JSON with OCaml

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Ocaml and JSON

Manipulating JSON (JavaScript Object Notation) elements is amazing and allows to move data around between different programs easily. The only thing you need is to be able to write and read JSON in all your programs. Though this can be simple in some languages, other will require some work to.

OCaml being strongly typed, serializing / deserializing object is less obvious than in, say, Python.

The tools

Yojson: low-level JSON library for OCaml

ppx_deriving_yojson : writing json

To get them, simply use opam:

opam install yojson
opam install ppx_deriving_yojson

Optionnal, for vim users

If you have not done it already, these tools will help you navigate through the different functions proposed by the above packages. They allow to have “intellisense” for OCaml.

Merlin

YouCompleteMe

Note that you have to create a .merlin file in the directory where you are working, to help vim find the relevant packages.

PKG find yojson

Type to JSON

What is PPX ?

Writing code for each type to turn it into JSON is particularly boring and can lead to errors. In many languages this is straightworard. Like using json.dumps(obj) in Python. The PPX language is a feature that provides a new API for syntactic extensions in OCaml. deriving yojson generates three functions per type:

# #require "ppx_deriving_yojson";;
# type ty = .. [@@deriving yojson];;
val ty_of_yojson : Yojson.Safe.json -> (ty, string) Result.result
val ty_of_yojson_exn : Yojson.Safe.json -> ty
val ty_to_yojson : ty -> Yojson.Safe.json

So what happens is that, during the compilation process, the functions ty_of_yojson and the others will be, automatically and silently, generated and the compiler will not complain that they are missing - though you never had to write them.

Nested types

Let’s give it a try ! And come up with a nested type.

type t = {x: int; y: int} [@@deriving to_yojson]
type u = {s: string; pos: t} [@@deriving to_yojson]
let () = print_endline (Yojson.Safe.pretty_to_string (u_to_yojson {s= "hello"; pos={x= 1; y= 2}}))
ocamlfind opt -package ppx_deriving_yojson -linkpkg -o testJson.byte testJson.ml
./testJson.byte
rm testJson.byte

And…

{ "s": "hello", "pos": { "x": 1, "y": 2 } }

It worked :)

Learning more

Real World OCaml: Functional programming for the masses by Yaron Minsky, Anil Madhavapeddy and Jason Hickey is a good introduction to OCaml.

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