Tutorial 5-promise does not support its rationale/might be deprecated

[pierre-ludmann-depinfonancy]

Hi!

I was going through the tutorial exemples to discover Dream and I keep thinking about 5-promise. Its rationale is to able computation after inner handling of the request; while previous example was limited to computation beforehand. It conveys the idea this is thanks to Lwt. Here is a trace log from the current example:

% dune exec --root . ./promise.exe
12.04.25 01:28:52.717                       Running at http://localhost:8080
12.04.25 01:28:52.717                       Type Ctrl+C to stop
12.04.25 01:28:56.081    dream.logger  INFO REQ 1 GET / ::1:55144 fd 8 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0
12.04.25 01:28:56.081    dream.logger  INFO REQ 1 200 in 128 μs
12.04.25 01:28:57.324    dream.logger  INFO REQ 2 GET / ::1:55144 fd 8 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0
12.04.25 01:28:57.324    dream.logger  INFO REQ 2 200 in 193 μs
12.04.25 01:29:01.223    dream.logger  INFO REQ 3 GET / ::1:55144 fd 8 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0
12.04.25 01:29:01.224    dream.logger  INFO REQ 3 200 in 111 μs
12.04.25 01:29:04.895    dream.logger  INFO REQ 4 GET /fail ::1:55144 fd 8 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0
12.04.25 01:29:04.895    dream.logger  WARN REQ 4 Aborted by: Failure("The Web app failed!")
12.04.25 01:29:04.895    dream.logger  WARN Raised at Dune__exe__Promise.count_requests.(fun) in file "promise.ml", line 12, characters 4-13
12.04.25 01:29:04.895    dream.logger  WARN Called from Lwt.Sequence_associated_storage.with_value in file "src/core/lwt.ml", line 804, characters 19-23
12.04.25 01:29:04.895    dream.logger  WARN Re-raised at Lwt.Sequence_associated_storage.with_value in file "src/core/lwt.ml", line 809, characters 6-15
12.04.25 01:29:04.895    dream.logger  WARN Called from Lwt.Sequential_composition.try_bind in file "src/core/lwt.ml", line 2139, characters 10-14
12.04.25 01:29:04.895      dream.http ERROR REQ 4 Failure("The Web app failed!")
12.04.25 01:29:04.895      dream.http ERROR REQ 4 Raised at Stdlib__Map.Make.find in file "map.ml", line 141, characters 10-25
12.04.25 01:29:04.895      dream.http ERROR REQ 4 Called from Logs.Tag.find in file "src/logs.ml", line 153, characters 14-32
12.04.25 01:29:07.056    dream.logger  INFO REQ 5 GET / ::1:55144 fd 8 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0
12.04.25 01:29:07.057    dream.logger  INFO REQ 5 200 in 134 μs

However, when I remove Lwt, the behavior is the same (save for some ERROR logs):

% git diff promise.ml             
diff --git a/example/5-promise/promise.ml b/example/5-promise/promise.ml
index ffa2790..bebfea3 100644
--- a/example/5-promise/promise.ml
+++ b/example/5-promise/promise.ml
@@ -2,10 +2,10 @@ let successful = ref 0
 let failed = ref 0
 
 let count_requests inner_handler request =
-  try%lwt
-    let%lwt response = inner_handler request in
+  try
+    let response = inner_handler request in
     successful := !successful + 1;
-    Lwt.return response
+    response
 
   with exn ->
     failed := !failed + 1;
% dune exec --root . ./promise.exe
12.04.25 01:31:42.248                       Running at http://localhost:8080
12.04.25 01:31:42.248                       Type Ctrl+C to stop
12.04.25 01:31:50.938    dream.logger  INFO REQ 1 GET / ::1:55152 fd 8 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0
12.04.25 01:31:50.938    dream.logger  INFO REQ 1 200 in 89 μs
12.04.25 01:31:52.675    dream.logger  INFO REQ 2 GET / ::1:55152 fd 8 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0
12.04.25 01:31:52.675    dream.logger  INFO REQ 2 200 in 79 μs
12.04.25 01:31:57.211    dream.logger  INFO REQ 3 GET / ::1:55152 fd 8 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0
12.04.25 01:31:57.212    dream.logger  INFO REQ 3 200 in 397 μs
12.04.25 01:32:01.724    dream.logger  INFO REQ 4 GET /fail ::1:55152 fd 8 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0
12.04.25 01:32:01.724    dream.logger  WARN REQ 4 Aborted by: Failure("The Web app failed!")
12.04.25 01:32:01.724    dream.logger  WARN Raised at Dune__exe__Promise.(fun) in file "promise.ml", line 22, characters 8-45
12.04.25 01:32:01.724    dream.logger  WARN Called from Dune__exe__Promise.count_requests in file "promise.ml", line 6, characters 19-40
12.04.25 01:32:01.724    dream.logger  WARN Re-raised at Dune__exe__Promise.count_requests in file "promise.ml", line 12, characters 4-13
12.04.25 01:32:01.724    dream.logger  WARN Called from Lwt.Sequence_associated_storage.with_value in file "src/core/lwt.ml", line 804, characters 19-23
12.04.25 01:32:01.725    dream.logger  WARN Re-raised at Lwt.Sequence_associated_storage.with_value in file "src/core/lwt.ml", line 809, characters 6-15
12.04.25 01:32:01.725    dream.logger  WARN Called from Lwt.Sequential_composition.try_bind in file "src/core/lwt.ml", line 2139, characters 10-14
12.04.25 01:32:01.725      dream.http ERROR REQ 4 Failure("The Web app failed!")
12.04.25 01:32:01.725      dream.http ERROR REQ 4 Raised at Stdlib__Map.Make.find in file "map.ml", line 141, characters 10-25
12.04.25 01:32:01.725      dream.http ERROR REQ 4 Called from Logs.Tag.find in file "src/logs.ml", line 153, characters 14-32
12.04.25 01:32:04.631    dream.logger  INFO REQ 5 GET / ::1:55152 fd 8 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0
12.04.25 01:32:04.631    dream.logger  INFO REQ 5 200 in 124 μs

As such, I feel misled by this tutorial example. Did that example deprecate? Am I daft and missing something entirely?

Thank you for reading and for the dream

[aantron]

Without awaiting the promise returned by inner_handler request, the observation that the behavior is the same is just an observation of a race condition. It's only accidentally the same in your testing. Depending on whether the computation represented by inner_promise gets descheduled or not, you would see the counter incremented before or after the computation completes.

[pierre-ludmann-depinfonancy]

Ok. My lack of familiarity with Lwt got me. With your answer I device the following bit to force the observation to differ:

diff --git a/example/5-promise/promise.ml b/example/5-promise/promise.ml
index ffa2790..6b8f86e 100644
--- a/example/5-promise/promise.ml
+++ b/example/5-promise/promise.ml
@@ -2,10 +2,10 @@ let successful = ref 0
 let failed = ref 0
 
 let count_requests inner_handler request =
-  try%lwt
-    let%lwt response = inner_handler request in
+  try
+    let response = inner_handler request in
     successful := !successful + 1;
-    Lwt.return response
+    response
 
   with exn ->
     failed := !failed + 1;
@@ -22,6 +22,7 @@ let () =
         raise (Failure "The Web app failed!"));
 
     Dream.get "/" (fun _ ->
+        let%lwt () = Lwt_unix.sleep 1. in
       Dream.html (Printf.sprintf
         "%3i request(s) successful<br>%3i request(s) failed"
         !successful !failed));

Could be nice to add it to the example, or not?

[aantron]

Yes, maybe I will add a note explaining this to the example. Thank you!