Opium

Executive Summary

Sinatra like web toolkit for OCaml based on cohttp, core, & lwt

Design Goals

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

• Opium should be extendable using indepedently 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, cow, a decent standard library.

Installation

NOTE: At this point there's a good chance this library will only work against cohttp master. Once cohttp 1.0 is released then this library will always be developed against OPAM version.

Stable

The latest stable version is available on opam

$ opam install opium

Master

If you'd like to live on the bleeding edge (which is sometimes more stable than stable)

$ opam pin add opium --dev-repo

Examples

Hello World

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

$ cat hello_world.ml

open Core_kernel.Std
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" begin fun req ->
  `String ("Hello " ^ param req "name") |> respond'
end

let print_person = get "/person/:name/:age" begin fun req ->
  let person = {
    name = param req "name";
    age = "age" |> param req |> Int.of_string;
  } in
  `Json (person |> json_of_person |> Ezjsonm.wrap) |> respond'
end

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

compile with:

$ corebuild -pkg opium,cow.syntax hello_world.native

Middleware

The two fundamental building blocks of opium are:

• Handlers: Rock.Request.t -> Rock.Response.t Deferred.t
• Middleware: Rock.Handler.t -> Rock.Handler.t

Almost every 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 Core_kernel.Std
open Opium.Std
(* don't open cohttp and opium since they both define
   request/response modules*)

let is_substring ~substring s =
  Option.is_some (String.substr_index s ~pattern:substring)

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:(Info.of_string "reject_ua")

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

Compile with:

$ corebuild -pkg opium middleware_ua.native

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

# run in debug mode on port 9000
$ ./middleware_ua.native -p 9000 -d