Improved --compare-messages to also compare the messages (if non-default).

src/LRijkstra.ml

@@ -1143,13 +1143,13 @@ let display (nt, w, s') : unit =
      blank line, followed with a proposed error message, followed with
      another blank line. *)
   Printf.printf
-    "%s: %s\n# Length: %d\n# Leads to an error in state: %d.\n%s\n%s\n\n"
+    "%s: %s\n# Length: %d\n# Leads to an error in state: %d.\n%s\n%s\n"
     (Nonterminal.print false nt)
     (W.print w)
     (W.length w)
     (Lr1.number s')
     (Lr0.print_closure "# " (Lr1.state s'))
-    "Syntax error."
+    Interpret.default_message
 
 (* Perform the forward search. *)
 

src/LRijkstra.mli

@@ -3,8 +3,8 @@
    an error, a minimal path (starting in some initial state) that actually
    triggers this error. *)
 
-(* The result of this analysis is written to the standard output channel.
-   No result is returned. *)
+(* The result of this analysis is a [.messages] file. It is written to the
+   standard output channel. No result is returned. *)
 
 module Run (X : sig
   (* If [verbose] is set, produce various messages on [stderr]. *)

src/interpret.ml

@@ -461,6 +461,9 @@ let () =
    state that appears on the left-hand side appears on the right-hand side as
    well. *)
 
+let default_message =
+  "<YOUR SYNTAX ERROR MESSAGE HERE>\n"
+
 let () =
   Settings.compare_errors |> Option.iter (fun (filename1, filename2) ->
 
@@ -484,15 +487,23 @@ let () =
 
     (* Check that [table1] is a subset of [table2], that is, for every state
        [s] in the domain of [table1], [s] is mapped by [table1] and [table2]
-       to the same error message. *)
+       to the same error message. As an exception, if the message found in
+       [table1] is the default message, then no comparison takes place. This
+       allows using [--list-errors] and [--compare-errors] in conjunction to
+       ensure that a [.messages] file is complete, without seeing warnings
+       about different messages. *)
     table1 |> Lr1.NodeMap.iter (fun s ((poss1, _), message1) ->
-      let (poss2, _), message2 = Lr1.NodeMap.find s table2 in
-      if message1 <> message2 then
-        Error.warning (poss1 @ poss2) (Printf.sprintf
-          "These sentences lead to an error in state %d.\n\
-           The corresponding messages in \"%s\" and \"%s\" differ."
-          (Lr1.number s) filename1 filename2
-        )
+      if message1 <> default_message then
+        try
+          let (poss2, _), message2 = Lr1.NodeMap.find s table2 in
+          if message1 <> message2 then
+            Error.warning (poss1 @ poss2) (Printf.sprintf
+              "These sentences lead to an error in state %d.\n\
+               The corresponding messages in \"%s\" and \"%s\" differ."
+              (Lr1.number s) filename1 filename2
+            )
+        with Not_found ->
+          ()
     );
 
     if Error.errors() then exit 1;

src/interpret.mli

-(* This module is in charge of handling the [--interpret] option,
-   if it is present. It offers no functionality. *)
+(* This module is in charge of handling several command line options, namely
+   [--interpret], [--interpret-error], [--compile-errors], [--compare-errors].
+   If any of these options is present, the execution of Menhir stops here. *)
+
+(* This default error message is produced by [--list-errors] when it creates a
+   [.messages] file, and is recognized by [--compare-errors] when it compares
+   two such files. *)
+
+val default_message: string