diff --git a/CHANGES.md b/CHANGES.md
index 1b730a0..a259183 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,7 @@
+## Unreleased
+
+- Add support of dune formatter. (#3)
+
 ## 0.1 (2019-02-19)
 
-- Initial release.
\ No newline at end of file
+- Initial release.
diff --git a/README.md b/README.md
index 75f223a..ef4c2a6 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,7 @@ code formatters.
 `merge-fmt` currently only knows about the following formatters:
 - [ocamlformat](https://github.com/ocaml-ppx/ocamlformat) for OCaml.
 - [refmt](https://github.com/facebook/reason) for reason.
+- [dune](https://github.com/ocaml/dune) for dune.
 
 Note that supporting new code formatters is trivial.
 
@@ -47,4 +48,4 @@ Install
 -------
 ```sh
 $ opam pin add merge-fmt git@github.com:hhugo/merge-fmt.git
-```
\ No newline at end of file
+```
diff --git a/merge-fmt-help.txt b/merge-fmt-help.txt
index b142c1f..c93030c 100644
--- a/merge-fmt-help.txt
+++ b/merge-fmt-help.txt
@@ -17,6 +17,9 @@ COMMANDS
            Register the [merge-fmt] mergetool in git
 
 OPTIONS
+       --dune=VAL
+           dune path
+
        --echo
            Echo all commands.
 
diff --git a/merge-fmt-mergetool-help.txt b/merge-fmt-mergetool-help.txt
index 2ef1e7a..c85055a 100644
--- a/merge-fmt-mergetool-help.txt
+++ b/merge-fmt-mergetool-help.txt
@@ -9,6 +9,9 @@ OPTIONS
 
        --current=<current-file>
 
+       --dune=VAL
+           dune path
+
        --echo
            Echo all commands.
 
diff --git a/src/fmters.ml b/src/fmters.ml
index ef4f092..e190f4b 100644
--- a/src/fmters.ml
+++ b/src/fmters.ml
@@ -4,26 +4,62 @@ open Common
 type config =
   { ocamlformat_path : string option
   ; refmt_path : string option
+  ; dune_path : string option
   }
 
-type t = string
+type t =
+  | Inplace of string
+  | Stdout of string
+
+let transfer ic oc =
+  let b = Bytes.create 4096 in
+  let rec loop () =
+    match Stdlib.input ic b 0 (Bytes.length b) with
+    | 0 -> ()
+    | l ->
+        Stdlib.output oc b 0 l;
+        loop ()
+  in
+  loop ()
 
 let ocamlformat ~bin ~name =
-  sprintf "%s -i %s"
-    (Option.value ~default:"ocamlformat" bin)
-    (Option.value_map ~default:"" ~f:(fun name -> " --name=" ^ name) name)
+  Inplace
+    (sprintf "%s -i %s"
+       (Option.value ~default:"ocamlformat" bin)
+       (Option.value_map ~default:"" ~f:(fun name -> " --name=" ^ name) name))
+
+let refmt ~bin =
+  Inplace (sprintf "%s --inplace" (Option.value ~default:"refmt" bin))
 
-let refmt ~bin = sprintf "%s --inplace" (Option.value ~default:"refmt" bin)
+let dune ~bin =
+  Stdout (sprintf "%s format-dune-file --" (Option.value ~default:"dune" bin))
 
 let find ~config ~filename ~name =
   let filename = Option.value ~default:filename name in
-  match (Caml.Filename.extension filename, config) with
-  | (".ml" | ".mli"), { ocamlformat_path; _ } ->
+  match (filename, Caml.Filename.extension filename, config) with
+  | _, (".ml" | ".mli"), { ocamlformat_path; _ } ->
       Some (ocamlformat ~bin:ocamlformat_path ~name)
-  | (".re" | ".rei"), { refmt_path; _ } -> Some (refmt ~bin:refmt_path)
+  | _, (".re" | ".rei"), { refmt_path; _ } -> Some (refmt ~bin:refmt_path)
+  | ("dune" | "dune-project" | "dune-workspace"), "", { dune_path; _ } ->
+      Some (dune ~bin:dune_path)
   | _ -> None
 
-let run t ~echo ~filename = system ~echo "%s %s" t filename
+let run t ~echo ~filename =
+  match t with
+  | Inplace t -> system ~echo "%s %s" t filename
+  | Stdout t -> (
+      let ic = open_process_in ~echo "%s %s" t filename in
+      let tmp_file, oc = Stdlib.Filename.open_temp_file "merge-fmt" "stdout" in
+      transfer ic oc;
+      Stdlib.close_out oc;
+      match Unix.close_process_in ic with
+      | WEXITED 0 ->
+          Stdlib.Sys.rename tmp_file filename;
+          Ok ()
+      | WEXITED n ->
