"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

Pforzheim

Possible duplicate of Difference between module and package Ocaml – Came

@Came it's not – Pforzheim

did you run 'eval $(opam config env)' before running your commands ? (it should solve 'command not found' issues). Regarding lwt and cohttp, you may miss some package on the ocamlbuild commands (-pkg lwt and so on) – Mule

@PierreG., no. Where is it said that I need to run it? – Pforzheim

@PierreG., see my update – Pforzheim

1st question : generally this is what opam tells after running 'opam switch <compiler_version>' , so not your case. 2nd question : from github.com/mirage/ocaml-cohttp : 'ocamlbuild -pkg cohttp-lwt-unix client_example.native'. so I guess 'ocamlfind ocamlc -package cohttp-lwt-unix -c my_test1.ml' might work (... I do not have ocaml environment under my hand currently) – Mule

@PierreG., thx. – Pforzheim

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

Copacetic

and cohttp_lwt_unix is available thx to 'opam install cohttp-lwt-unix' (underscore '_' of the package name becomes a dash '-'). – Mule

i'd run "opam init" before I posted my question – Pforzheim

opam init will update your shell profile to include the correct env settings. But the current shell will still have the old environment. So you need to run "eval opam config env" to load the environment in the current shell (with backquote around "opam config env"). – Copacetic

2) Unknown directive `require'. – Pforzheim

$ opam install topfind ====> [ERROR] No package named topfind found. – Pforzheim

I'm pretty sure your environment is not correctly set up. Do you have an an ~/.ocamlinit file with the following content? (* Added by OPAM. *) let () = try Topdirs.dir_directory (Sys.getenv "OCAML_TOPLEVEL_PATH") with Not_found -> () ;; – Copacetic

Run opam init and try adding "eval opam config env" to your shell's init file (~/bashrc if your'e using bash). – Copacetic

building with dune fixed it for me – Morgun

Answer 2

Try to compile it like this ...

ocamlfind ocamlopt -o my_test \ 
   -linkpkg \ 
   -package lwt,cohttp,cohttp-lwt-unix \
   -thread 
    my_test.ml

Truce