Why doesn't try catch work with Lwt?

Learning
1

I am executing this code which works fine.

trailsList
  |> List.map buildUrl
  |> split 50

If I add 1 more step to this pipeline.

trailsList
  |> List.map buildUrl
  |> split 50
  |> Lwt_list.map_p(make_http_call);;

The code runs for all 800 items in my list but right at the end prints

"Exception:

Failure
“TLS to non-TCP currently unsupported: host=api.weather.gov endp=(Unknown "name resolution failed")”."

And other times it fails with

“Fatal error: exception SSL read() error: error:00000000:lib(0)::reason(0)”

In each case, there is no way for me to troubleshoot because I don’t know which line in my 800 item input URL list is causing this failure.

The implementation of make_http_calls is a little long but I have put try catch block on every step but the code never hits my catch block

let make_http_call ?(wait=5) = function 
  | Process(list) -> 
    list 
    |> List.map(fun str -> 
      try 
        str |> Uri.of_string |> Option.some
      with 
      | exn -> 
        "could not generate URI " ^
        "because of exception " ^
        (exn |> Printexc.to_string) 
        |> print_endline;
        None
    ) 
    |> Util.remove_none
    |> Lwt_list.map_p (fun uri -> 
      try
        uri |> Client.get |> Lwt.map(Option.some)
      with 
      | exn -> 
        "could not get data for " ^ 
        (uri |> Uri.to_string) ^ 
        "because of exception " ^ 
        (exn |> Printexc.to_string) 
        |> Lwt_io.(write stdout) |> Lwt.map( fun _ -> None)
    )
    |> Lwt.map(Util.remove_none)
    >>= (fun list -> 
      list |> Lwt_list.map_p(fun (resp, body) -> 
        let code = resp |> Response.status |> Cohttp.Code.code_of_status in
        if code == 200 then 
          try 
            body |> Cohttp_lwt.Body.to_string |> Lwt.map (Option.some)
          with 
          | exn -> exn |> Printexc.to_string |> Lwt_io.(write stdout) |> Lwt.map ( fun _ -> None)
        else begin 
          "did not get 200 response" |> Lwt_io.(write stdout) |> Lwt.map ( fun _ -> None )
        end
      )
    )
  | Wait -> 
    print_endline("going to wait now for " ^ (wait |> Int.to_string) ^ " seconds");Unix.sleep(wait); [None] |> Lwt.return

Why is it that the code fails without hitting any of my catch blocks? how to even troubleshoot where my code is failing.

2

In Lwt code exceptions are not thrown, promises are rejected. They have to be handled using operations provided by the Lwt library. See Module Lwt

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