, in default operation, only accepts strings that meet the
constraints set out in the json spec (strings are sequences of unicode
codepoints deliminated by `"` (`u+0022`) that may not contain control codes
unless properly escaped with `\` (`u+005c`)) and that are encoded in `utf8`
the utf8 restriction means improperly paired surrogates are explicitly
disallowed. `u+d800` to `u+dfff` are allowed, but only when they form valid
surrogate pairs. surrogates encountered otherwise result in errors
json string escapes of the form `\uXXXX` will be converted to their
equivalent codepoints during parsing. this means control characters and
other codepoints disallowed by the json spec may be encountered in resulting
strings, but codepoints disallowed by the unicode spec will not be. in the
interest of pragmatism there is an [option](#option) for looser parsing
all erlang strings are represented by **valid** `utf8` encoded binaries. the
encoder will check strings for conformance. noncharacters (like `u+ffff`)
are allowed in erlang utf8 encoded binaries, but not in strings passed to
the encoder (although, again, see [options](#option))
this implementation performs no normalization on strings beyond that
detailed here. be careful when comparing strings as equivalent strings
may have different `utf8` encodings
* true, false and null
the json primitives `true`, `false` and `null` are represented by the
erlang atoms `true`, `false` and `null`. surprise
* arrays
json arrays are represented with erlang lists of json values as described
in this section
* objects
json objects are represented by erlang proplists. the empty object has the
special representation `[{}]` to differentiate it from the empty list.
ambiguities like `[true, false]` prevent the use of the shorthand form of
property lists using atoms as properties so all properties must be tuples.
all keys must be encoded as in `string` or as atoms (which will be escaped
and converted to binaries for presentation to handlers). values should be
valid json values
### incomplete input ###
jsx can handle incomplete json texts. if the option `stream` is passed to the decoder
or parser and if a partial json text is parsed, rather than returning a term from
your callback handler, jsx returns `{incomplete, F}` where `F` is a function with
an identical API to the anonymous fun returned from `decoder/3`, `encoder/3` or
`parser/3`. it retains the internal state of the parser at the point where input
was exhausted. this allows you to parse as you stream json over a socket or file
descriptor, or to parse large json texts without needing to keep them entirely in
memory
however, it is important to recognize that jsx is conservative by default. jsx will
not consider the parsing complete even when input is exhausted and the json text is
unambiguously incomplete. to end parsing call the `incomplete` function with the
argument `end_stream` like:
```erlang
1> {incomplete, F} = jsx:decode(<<"[">>, [stream]).
{incomplete,#Fun<jsx_decoder.1.122947756>}
2> F(end_stream).
** exception error: bad argument
3> {incomplete, G} = F(<<"]">>).
{incomplete,#Fun<jsx_decoder.1.122947756>}
4> G(end_stream).
[]
```
## data types ##
#### `json_term()` ####
```erlang
json_term() = [json_term()]
| [{binary() | atom(), json_term()}]
| true
| false
| null
| integer()
| float()
| binary()
```
the erlang representation of json. binaries should be `utf8` encoded, or close
at least
#### `json_text()` ####
```erlang
json_text() = binary()
```
a utf8 encoded binary containing a json string
#### `token()` ####
```erlang
event() = start_object
| end_object
| start_array
| end_array
| {key, binary()}
| {string, binary()}
| binary()
| {integer, integer()}
| integer()
| {float, float()}
| float()
| {literal, true}
| true
| {literal, false}
| false
| {literal, null}
| null
| end_json
```
the representation used during syntactic analysis. you can generate this
yourself and feed it to `jsx:parser/3` if you'd like to define your own
representations
#### `event()` ####
```erlang
event() = start_object
| end_object
| start_array
| end_array
| {key, binary()}
| {string, binary()}
| {integer, integer()}
| {float, float()}
| {literal, true}
| {literal, false}
| {literal, null}
| end_json
```
the subset of [`token()`](#token) emitted by the decoder and encoder to handlers
#### `option()` ####
```erlang
option() = escaped_forward_slashes
| escaped_strings
| unescaped_jsonp
| dirty_strings
| strict
| {strict, [strict_option()]}
| stream
| {incomplete_handler, fun()}
| {error_handler, fun()}
strict_option() = comments
| utf8
| single_quotes
| escapes
```
jsx functions all take a common set of options. not all flags have meaning
in all contexts, but they are always valid options. functions may have
additional options beyond these. see
[individual function documentation](#exports) for details
- `escaped_forward_slashes`
json strings are escaped according to the json spec. this means forward
slashes (solidus) are only escaped when this flag is present. otherwise they
