"Error: unbound module" in OCaml

Question

Here's a simple example of using the library Cohttp:

open Lwt
open Cohttp
open Cohttp_lwt_unix

let body =
  Client.get (Uri.of_string "http://www.reddit.com/") >>= fun (resp, body) ->
  let code = resp |> Response.status |> Code.code_of_status in
  Printf.printf "Response code: %d\n" code;
  Printf.printf "Headers: %s\n" (resp |> Response.headers |> Header.to_string);
  body |> Cohttp_lwt.Body.to_string >|= fun body ->
  Printf.printf "Body of length: %d\n" (String.length body);
  body

let () =
  let body = Lwt_main.run body in
  print_endline ("Received body\n" ^ body)

I'm trying to compile it

 ocaml my_test1.ml

Error:

Error: Unbound module Lwt

How to actually include/require the module Lwt into my app?

update

Also:

$ ocamlbuild
bash: ocamlbuild: command not found

But:

$ opam install ocamlbuild
[NOTE] Package ocamlbuild is already installed (current version is
       0.12.0).

And

$ opam install ocamlfind
[NOTE] Package ocamlfind is already installed (current version is
       1.7.3-1).

And

$ ocamlfind
bash: ocamlfind: command not found

Where are ocamlfind and ocamlbuild located?

update2

$ ocamlfind ocamlc -package lwt -c my_test1.ml 
 File "my_test1.ml", line 2, characters 5-11:
 Error: Unbound module Cohttp

Answer 1

You have several options depending on your needs.

1) If you want to create a full project for your binary I recommend looking at jbuilder. Here is a very nice guide that explains the environment/project configuration step-by-step: OCaml for the impatient.

2) Another option is to compile the binary directly as you were trying to do:

ocamlbuild -pkg lwt -pkg cohttp-lwt-unix my_test1.native

Note that you need to have a file named my_test1.ml to generate the requested my_test1.native.

3) And finally for quick scripts I find it handy to be able to ask the OCaml interpreter to load the dependencies directly in the source file. Just add the following to the beginning of your file:

#use "topfind";;
#require "lwt";;
#require "cohttp-lwt-unix";;

And then run ocaml my_test1.ml.

---

Hope this helps! :)

Also looking at the command not found errors you are getting I can suggest to make sure your environment is correctly configured. The Real World OCaml book has a wiki page for that: https://github.com/realworldocaml/book/wiki/Installation-Instructions