+          Stdlib.Printf.eprintf ">>> Exit with %d\n" n;
+          Error ()
+      | WSIGNALED _ | WSTOPPED _ -> Error ())
 
 module Flags = struct
   open Cmdliner
@@ -36,9 +72,13 @@ module Flags = struct
     let doc = "refmt path" in
     Arg.(value & opt (some string) None & info [ "refmt" ] ~doc)
 
+  let dune_path =
+    let doc = "dune path" in
+    Arg.(value & opt (some string) None & info [ "dune" ] ~doc)
+
   let t =
     Term.(
-      const (fun ocamlformat_path refmt_path ->
-          { ocamlformat_path; refmt_path })
-      $ ocamlformat_path $ refmt_path)
+      const (fun ocamlformat_path refmt_path dune_path ->
+          { ocamlformat_path; refmt_path; dune_path })
+      $ ocamlformat_path $ refmt_path $ dune_path)
 end
diff --git a/src/resolve_cmd.ml b/src/resolve_cmd.ml
index a8460b3..bd7f5e3 100644
--- a/src/resolve_cmd.ml
+++ b/src/resolve_cmd.ml
@@ -67,8 +67,10 @@ let show ~echo version versions =
 
 let create_tmp ~echo fn version versions =
   let content = show ~echo version versions in
-  let ext = Caml.Filename.extension fn
-  and base = Caml.Filename.chop_extension fn in
+  let ext = Caml.Filename.extension fn in
+  let base =
+    if String.equal ext "" then fn else Caml.Filename.chop_extension fn
+  in
   let fn' = sprintf "%s.%s%s" base (string_of_version version) ext in
   let oc = Out_channel.create fn' in
   Out_channel.output_string oc content;
@@ -123,7 +125,9 @@ let resolve config echo () =
                 ~f:(fun () -> git_add ~echo ~filename)
               |> (ignore : (unit, unit) Result.t -> unit);
               let n2 = conflict ~filename in
-              eprintf "Resolved %d/%d %s\n%!" (n1 - n2) n1 filename
+              if n2 > n1
+              then eprintf "Resolved ?? %s\n%!" filename
+              else eprintf "Resolved %d/%d %s\n%!" (n1 - n2) n1 filename
           | None -> eprintf "Ignore %s (no formatter register)\n%!" filename)
       | Error reason -> eprintf "Ignore %s (%s)\n%!" filename reason);
   let all = ls ~echo () in
diff --git a/test/merge_dune.ml b/test/merge_dune.ml
new file mode 100644
index 0000000..5dbdecf
--- /dev/null
+++ b/test/merge_dune.ml
@@ -0,0 +1,177 @@
+open! Base
+open! Stdio
+open! Common
+
+let%expect_test "default merge tool" =
+  within_temp_dir (fun () ->
+      git_init ();
+
+      write "dune" {|
+(executable
+(name aaaa)
+(libraries unix))
+|};
+      git_commit "first commit";
+      git_branch "branch1";
+
+      (* add public name *)
+      if false
+      then (
+        write "dune"
+          {|
+(executable
+(name aaaa)
+(libraries unix)
+(public_name pppp))
+|};
+        git_commit "second commit");
+
+      (* add library *)
+      write "dune"
+        {|
+(executable
+(name aaaa)
+(libraries unix))
+
+(library
+(name liblib))
+|};
+      git_commit "third commit";
+      git_branch "branch2";
+
+      git_checkout "branch1";
+      [%expect {| Switched to branch 'branch1' |}];
+      (* change name to b, reformat *)
+      write "dune" {|
+(executable
+ (name bbbb)
+ (libraries unix))
+|};
+      git_commit "second commit (fork)";
+
+      write "dune"
+        {|
+(alias
+ (name runtest)
+ (action
+  (diff rebase.diff rebase.diff.gen)))
+
+(executable
+ (name bbbb)
+ (libraries unix))
+|};
+
+      git_commit "third commit (fork)";
+      git_branch "old_branch1";
+      system "git rebase branch2 -q";
+      [%expect
+        {|
+        Auto-merging dune
+        CONFLICT (content): Merge conflict in dune
+        error: could not apply 2e00bc1... second commit (fork)
+        hint: Resolve all conflicts manually, mark them as resolved with
+        hint: "git add/rm <conflicted_files>", then run "git rebase --continue".
+        hint: You can instead skip this commit: run "git rebase --skip".
+        hint: To abort and get back to the state before "git rebase", run "git rebase --abort".
