Support ;%lwt as sequence syntax

src/ppx/ppx_lwt.ml

@@ -133,6 +133,14 @@ let gen_top_binds vbs =
   [Vb.mk (Pat.tuple (vbs |> List.map (fun { pvb_pat; _ } -> pvb_pat)))
      [%expr Lwt_main.run [%e gen_exp vbs 0]]]
 
+let lwt_sequence mapper ~lhs ~rhs =
+  let lhs, rhs = mapper.expr mapper lhs, mapper.expr mapper rhs in
+  if !debug then
+    [%expr Lwt.backtrace_bind (fun exn -> try raise exn with exn -> exn)
+              [%e lhs] (fun () -> [%e rhs])]
+  else
+    [%expr Lwt.bind [%e lhs] (fun () -> [%e rhs])]
+
 (** For expressions only *)
 (* We only expand the first level after a %lwt.
    After that, we call the mapper to expand sub-expressions. *)
@@ -141,6 +149,9 @@ let lwt_expression mapper exp attributes =
   let pexp_attributes = attributes @ exp.pexp_attributes in
   match exp.pexp_desc with
 
+  (* $e$;%lwt $e'$ ≡ [Lwt.bind $e$ (fun $p$ -> $e'$)] *)
+  | Pexp_sequence (lhs, rhs) ->
+     lwt_sequence mapper ~lhs ~rhs
   (* [let%lwt $p$ = $e$ in $e'$] ≡ [Lwt.bind $e$ (fun $p$ -> $e'$)] *)
   | Pexp_let (Nonrecursive, vbl , e) ->
     let new_exp =

src/ppx/ppx_lwt.mli

@@ -193,6 +193,16 @@ $ ocamlfind ocamlc -package lwt.ppx \
 
    {2 Sequence}
 
+   With OCaml 4.04.0 and on a convenient syntax is available for sequencing Lwt
+   operations using [;%lwt].  This allows free mixing of standard [;]
+   sequencing and [;%lwt] sequencing without extra parentheses:
+   {[
+     let i = ref 0 in
+     write stdout "Hello, ";%lwt
+     incr i;
+     write stdout "world!"
+   ]}
+
    It is also possible to sequence Lwt operations with the [>>] operator:
    {[
      write stdout "Hello, " >> write stdout "world!"

test/ppx/main.ml

@@ -163,6 +163,16 @@ let suite = suite "ppx" [
        (Lwt.return (!lst = [1;2]))
     ) ;
 
+  test "sequence with semicolon"
+    (fun () ->
+       let lst = ref [] in
+       lst := 2 :: !lst;
+       Lwt.return_unit;%lwt
+       lst := 1 :: !lst;
+       Lwt.return_unit;%lwt
+       Lwt.return (!lst = [1;2])
+    ) ;
+
   test "log"
     (fun () ->
        Lwt_log.ign_debug "bar";