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Module Yojson.Basic.Util

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.

Here is some sample JSON data: 

{
  "id": "398eb027",
  "name": "John Doe",
  "pages": [
    {
      "id": 1,
      "title": "The Art of Flipping Coins",
      "url": "http://example.com/398eb027/1"
    },
    {
      "id": 2,
      "deleted": true
    },
    {
      "id": 3,
      "title": "Artichoke Salad",
      "url": "http://example.com/398eb027/3"
    },
    {
      "id": 4,
      "title": "Flying Bananas",
      "url": "http://example.com/398eb027/4"
    }
  ]
}

In order to extract the "id" field, assuming it is mandatory, we would use the following OCaml code that operates on single JSON nodes: 

open Yojson.Basic.Util
...
  let id = json |> member "id" |> to_string in
  ...

In order to extract all the "title" fields, we would write the following OCaml code that operates on lists of JSON nodes, skipping undefined nodes and nodes of unexpected type: 

open Yojson.Basic.Util

let extract_titles (json : Yojson.Basic.json) : string list =
  [json]
    |> filter_member "pages"
    |> flatten
    |> filter_member "title"
    |> filter_string

exception Type_error of string * Yojson.Basic.json
Raised when the JSON value is not of the correct type to support an operation, e.g. member on an `Int. The string message explains the mismatch.
exception Undefined of string * Yojson.Basic.json
Raised when the equivalent JavaScript operation on the JSON value would return undefined. Currently this only happens when an array index is out of bounds.
val (|>) : 'a -> ('a -> 'b) -> 'b
Forward pipe operator; useful for composing JSON access functions without too many parentheses
val member : string -> Yojson.Basic.json -> Yojson.Basic.json
member k obj returns the value associated with the key k in the JSON object obj, or `Null if k is not present in obj.
val index : int -> Yojson.Basic.json -> Yojson.Basic.json
index i arr returns the value at index i in the JSON array arr. Negative indices count from the end of the list (so -1 is the last element).
val map : (Yojson.Basic.json -> Yojson.Basic.json) ->
       Yojson.Basic.json -> Yojson.Basic.json
map f arr calls the function f on each element of the JSON array arr, and returns a JSON array containing the results.
val to_assoc : Yojson.Basic.json -> (string * Yojson.Basic.json) list
Extract the items of a JSON array or raise Type_error.
val to_option : (Yojson.Basic.json -> 'a) -> Yojson.Basic.json -> 'a option
Return None if the JSON value is null or map the JSON value to Some value using the provided function.
val to_bool : Yojson.Basic.json -> bool
Extract a boolean value or raise Type_error.
val to_bool_option : Yojson.Basic.json -> bool option
Extract Some boolean value, return None if the value is null, or raise Type_error otherwise.
val to_number : Yojson.Basic.json -> float
Extract a number or raise Type_error.
val to_number_option : Yojson.Basic.json -> float option
Extract Some number, return None if the value is null, or raise Type_error otherwise.
val to_float : Yojson.Basic.json -> float
Extract a float value or raise Type_error. to_number is generally preferred as it also works with int literals.
val to_float_option : Yojson.Basic.json -> float option
Extract Some float value, return None if the value is null, or raise Type_error otherwise. to_number_option is generally preferred as it also works with int literals.
val to_int : Yojson.Basic.json -> int
Extract an int from a JSON int or raise Type_error.
val to_int_option : Yojson.Basic.json -> int option
Extract Some int from a JSON int, return None if the value is null, or raise Type_error otherwise.
val to_list : Yojson.Basic.json -> Yojson.Basic.json list
Extract a list from JSON array or raise Type_error.
val to_string : Yojson.Basic.json -> string
Extract a string from a JSON string or raise Type_error.
val to_string_option : Yojson.Basic.json -> string option
Extract Some string from a JSON string, return None if the value is null, or raise Type_error otherwise.
val convert_each : (Yojson.Basic.json -> 'a) -> Yojson.Basic.json -> 'a list
The conversion functions above cannot be used with map, because they do not return JSON values. This convenience function convert_each to_f arr is equivalent to List.map to_f (to_list arr).

Exception-free filters



The following functions operate on lists of JSON nodes. None of them raises an exception when a certain kind of node is expected but no node or the wrong kind of node is found. Instead of raising an exception, nodes that are not as expected are simply ignored.
val filter_map : ('a -> 'b option) -> 'a list -> 'b list
filter_map f l maps each element of the list l to an optional value using function f and unwraps the resulting values.
val flatten : Yojson.Basic.json list -> Yojson.Basic.json list
Expects JSON arrays and returns all their elements as a single list. flatten l is equivalent to List.flatten (filter_list l).
val filter_index : int -> Yojson.Basic.json list -> Yojson.Basic.json list
Expects JSON arrays and returns all their elements existing at the given position.
val filter_list : Yojson.Basic.json list -> Yojson.Basic.json list list
Expects JSON arrays and unwraps them.
val filter_member : string -> Yojson.Basic.json list -> Yojson.Basic.json list
Expects JSON objects and returns all the fields of the given name (at most one field per object).
val filter_assoc : Yojson.Basic.json list -> (string * Yojson.Basic.json) list list
Expects JSON objects and unwraps them.
val filter_bool : Yojson.Basic.json list -> bool list
Expects JSON booleans and unwraps them.
val filter_int : Yojson.Basic.json list -> int list
Expects JSON integers (`Int nodes) and unwraps them.
val filter_float : Yojson.Basic.json list -> float list
Expects JSON floats (`Float nodes) and unwraps them.
val filter_number : Yojson.Basic.json list -> float list
Expects JSON numbers (`Int or `Float) and unwraps them. Ints are converted to floats.
val filter_string : Yojson.Basic.json list -> string list
Expects JSON strings and unwraps them.