+        Could not apply 2e00bc1... second commit (fork)
+        Exit with 1 |}];
+      print_file "dune";
+      [%expect
+        {|
+        File dune
+
+        (executable
+        <<<<<<< HEAD
+        (name aaaa)
+        (libraries unix))
+
+        (library
+        (name liblib))
+        =======
+         (name bbbb)
+         (libraries unix))
+        >>>>>>> 2e00bc1 (second commit (fork)) |}])
+
+let%expect_test "custom merge tool" =
+  within_temp_dir (fun () ->
+      git_init ();
+      system "%s setup-merge --merge-fmt-path %s --update" merge_fmt merge_fmt;
+      [%expect {||}];
+
+      write "dune" {|
+(executable
+(name aaaa)
+(libraries unix))
+|};
+      git_commit "first commit";
+      git_branch "branch1";
+
+      (* add public name *)
+      if false
+      then (
+        write "dune"
+          {|
+(executable
+(name aaaa)
+(libraries unix)
+(public_name pppp))
+|};
+        git_commit "second commit");
+
+      (* add library *)
+      write "dune"
+        {|
+(executable
+(name aaaa)
+(libraries unix))
+
+(library
+(name liblib))
+|};
+      git_commit "third commit";
+      git_branch "branch2";
+
+      git_checkout "branch1";
+      [%expect {| Switched to branch 'branch1' |}];
+      (* change name to b, reformat *)
+      write "dune" {|
+(executable
+ (name bbbb)
+ (libraries unix))
+|};
+      git_commit "second commit (fork)";
+
+      write "dune"
+        {|
+(alias
+ (name runtest)
+ (action
+  (diff rebase.diff rebase.diff.gen)))
+
+(executable
+ (name bbbb)
+ (libraries unix))
+|};
+      git_commit "third commit (fork)";
+      git_branch "old_branch1";
+
+      system "git rebase branch2 -q";
+      [%expect {| |}];
+      system "%s" merge_fmt;
+      print_file "dune";
+      [%expect
+        {|
+        Nothing to resolve
+        Exit with 1
+        File dune
+        (alias
+         (name runtest)
+         (action
+          (diff rebase.diff rebase.diff.gen)))
+
+        (executable
+         (name bbbb)
+         (libraries unix))
+
+        (library
+         (name liblib)) |}])
diff --git a/test/rebase.diff b/test/rebase.diff
index 1884675..c9c9764 100644
--- a/test/rebase.diff
+++ b/test/rebase.diff
@@ -28,19 +28,20 @@
 ---
 >           }
 >         >>>>>>> 6070a8f (second commit (fork)) |}];
-59,62c69
+59,62c69,70
 <       [%expect
 <         {|
 <         Ignore a.ml (not a 3-way merge)
 <         Exit with 1 |}];
 ---
->       [%expect {| Resolved 1/1 b.ml |}];
-66,67c73
+>       [%expect {|
+>         Resolved 1/1 b.ml |}];
+66,67c74
 <         DU File a.ml
 < 
 ---
 >         M File b.ml
-69,71c75,78
+69,71c76,79
 <           { a : int option;
 <             b : string;
 <             c : float;
@@ -49,7 +50,7 @@
 >           ; b : string
 >           ; c : float
 >           ; d : unit option
-76,79c83,84
+76,79c84,85
 <         a.ml: needs merge
 <         You must edit all merge conflicts and then
 <         mark them as resolved using git add
diff --git a/test/rebase_b.ml b/test/rebase_b.ml
index c1fa608..e6ec9fc 100644
--- a/test/rebase_b.ml
+++ b/test/rebase_b.ml
@@ -66,7 +66,8 @@ type t =
           }
         >>>>>>> 6070a8f (second commit (fork)) |}];
       resolve ();
-      [%expect {| Resolved 1/1 b.ml |}];
+      [%expect {|
+        Resolved 1/1 b.ml |}];
       print_status ();
       [%expect
         {|
diff --git a/test/resolve1.ml b/test/resolve1.ml
index ee3ce93..a039386 100644
--- a/test/resolve1.ml
+++ b/test/resolve1.ml
@@ -149,7 +149,8 @@ type t =
           | A
           | B of int |}];
       resolve ();
-      [%expect {| Resolved 1/1 b.ml |}];
+      [%expect {|
+        Resolved 1/1 b.ml |}];
       print_status ();
       [%expect
         {|
diff --git a/test/resolve2.ml b/test/resolve2.ml
index fde9ec3..742b544 100644
--- a/test/resolve2.ml
+++ b/test/resolve2.ml
@@ -69,7 +69,8 @@ type t =
           }
         >>>>>>> ead71ee (second commit (fork)):a.ml |}];
       resolve ();
-      [%expect {| Resolved 1/1 b.ml |}];
+      [%expect {|
+        Resolved 1/1 b.ml |}];
       print_status ();
       [%expect
         {|