Use a polymorphic variant result type

This allows us to easily use the same result type from other libraries,
without them being different types, needing different binds etc.

Signed-off-by: David Scott <[email protected]>

CHANGES.md

@@ -1,4 +1,4 @@
 v0.2 (unreleased)
 * add Mbr_partition: V1_LWT.BLOCK, for easy access to partitions via
   the standard Mirage block interface.
-
+* use a polymorphic variant result type [`Ok of 'a | `Error of 'b]

cli/impl.ml

@@ -30,10 +30,10 @@ let info common filename =
       Lwt_unix.openfile filename [ Lwt_unix.O_RDONLY ] 0o0 >>= fun fd ->
       Mbr_lwt.really_read fd mbr >>= fun () ->
       let mbr = match Mbr.unmarshal mbr with
-      | Mbr.Error reason ->
+      | `Error reason ->
         Printf.fprintf stderr "Failed to unmarshal MBR: %s\n%!" reason;
         exit 1
-      | Mbr.Ok x -> x in
+      | `Ok x -> x in
       let all = List.map (fun f ->
         match Mbr.get mbr f with
         | Some v -> [ f; v ]

lib/mbr.ml

@@ -14,16 +14,17 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  *)
 
-type ('a, 'b) result =
-  | Ok of 'a
-  | Error of 'b
+type ('a, 'b) result = [
+  | `Ok of 'a
+  | `Error of 'b
+]
 
 let ( >>= ) x f = match x with
-  | Error y -> Error y
-  | Ok z -> f z
+  | `Error y -> `Error y
+  | `Ok z -> f z
 
-let return x = Ok x
-let fail y = Error y
+let return x = `Ok x
+let fail y = `Error y
 
 let kib = 1024L
 let mib = Int64.mul kib 1024L

lib/mbr.mli

@@ -14,9 +14,10 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  *)
 
-type ('a, 'b) result =
-  | Ok of 'a
-  | Error of 'b
+type ('a, 'b) result = [
+  | `Ok of 'a
+  | `Error of 'b
+]
 
 module Geometry : sig
   type t = {