jsx (v1.0)

a sane json implementation for erlang, inspired by yajl

copyright 2011, 2012 alisdair sullivan

jsx is released under the terms of the MIT license

to build jsx, make or ./rebar compile

api

converting json to erlang terms

parses a JSON text (a utf8 encoded binary) and produces an erlang term (see json <-> erlang mapping details below)

to_term(JSON) -> Term

to_term(JSON, Opts) -> Term

types:

• JSON = binary()
• Term = [] | [{}] | [any()] | {incomplete, Fun}
• Fun = fun(JSON) -> Term
• Opts = [] | [Opt]
• Opt =
  • loose_unicode
  • labels
  • {labels, Label}
  • Label =
    • binary
    • atom
    • existing_atom
  • explicit_end

JSON SHOULD be a utf8 encoded binary. if the option loose_unicode is present attempts are made to replace invalid codepoints with u+FFFD but badly encoded binaries may, in either case, result in badarg errors

the option labels controls how keys are converted from json to erlang terms. binary does no conversion beyond normal escaping. atom converts keys to erlang atoms, and results in a badarg error if 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

see the note below about streaming mode for details of explicit_end

converting erlang terms to json

produces a JSON text from an erlang term (see json <-> erlang mapping details below)

to_json(Term) -> JSON

to_json(Term, Opts) -> JSON

types:

• JSON = binary()
• Term = [] | [{}] | [any()]
• Opts = [] | [Opt]
• Opt =
  • space
  • {space, N}
  • indent
  • {indent, N}
  • escape_forward_slash

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}

if the option escape_forward_slash is enabled, $/ is escaped. this is not normally required but is necessary for compatibility with microsoft's json date format

formatting json texts

produces a JSON text from JSON text, reformatted

format(JSON) -> JSON

format(JSON, Opts) -> JSON

types:

• JSON = binary()
• Term = [] | [{}] | [any()] | {incomplete, Fun}
• Fun = fun(JSON) -> Term
• Opts = [] | [Opt]
• Opt =
  • space
  • {space, N}
  • indent
  • {indent, N}
  • loose_unicode
  • escape_forward_slash
  • explicit_end

JSON SHOULD be a utf8 encoded binary. if the option loose_unicode is present attempts are made to replace invalid codepoints with u+FFFD but badly encoded binaries may, in either case, result in badarg errors

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}

if the option escape_forward_slash is enabled, $/ is escaped. this is not normally required but is necessary for compatibility with microsoft's json date format

see the note below about streaming mode for details of explicit_end

verifying json texts

returns true if input is a valid JSON text, false if not

is_json(MaybeJSON) -> Term

is_json(MaybeJSON, Opts) -> Term

types:

• MaybeJSON = any()
• Term = true | false | {incomplete, Fun}
• Opts = [] | [Opt]
• Opt =
  • loose_unicode
  • explicit_end

see json_to_term for details of options

verifying json texts

returns true if input is a valid erlang term that represents a JSON text, false if not

is_term(MaybeJSON) -> Term

types:

• MaybeJSON = any()
• Term = true | false

streaming mode

this implementation is interruptable and reentrant and may be used to incrementally parse json texts. it's greedy and will exhaust input, returning when the stream buffer is empty. if the json text is so far valid, but incomplete (or if the option explicit_end has been selected), {incomplete, Fun} will be returned. Fun/1 may be called with additional input (or the atom end_stream to force the end of parsing)

explicit_end is of use when parsing bare numbers (like 123 or -0.987 for example) as they may have no unambiguous end when encountered in a stream. it is also of use when reading from a socket or file and there may be unprocessed white space (or errors) left in the stream

json <-> erlang

json	erlang
number	integer() OR float()
string	binary()
true, false and null	true, false and null
array	list()
object	[{}] OR [{binary(), JSON}]

json

json must be encoded in utf8. if it's invalid utf8, it probably won't parse without errors. one optional exception is made for json strings that are otherwise utf8, see under strings below.

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 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 and 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 (properly escaped control characters and quotes) and that are encoded in utf8. in the interests of pragmatism, however, the parser option loose_unicode attempts to replace invalid utf8 sequences with the replacement codepoint u+fffd when possible

all erlang strings are represented by valid utf8 encoded binaries

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 document

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 using the shorthand form of property lists using atoms as properties. all properties must be tuples. all keys must be encoded as in string, above, or as atoms (which will be escaped and converted to binaries for presentation to handlers)

acknowledgements

paul davis, lloyd hilaiel, john engelhart, bob ippolito, fernando benavides and alex kropivny have all contributed to the development of jsx, whether they know it or not