Sinatra like web toolkit based on Lwt + Cohttp

Executive Summary

Sinatra like web toolkit for OCaml based on cohttp & lwt

Design Goals

  • Opium should be very small and easily learnable. A programmer should
    be instantly productive when starting out.

  • Opium should be extensible using independently developed plugins. This is a
    Rack inspired mechanism borrowed from Ruby. The middleware mechanism in
    Opium is called Rock.

  • It should maximize use of creature comforts people are used to in
    other languages. Such as sexplib, fieldslib, a decent
    standard library.



The latest stable version is available on opam

$ opam install opium


$ opam pin add opium_kernel --dev-repo
$ opam pin add opium --dev-repo


For the API documentation:

The following tutorials walk through various usecases of Opium:

For examples of idiomatic usage, see the ./examples directory
and the simple examples below.


Assuming the necessary dependencies are installed, $ dune build @examples will
compile all examples. The binaries are located in _build/default/examples/.

You can execute these binaries directly, though in the examples below we use
dune exec to run them.

Hello World

Here's a simple hello world example to get your feet wet:

$ cat hello_world.ml

open Opium.Std

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

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 streaming =
  let open Lwt.Infix in
  get "/hello/stream" (fun _req ->
      (* [create_stream] returns a push function that can be used to
         push new content onto the stream. [f] is function that
         expects to receive a promise that gets resolved when the user
         decides that they have pushed all their content onto the stream.
         When the promise forwarded to [f] gets resolved, the stream will be
         closed. *)
      let f, push = App.create_stream () in
      let timers =
          (fun t ->
            Lwt_unix.sleep t
            >|= fun () -> push (Printf.sprintf "Hello after %f seconds\n" t))
          [1.; 2.; 3.]
      f (Lwt.join timers))

let default =
  not_found (fun _req ->
      `Json Ezjsonm.(dict [("message", string "Route not found")]) |> 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')

let _ =
  App.empty |> print_param |> print_person |> streaming |> default
  |> App.run_command

compile and run with:

$ dune exec examples/hello_world.exe &

then call

curl http://localhost:3000/person/john_doe/42

You should see the JSON message



The two fundamental building blocks of opium are:

  • Handlers: Rock.Request.t -> Rock.Response.t Lwt.t

  • Middleware: Rock.Handler.t -> Rock.Handler.t

Almost all of opium's functionality is assembled through various
middleware. For example: debugging, routing, serving static files,
etc. Creating middleware is usually the most natural way to extend an
opium app.

Here's how you'd create a simple middleware turning away everyone's
favourite browser.

open Opium.Std

(* don't open cohttp and opium since they both define request/response modules*)

let is_substring ~substring =
  let re = Re.compile (Re.str substring) in
  Re.execp re

let reject_ua ~f =
  let filter handler req =
    match Cohttp.Header.get (Request.headers req) "user-agent" with
    | Some ua when f ua -> `String "Please upgrade your browser" |> respond'
    | _ -> handler req
  Rock.Middleware.create ~filter ~name:"reject_ua"

let _ =
  |> get "/" (fun _ -> `String "Hello World" |> respond')
  |> middleware (reject_ua ~f:(is_substring ~substring:"MSIE"))
  |> App.cmd_name "Reject UA" |> App.run_command

Compile with:

$ dune build examples/middleware_ua.ml

Here we also use the ability of Opium to generate a cmdliner term to run your
app. Run your executable with -h to see the options that are available to you.
For example:

# run in debug mode on port 9000
$ dune exec examples/middleware_ua.exe -- -p 9000 -d
26 Mar 2020
Reverse Dependencies
< "0.2.0"