Use failwith instead of Lwt.fail_with

Lwt's documentation reads:

> In most cases, it is better to use `failwith s` from the standard
> library.

and

> Whenever possible, it is recommended to use `raise exn` instead, as
> raise captures a backtrace, while `Lwt.fail` does not. If you call
> `raise exn` in a callback that is expected by Lwt to return a
> promise, Lwt will automatically wrap `exn` in a rejected promise,
> but the backtrace will have been recorded by the OCaml runtime.
>
> For example, `bind`'s second argument is a callback which returns a
> promise. And so it is recommended to use `raise` in the body of that
> callback.
>
> Use `Lwt.fail` only when you specifically want to create a rejected
> promise, to pass to another function, or store in a data structure.

Prefer to capture backtraces to improve debugability.

README.md

@@ -154,7 +154,7 @@ let compute ~time ~f =
 let body =
   let get () = Client.get (Uri.of_string "https://www.reddit.com/") in
   compute ~time:0.1 ~f:get >>= function
-  | `Timeout -> Lwt.fail_with "Timeout expired"
+  | `Timeout -> failwith "Timeout expired"
   | `Done (resp, body) -> Lwt.return (resp, body)
 ```
 
@@ -174,7 +174,7 @@ For example,
 ```ocaml
 let get_body ~uri ~timeout =
     let%bind _, body = Cohttp_async.Client.get ~interrupt:(after (sec timeout)) uri in
-    Body.to_string body    
+    Body.to_string body
 
 let body =
   let uri = Uri.of_string "https://www.reddit.com/" in
@@ -275,19 +275,18 @@ and follow_redirect ~max_redirects request_uri (response, body) =
       handle_redirect ~permanent:true ~max_redirects request_uri response
   | `Found | `Temporary_redirect ->
       handle_redirect ~permanent:false ~max_redirects request_uri response
-  | `Not_found | `Gone -> Lwt.fail_with "Not found"
+  | `Not_found | `Gone -> failwith "Not found"
   | status ->
-      Lwt.fail_with
-        (Printf.sprintf "Unhandled status: %s"
-           (Cohttp.Code.string_of_status status))
+      Printf.ksprintf failwith "Unhandled status: %s"
+          (Cohttp.Code.string_of_status status)
 
 and handle_redirect ~permanent ~max_redirects request_uri response =
-  if max_redirects <= 0 then Lwt.fail_with "Too many redirects"
+  if max_redirects <= 0 then failwith "Too many redirects"
   else
     let headers = Http.Response.headers response in
     let location = Http.Header.get headers "location" in
     match location with
-    | None -> Lwt.fail_with "Redirection without Location header"
+    | None -> failwith "Redirection without Location header"
     | Some url ->
         let open Lwt.Syntax in
         let uri = Uri.of_string url in

cohttp-lwt-jsoo/src/cohttp_lwt_jsoo.ml

@@ -269,7 +269,7 @@ module Make_client_async (P : Params) = Make_api (struct
         CLB.to_string body >>= fun body ->
         let bs = binary_string body in
         (*Js.Opt.case (File.CoerceTo.blob (Obj.magic blob))
-          (fun () -> Lwt.fail_with "could not coerce to blob")
+          (fun () -> failwith "could not coerce to blob")
           (fun blob -> Lwt.return (xml##(send_blob blob)))*)
         (*Lwt.return (xml##send (Js.Opt.return bs)) *)
         Lwt.return (xml##send (Js.Opt.return (Obj.magic bs))))

cohttp-lwt-unix/examples/client_lwt_timeout.ml

@@ -11,7 +11,7 @@ let compute ~time ~f =
 let body =
   let get () = Client.get (Uri.of_string "https://www.reddit.com/") in
   compute ~time:0.1 ~f:get >>= function
-  | `Timeout -> Lwt.fail_with "Timeout expired"
+  | `Timeout -> failwith "Timeout expired"
   | `Done (resp, body) ->
       let code = resp |> Response.status |> Code.code_of_status in
       Printf.printf "Response code: %d\n" code;

cohttp-lwt-unix/test/cohttp_lwt_unix_test/src/cohttp_lwt_unix_test.ml

@@ -20,7 +20,7 @@ let expert ?(rsp = Http.Response.make ()) f _req _body =
   return (`Expert (rsp, f))
 
 let const rsp _req _body = rsp >|= response
-let response_sequence = Cohttp_test.response_sequence Lwt.fail_with
+let response_sequence = Cohttp_test.response_sequence failwith
 let () = Debug.activate_debug ()
 let () = Logs.set_level (Some Info)
 

cohttp-lwt-unix/test/test_client.ml

@@ -54,7 +54,7 @@ let methods (handler : Cohttp_lwt.S.call) uri =
     Body.drain_body body >>= fun () ->
     match Response.status res with
     | `Created | `No_content | `OK -> Lwt.return_unit
-    | _ -> Lwt.fail_with "put failed"
+    | _ -> failwith "put failed"
   and get k =
     handler `GET Uri.(with_path uri k) >>= fun (res, body) ->
     match Response.status res with

cohttp-mirage/src/io.ml

@@ -54,7 +54,7 @@ module Make (Channel : Mirage_channel.S) = struct
     Channel.write_string oc buf 0 (String.length buf);
     Channel.flush oc >>= function
     | Ok () -> Lwt.return_unit
-    | Error `Closed -> Lwt.fail_with "Trying to write on closed channel"
+    | Error `Closed -> failwith "Trying to write on closed channel"
     | Error e -> Lwt.fail (Write_exn e)
 
   let flush _ =

cohttp-mirage/src/static.ml

@@ -24,7 +24,7 @@ module HTTP (FS : Mirage_kv.RO) (S : Cohttp_lwt.S.Server) = struct
   open Lwt.Infix
   open Astring
 
-  let failf fmt = Fmt.kstr Lwt.fail_with fmt
+  let failf fmt = Fmt.failwith fmt
 
   let read_fs t name =
     FS.get t (Key.v name) >>= function