5 files changed, 35 insertions, 4 deletions
diff --git a/lib/protocols/Addressable.ml b/lib/protocols/Addressable.ml
new file mode 100644
index 0000000..7e97f08
--- /dev/null
+++ b/lib/protocols/Addressable.ml
@@ -0,0 +1,13 @@
+class type implementation = object
+  method address : Concepts.Hash.hash
+end
+
+type Handle.protocol += Addressable of implementation
+
+type t = implementation
+
+let make impl = Addressable (impl :> implementation)
+
+let from handle = Handle.into handle (function Addressable impl -> Some impl | _ -> None)
+
+let address i = Handle.invoke i (fun o -> o#address)
diff --git a/lib/protocols/Addressable.mli b/lib/protocols/Addressable.mli
new file mode 100644
index 0000000..2d9c14d
--- /dev/null
+++ b/lib/protocols/Addressable.mli
@@ -0,0 +1,11 @@
+type t
+
+class type implementation = object
+  method address : Concepts.Hash.hash
+end
+
+val make : #implementation -> Handle.protocol
+
+val from : Handle.t -> t Handle.interface option
+
+val address : t Handle.interface -> Concepts.Hash.hash
diff --git a/lib/protocols/Context.ml b/lib/protocols/Context.ml
deleted file mode 100644
index b7c8a37..0000000
--- a/lib/protocols/Context.ml
+++ /dev/null
@@ -1,4 +0,0 @@
-type t =
-  { snapshot : Concepts.Hash.hash
-  ; root : Handle.t
-  ; status : unit -> [`Live | `Cancelled | `Exhausted] }
diff --git a/lib/protocols/Handle.ml b/lib/protocols/Handle.ml
index 524386b..313a3d4 100644
--- a/lib/protocols/Handle.ml
+++ b/lib/protocols/Handle.ml
@@ -1,5 +1,12 @@
 type protocol = ..
 
+module Error = struct
+  open Concepts.Condition
+
+  let unimplemented_protocol () = condition "unimplemented-protocol" "A required protocol was not implemented by the specified object"
+                                    empty
+end
+
 class type obj = object
   method reference : bool
   method release : unit
@@ -45,3 +52,5 @@ let release ({ valid; _ } as handle) =
   let o = object_of handle in
   valid := false;
   o#release
+
+let require f h = Option.to_result ~none:(Error.unimplemented_protocol ()) (f h)
diff --git a/lib/protocols/Handle.mli b/lib/protocols/Handle.mli
index f36c95e..f63bbea 100644
--- a/lib/protocols/Handle.mli
+++ b/lib/protocols/Handle.mli
@@ -22,3 +22,5 @@ val equal : t -> t -> bool
 val release : t -> unit
 
 val hash : t -> Concepts.Hash.hash
+
+val require : (t -> 'a interface option) -> t -> ('a interface, Concepts.Condition.condition) result