MirageOS - Http-fetch example

Question

I'm trying to modify a bit the MirageOS http-fetch example (https://github.com/mirage/mirage-skeleton) that can be found inside mirage-skeleton but I'm having some problems understanding why I can't move some of the function executed inside the config.ml file to my unikernel.ml file. The original config.ml file follows (I'll copy just the interesting part) :

[...]
let client =
      foreign "Unikernel.Client" @@ console @-> resolver @-> conduit @-> job

let () =
  add_to_ocamlfind_libraries ["mirage-http"];
  add_to_opam_packages ["mirage-http"];
  let sv4 = stack default_console in
  let res_dns = resolver_dns sv4 in
  let conduit = conduit_direct sv4 in
  let job =  [ client $ default_console $ res_dns $ conduit ] in
  register "http-fetch" job

What I'm trying to do is move these two lines :

  let res_dns = resolver_dns sv4 in
  let conduit = conduit_direct sv4 in

into my unikernel.ml start method. Basically I want to pass to my module just the stack and let it create a dns resolver and a conduit. My start function follows:

  let start c s =
    C.log_s c (sprintf "Resolving in 1s using DNS server %s" ns) >>= fun () ->
    OS.Time.sleep 1.0 >>= fun () ->
    let res_dns = resolver_dns s in
        let conduit = conduit_direct s in 
            http_fetch c res_dns conduit >>= fun (data) ->
            Lwt.return(dump_to_db data);

Right now I'm getting this error at http_fetch parameters submission:

Error: This expression has type Mirage.resolver Mirage.impl
       but an expression was expected of type Resolver_lwt.t

What I'm asking here is mostly a conceptual question because I'm clearly missing something. I'm not an expert in OCaml/MirageOS but this controversial behaviour of type mismatch is hard to understand considering that I'm just calling the same function from a different file.

Answer 1

config.ml is used to generate main.ml. You can copy the generated code from there if you want.