diff --git a/.ocamlformat b/.ocamlformat
index 15514ed..c365faf 100644
--- a/.ocamlformat
+++ b/.ocamlformat
@@ -1,4 +1,4 @@
-version=0.26.1
+version=0.27.0
 assignment-operator=end-line
 break-cases=fit
 break-fun-decl=wrap
diff --git a/ccbg.opam b/ccbg.opam
index b60f804..0fabd41 100644
--- a/ccbg.opam
+++ b/ccbg.opam
@@ -15,6 +15,7 @@ depends: [
   "bos" {>= "0.2.0"}
   "directories"
   "fpath"
+  "prelude"
   "scfg" {>= "0.2"}
   "odoc" {with-doc}
 ]
diff --git a/dune-project b/dune-project
index 0c3e4f8..1ea1446 100644
--- a/dune-project
+++ b/dune-project
@@ -33,5 +33,6 @@
    (>= 0.2.0))
   directories
   fpath
+  prelude
   (scfg
    (>= 0.2))))
diff --git a/src/ccbg.ml b/src/ccbg.ml
index 2874990..fdf2bb2 100644
--- a/src/ccbg.ml
+++ b/src/ccbg.ml
@@ -9,11 +9,12 @@ end
 module Project_dirs = Directories.Project_dirs (App_id)
 module User_dirs = Directories.User_dirs ()
 
-let error msg =
-  Format.eprintf "error: %s@." msg;
-  exit 1
+let error = function
+  | `Msg msg ->
+    Fmt.epr "error: %s@." msg;
+    exit 1
 
-let some_or_fail msg = function None -> error msg | Some v -> v
+let some_or_fail msg = function None -> error (`Msg msg) | Some v -> v
 
 let ok_or_fail r = Result.fold ~error ~ok:Fun.id r
 
@@ -24,10 +25,13 @@ let config =
     some_or_fail "can't compute configuration directory path"
       Project_dirs.config_dir
   in
-  let file_name = Filename.concat config_dir "config.scfg" in
+  let config_dir = Fpath.of_string config_dir |> ok_or_fail in
+  let file_name = Fpath.(config_dir / "config.scfg") in
   (* if the config file doesn't exist, then we use the empty configuration *)
-  if not @@ Sys.file_exists file_name then []
-  else ok_or_fail (Scfg.Parse.from_file file_name)
+  match Bos.OS.File.exists file_name with
+  | Error (`Msg e) -> Fmt.failwith "%s" e
+  | Ok false -> []
+  | Ok true -> ok_or_fail @@ Scfg.Parse.from_file file_name
 
 let wallpaper_dir =
   match Scfg.Query.get_dir "wallpaper_dir" config with
@@ -41,7 +45,7 @@ let interval =
     | None -> 600
     | Some interval -> ok_or_fail (Scfg.Query.get_param_pos_int 0 interval)
   in
-  Format.sprintf "%ds" interval
+  Fmt.str "%ds" interval
 
 let mode =
   match Scfg.Query.get_dir "mode" config with
@@ -51,11 +55,11 @@ let mode =
     | ("center" | "fill" | "fit" | "solid_color" | "stretch" | "tile") as mode
       ->
       mode
-    | invalid_mode -> Format.ksprintf error "invalid mode `%s`" invalid_mode )
+    | invalid_mode -> Fmt.failwith "invalid mode `%s`" invalid_mode )
 
 let wallpaper_files =
   match Bos.OS.Dir.contents (Fpath.v wallpaper_dir) with
-  | Error (`Msg e) -> error e
+  | Error e -> error e
   | Ok cnt -> (
     (* remove directories from the list... *)
     let cnt =
@@ -71,7 +75,7 @@ let wallpaper_files =
         cnt
     in
     match Array.of_list cnt with
-    | [||] -> Format.ksprintf error "no wallpaper found in %s" wallpaper_dir
+    | [||] -> Fmt.failwith "no wallpaper found in %s" wallpaper_dir
     | wallpaper_files -> wallpaper_files )
 
 let random_wallpaper =
@@ -86,7 +90,7 @@ let rec new_wallpaper () =
         v "timeout" % interval % "swaybg" % "--mode" % mode % "--image"
         % random_wallpaper () )
   with
-  | Ok () -> error "timeout failed"
+  | Ok () -> error (`Msg "timeout failed")
   | Error (`Msg (_s : string)) ->
     (* timeout killed the swaybg process, we can launch a new one *)
     new_wallpaper ()
diff --git a/src/dune b/src/dune
index 636755d..b9b0880 100644
--- a/src/dune
+++ b/src/dune
@@ -1,4 +1,6 @@
 (executable
  (public_name ccbg)
  (modules ccbg)
- (libraries bos directories fpath scfg))
+ (libraries bos directories fpath prelude scfg)
+(flags
+  (:standard -open Prelude)))