are left unescaped. you may want to use this if you are embedding json
directly into a html or xml document
- `escaped_strings`
by default both the encoder and decoder return strings as utf8 binaries
appropriate for use in erlang. escape sequences that were present in decoded
terms are converted into the appropriate codepoint while encoded terms are
unaltered. this flag escapes strings as if for output in json, removing
control codes and problematic codepoints and replacing them with the
appropriate escapes
- `unescaped_jsonp`
javascript interpreters treat the codepoints `u+2028` and `u+2029` as
significant whitespace. json strings that contain either of these codepoints
will be parsed incorrectly by some javascript interpreters. by default,
these codepoints are escaped (to `\u2028` and `\u2029`, respectively) to
retain compatibility. this option simply removes that escaping
- `dirty_strings`
json escaping is lossy; it mutates the json string and repeated application
can result in unwanted behaviour. if your strings are already escaped (or
you'd like to force invalid strings into "json" you monster) use this flag
to bypass escaping. this can also be used to read in **really** invalid json
strings. everything between unescaped quotes are passed as is to the resulting
string term. note that this takes precedence over any other options
- `strict`
as mentioned [earlier](#description), jsx is pragmatic. if you're more of a
json purist or you're really into bdsm stricter adherence to the spec is
possible. the following restrictions are available
* `comments`
comments are disabled and result in a `badarg` error
* `utf8`
invalid codepoints and malformed unicode result in `badarg` errors
* `single_quotes`
only keys and strings delimited by double quotes (`u+0022`) are allowed. the
single quote (`u+0027`) results in a `badarg` error
* `escapes`
escape sequences not adhering to the json spec result in a `badarg` error
any combination of these can be passed to jsx by using `{strict, [strict_option()]}`.
`strict` is equivalent to `{strict, [comments, bad_utf8, single_quotes, escapes]}`
- `stream`
see [incomplete input](#incomplete-input)
- `incomplete_handler` & `error_handler`
the default incomplete and error handlers can be replaced with user defined
handlers. if options include `{error_handler, F}` and/or
`{incomplete_handler, F}` where `F` is a function of arity 3 they will be
called instead of the default handler. the spec for `F` is as follows
```erlang
F(Remaining, InternalState, Config) -> any()
Remaining = binary() | term()
InternalState = opaque()
Config = list()
```
`Remaining` is the binary fragment or term that caused the error
`InternalState` is an opaque structure containing the internal state of the
parser/decoder/encoder
`Config` is a list of options/flags in use by the parser/decoder/encoder
these functions should be considered experimental for now
## exports ##
#### `encoder/3`, `decoder/3` & `parser/3` ####
```erlang
decoder(Module, Args, Opts) -> Fun((JSONText) -> any())
encoder(Module, Args, Opts) -> Fun((JSONTerm) -> any())
parser(Module, Args, Opts) -> Fun((Tokens) -> any())
Module = atom()
Args = any()
Opts = [option()]
JSONText = json_text()
JSONTerm = json_term()
Tokens = event() | [event()]
```
jsx is a json compiler with interleaved tokenizing, syntactic analysis and
semantic analysis stages. included are two tokenizers; one that handles json
texts (`decoder/3`) and one that handles erlang terms (`encoder/3`). there is
also an entry point to the syntactic analysis stage for use with user-defined
tokenizers (`parser/3`)
all three functions return an anonymous function that takes the appropriate type
of input and returns the result of performing semantic analysis, the tuple
`{incomplete, F}` where `F` is a new anonymous function (see
[incomplete input](#incomplete_input)) or a `badarg` error exception if
syntactic analysis fails
`Module` is the name of the callback module
`Args` is any term that will be passed to `Module:init/1` prior to syntactic
analysis to produce an initial state
`Opts` are detailed [here](#option)
check out [callback module documentation](#callback_exports) for details of
the callback module interface
#### `decode/1,2` ####
```erlang
decode(JSON) -> Term
decode(JSON, Opts) -> Term
JSON = json_text()
Term = json_term()
Opts = [option() | labels | {labels, Label}]
Label = binary | atom | existing_atom | attempt_atom
F = fun((any()) -> any())
```
`decode` parses a json text (a `utf8` encoded binary) and produces an erlang
term
the option `labels` controls how keys are converted from json to
erlang terms. `binary` (the default behavior) does no conversion
beyond normal escaping. `atom` converts keys to erlang atoms and
results in a `badarg` error if the keys fall outside the range of erlang
atoms. `existing_atom` is identical to `atom` except it will not add
new atoms to the atom table and will result in a `badarg` error if the atom
does not exist. `attempt_atom` will convert keys to atoms when they exist,
and leave them as binary otherwise
