Opium

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.

Installation

Stable

The latest stable version is available on opam

$ opam install opium

Master

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

Documentation

For the API documentation:

• Read the hosted documentation for the latest version.
• Build and view the docs for version installed locally using odig: odig doc opium.

The following tutorials walk through various usecases of Opium:

• A Lightweight OCaml Webapp Tutorial covers a simple webapp generating dynamic HTML on the backend and interfacing with PostgreSQL.

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

Examples

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 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}
      in
      `Json (person |> json_of_person) |> respond')

let _ = App.empty |> print_param |> print_person |> 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

{"name":"john_doe","age":42}

Middleware

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
  in
  Rock.Middleware.create ~filter ~name:"reject_ua"

let _ =
  App.empty
  |> 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