let () =
  Dream.run ~debug:true
  @@ Dream.logger
  @@ Dream.router [

    Dream.get "/bad"
      (fun _ ->
        Dream.empty `Bad_Request);

    Dream.get "/fail"
      (fun _ ->
        raise (Failure "The web app failed!"));

  ]
  @@ Dream.not_found

$ dune exec --root . ./debug.exe

The rest of the app just adds two routes for triggering two kinds of failures that the debugger will detail. Visit http://localhost:8080/bad to trigger a 400 Bad Request response, and http://localhost:8080/fail to trigger an exception. The debugger will show reports like this:

(Failure "The web app failed!")
Raised at Stdlib__string.index_rec in file "string.ml", line 115, characters 19-34
Called from Sexplib0__Sexp.Printing.index_of_newline in file "src/sexp.ml", line 113, characters 13-47

From: Application
Blame: Server
Severity: Error

Client: 127.0.0.1:61988

GET /fail HTTP/1.1
Host: localhost:8080
Connection: keep-alive
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 [...snip...]
Accept: text/html,application/xhtml+xml, [...snip...]
Sec-GPC: 1
Sec-Fetch-Site: none
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Sec-Fetch-Dest: document
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US;q=0.9,en;q=0.8

dream.request_id.last_id: 2

As you can see, the report includes:

the error message,
a stack trace, if the error is an exception,
From: which part of the HTTP stack reported the error (TLS, HTTP, HTTP/2, WebSockets, or the app),
Blame: who is likely responsible for the error, the server or the client,
Severity: a suggested log level for the error,
Client: the client address,
request headers,
any request-scoped and application-scoped variables set in the request.

The debugger is disabled by default to avoid leaking information by accident in a production environment. Whether the debugger is enabled or disabled, Dream still writes error messages to the server-side log. The debugger is only about also sending error details to the client as reponses, which can be easier to work with in development.

Both the debugger's output and the non-debug error page are fully customizable — we will do this in the very next example!

Next steps:

9-error handles all errors in one place, also customizing the debugger.
a-log shows log levels and sub-logs.

Up to the tutorial index