package mjson

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Composable, applicative and monadic DSL for decoding Yojson values


Dune Dependency





mjson is a small OCaml library for decoding JSON values from more low-level JSON libraries such as Yojson.

It is just a thin layer on top of Yojson (Safe and Basic).


type person =
  { name : string
  ; age : int

let make_person name age = { name; age }

(* (monadic) decoder for Yojson.Safe.t values into a person. *)
let person_decoder =
  let open Mjson.Decoder.Yojson.Safe in
  let open Mjson.Decoder.Yojson.Safe.Syntax in
  (* decode the field "name" into a string *)
  let* name = field "name" string in
  (* decode the field "age" into an int *)
  let* age = field "age" int in
  (* if all went well [return] the person (or any error that might have
     occured along the way. *)
  return @@ make_person name age

Example utop session

utop # person_decoder @@ `Assoc [ "name", `String "Ursula"; "age", `Float 88.0];;
- : (person, Mjson.Decoder.Yojson.Safe.Error.t) result =
Ok {name = "Ursula"; age = 88}

utop # person_decoder @@ `Assoc [ "name", `String "Ursula"; "age", `Bool true];;
- : (person, Mjson.Decoder.Yojson.Safe.Error.t) result =
 (Mjson.Decoder.Yojson.Safe.Error.Field ("age",
   Mjson.Decoder.Yojson.Safe.Error.Failure ("not an int", `Bool true)))

See the tests for more examples.

Building the Code

This packages comes with a nix DevShell. All commands are intended for use within the provided (default) devShell.

$ make repl   # opens a utop repl with the project loaded
$ make test   # runs all tests
$ make doc    # generate all documentation