raises a `badarg` error exception if input is not valid json
#### `encode/1,2` ####
```erlang
encode(Term) -> JSON
encode(Term, Opts) -> JSON
Term = json_term()
JSON = json_text()
Opts = [option() | space | {space, N} | indent | {indent, N}]
F = fun((any()) -> any())
N = pos_integer()
```
`encode` converts an erlang term into json text (a `utf8` encoded binary)
the option `{space, N}` inserts `N` spaces after every comma and colon in your
json output. `space` is an alias for `{space, 1}`. the default is `{space, 0}`
the option `{indent, N}` inserts a newline and `N` spaces for each level of
indentation in your json output. note that this overrides spaces inserted after
a comma. `indent` is an alias for `{indent, 1}`. the default is `{indent, 0}`
raises a `badarg` error exception if input is not a valid
[erlang representation of json](#json---erlang-mapping)
#### `format/1,2` ####
```erlang
format(JSON) -> JSON
format(JSON, Opts) -> JSON
JSON = json_text()
Opts = [option() | space | {space, N} | indent | {indent, N}]
N = pos_integer()
```
`format` parses a json text (a `utf8` encoded binary) and produces a new json
text according to the format rules specified by `Opts`
the option `{space, N}` inserts `N` spaces after every comma and colon in your
json output. `space` is an alias for `{space, 1}`. the default is `{space, 0}`
the option `{indent, N}` inserts a newline and `N` spaces for each level of
indentation in your json output. note that this overrides spaces inserted after
a comma. `indent` is an alias for `{indent, 1}`. the default is `{indent, 0}`
raises a `badarg` error exception if input is not valid json
#### `minify/1` ####
```erlang
minify(JSON) -> JSON
JSON = json_text()
```
`minify` parses a json text (a `utf8` encoded binary) and produces a new json
text stripped of whitespace
raises a `badarg` error exception if input is not valid json
#### `prettify/1` ####
```erlang
prettify(JSON) -> JSON
JSON = json_text()
```
`prettify` parses a json text (a `utf8` encoded binary) and produces a new json
text equivalent to `format(JSON, [{space, 1}, {indent, 2}])`
raises a `badarg` error exception if input is not valid json
#### `is_json/1,2` ####
```erlang
is_json(MaybeJSON) -> true | false
is_json(MaybeJSON, Opts) -> true | false
MaybeJSON = any()
Opts = options()
```
returns true if input is a valid json text, false if not
what exactly constitutes valid json may be [altered](#option)
#### `is_term/1,2` ####
```erlang
is_term(MaybeJSON) -> true | false
is_term(MaybeJSON, Opts) -> true | false
MaybeJSON = any()
Opts = options()
```
returns true if input is a valid erlang representation of json, false if not
what exactly constitutes valid json may be altered via [options](#option)
## callback exports ##
the following functions should be exported from a jsx callback module
#### `Module:init/1` ####
```erlang
Module:init(Args) -> InitialState
Args = any()
InitialState = any()
```
whenever any of `encoder/3`, `decoder/3` or `parser/3` are called, this function
is called with the `Args` argument provided in the calling function to obtain
`InitialState`
#### `Module:handle_event/2` ####
```erlang
Module:handle_event(Event, State) -> NewState
Event = [event()]
State = any()
NewState = any()
```
semantic analysis is performed by repeatedly calling `handle_event/2` with a
stream of events emitted by the tokenizer and the current state. the new state
returned is used as the input to the next call to `handle_event/2`. the
following events must be handled:
- `start_object`
the start of a json object
- `end_object`
the end of a json object
- `start_array`
the start of a json array
- `end_array`
the end of a json array
- `{string, binary()}`
a json string. it will usually be a `utf8` encoded binary. see the
[options](#option) for possible exceptions. note that keys are also
json strings
- `{integer, integer()}`
an erlang integer (bignum)
- `{float, float()}`
an erlang float
- `{literal, true}`
the atom `true`
- `{literal, false}`
the atom `false`
- `{literal, null}`
the atom `null`
- `end_json`
this event is emitted when syntactic analysis is completed. you should
do any cleanup and return the result of your semantic analysis
## acknowledgements ##
jsx wouldn't be what it is without the contributions of [paul davis](https://github.com/davisp), [lloyd hilaiel](https://github.com/lloyd), [john engelhart](https://github.com/johnezang), [bob ippolito](https://github.com/etrepum), [fernando benavides](https://github.com/elbrujohalcon), [alex kropivny](https://github.com/amtal), [steve strong](https://github.com/srstrong), [michael truog](https://github.com/okeuday), [dmitry kolesnikov](https://github.com/fogfish) and [emptytea](https://github.com/emptytea)
[json]: http://json.org
[yajl]: http://lloyd.github.com/yajl
[MIT]: http://www.opensource.org/licenses/mit-license.html
[rebar]: https://github.com/rebar/rebar
[meck]: https://github.com/eproxus/meck
[rfc4627]: http://tools.ietf.org/html/rfc4627
[travis]: https://travis-ci.org/
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