Module Yojson

The Yojson library provides runtime functions for reading and writing JSON data from OCaml. It addresses a few shortcomings of its predecessor json-wheel and is about twice as fast (2.7x reading, 1.3x writing; results may vary). The design goals of Yojson are the following:

Shared types and functions

val version : string
exception Json_error of string
val json_error : string -> 'a
type lexer_state = {
buf : Bi_outbuf.t;(*

Buffer used to accumulate substrings

*)
mutable lnum : int;(*

Current line number (counting from 1)

*)
mutable bol : int;(*

Absolute position of the first character of the current line (counting from 0)

*)
mutable fname : string option;(*

Name referencing the input file in error messages

*)
}
module Lexer_state : sig ... end
val init_lexer : ?buf:Bi_outbuf.t -> ?fname:string -> ?lnum:int -> unit -> lexer_state

Create a fresh lexer_state record.

Basic JSON tree type

module Basic : sig ... end

This module supports standard JSON nodes only, i.e. no special syntax for variants or tuples as supported by Yojson.Safe. Arbitrary integers are not supported as they must all fit within the standard OCaml int type (31 or 63 bits depending on the platform).

Multipurpose JSON tree type

module Safe : sig ... end

This module supports a specific syntax for variants and tuples in addition to the standard JSON nodes. Arbitrary integers are supported and represented as a decimal string using `Intlit when they cannot be represented using OCaml's int type.

JSON tree type with literal int/float/string leaves

module Raw : sig ... end

Ints, floats and strings literals are systematically preserved using `Intlit, `Floatlit and `Stringlit. This module also supports the specific syntax for variants and tuples supported by Yojson.Safe.

Supertype of all JSON tree types

Type of the JSON tree

type t = [
| `Null
| `Bool of bool
| `Int of int
| `Intlit of string
| `Float of float
| `Floatlit of string
| `String of string
| `Stringlit of string
| `Assoc of (string * t) list
| `List of t list
| `Tuple of t list
| `Variant of string * t option
]

All possible cases defined in Yojson:

  • `Null: JSON null
  • `Bool of bool: JSON boolean
  • `Int of int: JSON number without decimal point or exponent.
  • `Intlit of string: JSON number without decimal point or exponent, preserved as a string.
  • `Float of float: JSON number, Infinity, -Infinity or NaN.
  • `Floatlit of string: JSON number, Infinity, -Infinity or NaN, preserved as a string.
  • `String of string: JSON string. Bytes in the range 128-255 are preserved as-is without encoding validation for both reading and writing.
  • `Stringlit of string: JSON string literal including the double quotes.
  • `Assoc of (string * json) list: JSON object.
  • `List of json list: JSON array.
  • `Tuple of json list: Tuple (non-standard extension of JSON). Syntax: ("abc", 123).
  • `Variant of (string * json option): Variant (non-standard extension of JSON). Syntax: <"Foo"> or <"Bar":123>.
type json = t

* Compatibility type alias for type `t`

val pp : Format.formatter -> t -> unit

Pretty printer, useful for debugging

val show : t -> string

Convert value to string, useful for debugging

val equal : t -> t -> bool

equal a b is the monomorphic equality. Determines whether two JSON values are considered equal. In the case of JSON objects, the order of the keys does not matter, except for duplicate keys which will be considered equal as long as they are in the same input order.

type json_max = t

JSON writers

val to_string : ?buf:Bi_outbuf.t -> ?len:int -> ?std:bool -> t -> string

Write a compact JSON value to a string.

  • parameter buf

    allows to reuse an existing buffer created with Bi_outbuf.create. The buffer is cleared of all contents before starting and right before returning.

  • parameter len

    initial length of the output buffer.

  • parameter std

    use only standard JSON syntax, i.e. convert tuples and variants into standard JSON (if applicable), refuse to print NaN and infinities, require the root node to be either an object or an array. Default is false.

val to_channel : ?buf:Bi_outbuf.t -> ?len:int -> ?std:bool -> out_channel -> t -> unit

Write a compact JSON value to a channel.

  • parameter buf

    allows to reuse an existing buffer created with Bi_outbuf.create_channel_writer on the same channel. buf is flushed right before to_channel returns but the out_channel is not flushed automatically.

    See to_string for the role of the other optional arguments.

val to_output : ?buf:Bi_outbuf.t -> ?len:int -> ?std:bool -> < output : string -> int -> int -> int; .. > -> t -> unit

Write a compact JSON value to an OO channel.

  • parameter buf

    allows to reuse an existing buffer created with Bi_outbuf.create_output_writer on the same channel. buf is flushed right before to_output returns but the channel itself is not flushed automatically.

    See to_string for the role of the other optional arguments.

val to_file : ?len:int -> ?std:bool -> string -> t -> unit

Write a compact JSON value to a file. See to_string for the role of the optional arguments.

val to_outbuf : ?std:bool -> Bi_outbuf.t -> t -> unit

Write a compact JSON value to an existing buffer. See to_string for the role of the optional argument.

val stream_to_string : ?buf:Bi_outbuf.t -> ?len:int -> ?std:bool -> t Stream.t -> string

Write a newline-separated sequence of compact one-line JSON values to a string. See to_string for the role of the optional arguments.

val stream_to_channel : ?buf:Bi_outbuf.t -> ?len:int -> ?std:bool -> out_channel -> t Stream.t -> unit

Write a newline-separated sequence of compact one-line JSON values to a channel. See to_channel for the role of the optional arguments.

val stream_to_file : ?len:int -> ?std:bool -> string -> t Stream.t -> unit

Write a newline-separated sequence of compact one-line JSON values to a file. See to_string for the role of the optional arguments.

val stream_to_outbuf : ?std:bool -> Bi_outbuf.t -> t Stream.t -> unit

Write a newline-separated sequence of compact one-line JSON values to an existing buffer. See to_string for the role of the optional arguments.

val write_t : Bi_outbuf.t -> t -> unit

Write the given JSON value to the given buffer. Provided as a writer function for atdgen.

Miscellaneous

val sort : t -> t

Sort object fields (stable sort, comparing field names and treating them as byte sequences)

JSON pretty-printing

val pretty_format : ?std:bool -> t -> Easy_format.t

Convert into a pretty-printable tree. See to_string for the role of the optional std argument.

val pretty_print : ?std:bool -> Format.formatter -> t -> unit

Pretty-print into a Format.formatter. See to_string for the role of the optional std argument.

  • since 1.3.1
val pretty_to_string : ?std:bool -> t -> string

Pretty-print into a string. See to_string for the role of the optional std argument.

val pretty_to_channel : ?std:bool -> out_channel -> t -> unit

Pretty-print to a channel. See to_string for the role of the optional std argument.