Module Yojson.Basic

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).

The main advantage of this module is its simplicity.

Type of the JSON tree

type t = [
| `Null
| `Bool of bool
| `Int of int
| `Float of float
| `String of string
| `Assoc of (string * t) list
| `List of t list
]

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`

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)

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.

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.

val prettify : ?std:bool -> string -> string

Combined parser and pretty-printer. See to_string for the role of the optional std argument.

val compact : ?std:bool -> string -> string

Combined parser and printer. See to_string for the role of the optional std argument.

JSON readers

exception Finally of exn * exn

Exception describing a failure in both finalizer and parsing.

val from_string : ?buf:Bi_outbuf.t -> ?fname:string -> ?lnum:int -> string -> t

Read a JSON value from a string.

  • parameter buf

    use this buffer at will during parsing instead of creating a new one.

  • parameter fname

    data file name to be used in error messages. It does not have to be a real file.

  • parameter lnum

    number of the first line of input. Default is 1.

val from_channel : ?buf:Bi_outbuf.t -> ?fname:string -> ?lnum:int -> in_channel -> t

Read a JSON value from a channel. See from_string for the meaning of the optional arguments.

val from_file : ?buf:Bi_outbuf.t -> ?fname:string -> ?lnum:int -> string -> t

Read a JSON value from a file. See from_string for the meaning of the optional arguments.

type lexer_state = Lexer_state.t = {
buf : Bi_outbuf.t;
mutable lnum : int;
mutable bol : int;
mutable fname : string option;
}

This alias is provided for backward compatibility. New code should refer to Yojson.lexer_state directly.

val init_lexer : ?buf:Bi_outbuf.t -> ?fname:string -> ?lnum:int -> unit -> lexer_state

This alias is provided for backward compatibility. New code should use Yojson.init_lexer directly.

val from_lexbuf : lexer_state -> ?stream:bool -> Lexing.lexbuf -> t

Read a JSON value from a lexbuf. A valid initial lexer_state can be created with init_lexer. See from_string for the meaning of the optional arguments.

  • parameter stream

    indicates whether more data may follow. The default value is false and indicates that only JSON whitespace can be found between the end of the JSON value and the end of the input.

val stream_from_string : ?buf:Bi_outbuf.t -> ?fname:string -> ?lnum:int -> string -> t Stream.t

Input a sequence of JSON values from a string. Whitespace between JSON values is fine but not required. See from_string for the meaning of the optional arguments.

val stream_from_channel : ?buf:Bi_outbuf.t -> ?fin:(unit -> unit) -> ?fname:string -> ?lnum:int -> in_channel -> t Stream.t

Input a sequence of JSON values from a channel. Whitespace between JSON values is fine but not required.

  • parameter fin

    finalization function executed once when the end of the stream is reached either because there is no more input or because the input could not be parsed, raising an exception.

  • raises Finally

    When the parsing and the finalizer both raised, Finally (exn, fin_exn) is raised, exn being the parsing exception and fin_exn the finalizer one.

    See from_string for the meaning of the other optional arguments.

val stream_from_file : ?buf:Bi_outbuf.t -> ?fname:string -> ?lnum:int -> string -> t Stream.t

Input a sequence of JSON values from a file. Whitespace between JSON values is fine but not required.

See from_string for the meaning of the optional arguments.

val stream_from_lexbuf : lexer_state -> ?fin:(unit -> unit) -> Lexing.lexbuf -> t Stream.t

Input a sequence of JSON values from a lexbuf. A valid initial lexer_state can be created with init_lexer. Whitespace between JSON values is fine but not required.

  • raises Finally

    When the parsing and the finalizer both raised, Finally (exn, fin_exn) is raised, exn being the parsing exception and fin_exn the finalizer one.

    See stream_from_channel for the meaning of the optional fin argument.

type json_line = [
| `Json of t
| `Exn of exn
]

The type of values resulting from a parsing attempt of a JSON value.

val linestream_from_channel : ?buf:Bi_outbuf.t -> ?fin:(unit -> unit) -> ?fname:string -> ?lnum:int -> in_channel -> json_line Stream.t

Input a sequence of JSON values, one per line, from a channel. Exceptions raised when reading malformed lines are caught and represented using `Exn.

See stream_from_channel for the meaning of the optional fin argument. See from_string for the meaning of the other optional arguments.

val linestream_from_file : ?buf:Bi_outbuf.t -> ?fname:string -> ?lnum:int -> string -> json_line Stream.t

Input a sequence of JSON values, one per line, from a file. Exceptions raised when reading malformed lines are caught and represented using `Exn.

See stream_from_channel for the meaning of the optional fin argument. See from_string for the meaning of the other optional arguments.

val read_t : lexer_state -> Lexing.lexbuf -> t

Read a JSON value from the given lexer_state and lexing buffer and return it. Provided as a reader function for atdgen.

module Util : sig ... end

This module provides combinators for extracting fields from JSON values. This approach is recommended for reading a few fields from data returned by public APIs. However for more complex applications we recommend Atdgen.