Module Basic.Util

This module provides combinators for extracting fields from JSON values.

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 keys : t -> string list

Returns all the key names in the given JSON object.

  • raises Type_error

    if argument is not a JSON object.

val values : t -> t list

Return all the value in the given JSON object.

  • raises Type_error

    if argument is not a JSON object.

val combine : t -> t -> t

Combine two JSON objects together.

  • raises Invalid_argument

    if either argument is not a JSON object.

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.

  • raises Type_error

    if obj is not a JSON object.

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).

  • raises Type_error

    if arr is not a JSON array.

  • raises Undefined

    if index is out of bounds.

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.

  • raises Type_error

    if arr is not an JSON array.

val to_assoc : t -> (string * t) list

Extract the items of a JSON object.

  • raises Type_error

    if argument is not a JSON object.

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.

  • raises Type_error

    if argument is not a JSON boolean.

val to_bool_option : t -> bool option

Extract Some boolean value, return None if the value is null.

  • raises Type_error

    if argument is neither.

val to_number : t -> float

Extract a number.

  • raises Type_error

    if argument is not a JSON number.

val to_number_option : t -> float option

Extract Some number, return None if the value is null.

  • raises Type_error

    if argument is neither.

val to_float : t -> float

Extract a float value. to_number is generally preferred as it also works with int literals.

  • raises Type_error

    if argument is not a JSON float.

val to_float_option : t -> float option

Extract Some float value, return None if the value is null. to_number_option is generally preferred as it also works with int literals.

  • raises Type_error

    if argument is neither.

val to_int : t -> int

Extract an int from a JSON int.

  • raises Type_error

    if argument is not a JSON int.

val to_int_option : t -> int option

Extract Some int from a JSON int, return None if the value is null.

  • raises Type_error

    if argument is neither.

val to_list : t -> t list

Extract a list from JSON array.

  • raises Type_error

    if argument is not a JSON array.

val to_string : t -> string

Extract a string from a JSON string.

  • raises Type_error

    if argument is not a JSON string.

val to_string_option : t -> string option

Extract Some string from a JSON string, return None if the value is null.

  • raises Type_error

    if argument is neither.

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).

  • raises Type_error

    if arr is not a JSON array.

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 : 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.