https://github.com/facebook/reason/wiki/Newcomer-Tips-&-Tricks

hello_world.re

/* Converting https://github.com/rgrinberg/opium#hello-world to Reason */
open Opium.Std;

type person = {
  name: string,
  age: int,
};

let json_of_person {name, age} => Ezjsonm.(dict [("name", string name), ("age", int age)]);

/* Original Ocaml
let json_of_person { name ; age } =
  let open Ezjsonm in
  dict [ "name", (string name)
       ; "age", (int age) ]
*/


let print_param = put "/hello/:name" (fun req => `String ("Hello " ^ param req "name") |> respond');
let print_hello = get "/" (fun req => `String ("Hello World") |> respond');
let print_person = get "/person/:name/:age" (
  fun req => {
    let person = {
      name: param req "name",
      age: "age" |> param req |> int_of_string
    };
    `Json (person |> json_of_person) |> respond'
  }
);

/* |> get "/" (fun req -> `String ("Hello World") |> respond') */
App.empty
  |> print_hello
  |> print_param
  |> print_person
  |> App.run_command;