When there are extra parentheses, display the exn

Dune tries to be helpful when there are extra parentheses, but sometimes
it is a bit too eager. This wraps the exception so that the original one
is displayed.

Closes #1173
Closes #1181

Signed-off-by: Etienne Millon <me@emillon.org>

CHANGES.md

@@ -36,6 +36,8 @@ next
 - Add an emacs mode providing helpers to promote correction files
   (#1192, @diml)
 
+- Improve message suggesting to remove parentheses (#1196, fix #1173, @emillon)
+
 1.1.1 (08/08/2018)
 ------------------
 

src/report_error.ml

@@ -17,6 +17,12 @@ let make_printer ?(backtrace=false) ?hint ?loc pp =
   ; backtrace
   }
 
+let set_loc p ~loc =
+  {p with loc = Some loc}
+
+let set_hint p ~hint =
+  {p with hint = Some hint}
+
 let builtin_printer = function
   | Dsexp.Of_sexp.Of_sexp (loc, msg, hint') ->
     let loc =

src/report_error.mli

@@ -20,8 +20,15 @@ val make_printer :
   (Format.formatter -> unit) ->
   printer
 
-(** Register an error reporter. *)
+val set_loc : printer -> loc:Loc.t -> printer
+
+val set_hint : printer -> hint:string -> printer
+
+(** Register an error printer. *)
 val register : (exn -> printer option) -> unit
 
+(** Find an error printer *)
+val find_printer : exn -> printer option
+
 (**/**)
 val map_fname : (string -> string) ref

src/stanza.ml

@@ -27,18 +27,22 @@ let file_kind () =
 module Of_sexp = struct
   include Dsexp.Of_sexp
 
-  exception Parens_no_longer_necessary of Loc.t
+  exception Parens_no_longer_necessary of Loc.t * exn
 
   let () =
     Report_error.register
       (function
-        | Parens_no_longer_necessary loc ->
-          let pp ppf =
-            Format.fprintf ppf
-              "These parentheses are no longer necessary with dune, \
-               please remove them.@\n"
+        | Parens_no_longer_necessary (loc, exn) ->
+          let hint =
+            "dune files require less parentheses than jbuild files.\n\
+             If you just converted this file from a jbuild file, try removing these parentheses."
           in
-          Some (Report_error.make_printer ~loc pp)
+          Option.map (Report_error.find_printer exn)
+            ~f:(fun printer ->
+              printer
+              |> Report_error.set_loc ~loc
+              |> Report_error.set_hint ~hint
+            )
         | _ -> None)
 
   let switch_file_kind ~jbuild ~dune =
@@ -61,12 +65,12 @@ module Of_sexp = struct
                   (if is_record then
                      peek >>= function
                      | Some (List _) ->
-                       raise (Parens_no_longer_necessary loc)
+                       raise (Parens_no_longer_necessary (loc, exn))
                      | _ -> t
                    else
                      t)
                   >>= fun _ ->
-                  raise (Parens_no_longer_necessary loc)))
+                  raise (Parens_no_longer_necessary (loc, exn))))
               (function
                 | Parens_no_longer_necessary _ as exn -> raise exn
                 | _ -> raise exn))

test/blackbox-tests/test-cases/too-many-parens/e/dune

@@ -0,0 +1,5 @@
+(alias
+ (name a)
+ (deps (glob *)) ; this form doesn't exist
+ (action (echo test))
+)

test/blackbox-tests/test-cases/too-many-parens/e/dune-project

@@ -0,0 +1 @@
+(lang dune 1.0)

test/blackbox-tests/test-cases/too-many-parens/run.t

@@ -8,25 +8,44 @@ are readable.
   3:   (public_name hello)
   4:   (libraries (lib))
   5: ))
-  These parentheses are no longer necessary with dune, please remove them.
+  Error: Atom expected
+  Hint: dune files require less parentheses than jbuild files.
+  If you just converted this file from a jbuild file, try removing these parentheses.
   [1]
 
   $ dune build --root b
   File "dune", line 4, characters 12-17:
    (libraries (lib)))
               ^^^^^
-  These parentheses are no longer necessary with dune, please remove them.
+  Error: 'select' expected
+  Hint: dune files require less parentheses than jbuild files.
+  If you just converted this file from a jbuild file, try removing these parentheses.
   [1]
 
   $ dune build --root c
   File "dune", line 3, characters 7-14:
    (deps (x y z)))
          ^^^^^^^
-  These parentheses are no longer necessary with dune, please remove them.
+  Error: Unknown constructor x
+  Hint: dune files require less parentheses than jbuild files.
+  If you just converted this file from a jbuild file, try removing these parentheses.
   [1]
 
 Checking that extra long stanzas (over 10 lines) are not printed
   $ dune build --root d
   File "dune", line 3, characters 13-192:
-  These parentheses are no longer necessary with dune, please remove them.
+  Error: 'select' expected
+  Hint: dune files require less parentheses than jbuild files.
+  If you just converted this file from a jbuild file, try removing these parentheses.
+  [1]
+
+When the inner syntax is wrong, do not warn about the parens:
+
+  $ dune build --root e
+  File "dune", line 3, characters 7-15:
+   (deps (glob *)) ; this form doesn't exist
+         ^^^^^^^^
+  Error: Unknown constructor glob
+  Hint: dune files require less parentheses than jbuild files.
+  If you just converted this file from a jbuild file, try removing these parentheses.
   [1]