Module `Basic.Util`

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.t) : string list =
  [json]
    |> filter_member "pages"
    |> flatten
    |> filter_member "title"
    |> filter_string

exception Type_error of string * t: 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 * t: 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

deprecated: Forward pipe operator; useful for composing JSON access functions without too many parentheses

val keys : t -> string list: Returns all the key names in the given JSON object

val values : t -> t list: Return all the value in the given JSON object

val combine : t -> t -> t: Combine two JSON Objects together

val member : string -> t -> t: 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 -> t -> t: 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 : (t -> t) -> t -> t: 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 : t -> (string * t) list: Extract the items of a JSON object or raise Type_error.

val to_option : (t -> 'a) -> t -> '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 : t -> bool: Extract a boolean value or raise Type_error.

val to_bool_option : t -> bool option: Extract Some boolean value, return None if the value is null, or raise Type_error otherwise.

val to_number : t -> float: Extract a number or raise Type_error.

val to_number_option : t -> float option: Extract Some number, return None if the value is null, or raise Type_error otherwise.

val to_float : t -> 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 : t -> 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 : t -> int: Extract an int from a JSON int or raise Type_error.

val to_int_option : t -> int option: Extract Some int from a JSON int, return None if the value is null, or raise Type_error otherwise.

val to_list : t -> t list: Extract a list from JSON array or raise Type_error.

val to_string : t -> string: Extract a string from a JSON string or raise Type_error.

val to_string_option : t -> string option: Extract Some string from a JSON string, return None if the value is null, or raise Type_error otherwise.

val convert_each : (t -> 'a) -> t -> '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

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 : t list -> t 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 -> t list -> t list: Expects JSON arrays and returns all their elements existing at the given position.

val filter_list : t list -> t list list: Expects JSON arrays and unwraps them.

val filter_member : string -> t list -> t list: Expects JSON objects and returns all the fields of the given name (at most one field per object).

val filter_assoc : t list -> (string * t) list list: Expects JSON objects and unwraps them.

val filter_bool : t list -> bool list: Expects JSON booleans and unwraps them.

val filter_int : t list -> int list: Expects JSON integers (`Int nodes) and unwraps them.

val filter_float : t list -> float list: Expects JSON floats (`Float nodes) and unwraps them.

val filter_number : t list -> float list: Expects JSON numbers (`Int or `Float) and unwraps them. Ints are converted to floats.

val filter_string : t list -> string list: Expects JSON strings and unwraps them.