A variant of https://pastebin.com/FNC2PMQP that works

dune

(executable
  (name FNC2PMQP)
  (public_name FNC2PMQP)
  (libraries cohttp cohttp-lwt cohttp-lwt-unix lwt))

FNC2PMQP.ml

open Lwt.Infix
open Cohttp
open Cohttp_lwt_unix
 
let body () =
  Client.get (Uri.of_string "https://hacker-news.firebaseio.com/v0/maxitem.json") >>= fun (resp, body) ->
  let code = resp |> Response.status |> Code.code_of_status in
  Lwt_io.printf "Response Code: %d\n" code >>= fun () ->
  Lwt_io.printf "Headers: %s\n" (resp |> Response.headers |> Header.to_string) >>= fun () ->
  body |> Cohttp_lwt.Body.to_string >>= fun body ->
  Lwt_io.printf "Body of length: %d\n" (String.length body) >|= fun () ->
  body
 
let getItem item =
  Client.get (Uri.of_string (String.concat "" ["https://hacker-news.firebaseio.com/v0/item/"; item; ".json?print=pretty"])) >>= fun (resp, body) ->
  let code = resp |> Response.status |> Code.code_of_status in
  Lwt_io.printf "Response Code: %d\n" code >>= fun () ->
  body |> Cohttp_lwt.Body.to_string >|= fun body ->
  body

let main () =
  body ()
  >>= getItem
  >>= Lwt_io.printf "Result: %s"
 
let () =
  Lwt_main.run @@ main ()