add result.ml; polymorphic variant errors + refacto

src/fat.ml

@@ -19,6 +19,14 @@ module Path = struct
   let equal a b = 0 = compare a b
 end
 
+type t = Mkernel.Block.t Mfat.t
+
+type entry = Mfat.entry = {
+  name: string;
+  is_dir: bool;
+  size: int32;
+}
+
 module Fat = Mfat.Make (struct
   include Mkernel.Block
 
@@ -28,18 +36,21 @@ end)
 
 include Fat
 
-type entry = Mfat.entry = {
-  name: string;
-  is_dir: bool;
-  size: int32;
-}
-
 let create blk =
-  match Fat.create blk with
-  | Error (`Msg e) -> Fmt.failwith "FAT file system failure: %s." e
+  match create blk with
+  | Error (`Msg e) ->
+      Fmt.epr "FAT file system initialization failure: %s@." e;
+      exit 1
   | Ok fs -> fs
 
-type t = Mkernel.Block.t Mfat.t
+let map_err r = Result.map_error (fun (`Msg e) -> `Mfat e) r
+let ls t p = ls t p |> map_err
+let read t p = read t p |> map_err
+let write t p s = write t p s |> map_err
+let mkdir t p = mkdir t p |> map_err
+let remove t p = remove t p |> map_err
+let exists t p = exists t p
+let stat t p = stat t p |> map_err
 
 module type FS = sig
   val t : t