"Error: unbound module" in OCaml - 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

Answer 2

Try to compile it like this ...
ocamlfind ocamlopt -o my_test \
-linkpkg \
-package lwt,cohttp,cohttp-lwt-unix \
-thread
my_test.ml