1 files changed, 20 insertions, 0 deletions
diff --git a/lib/protocols/Indexed.ml b/lib/protocols/Indexed.ml
new file mode 100644
index 0000000..7cf1ea3
--- /dev/null
+++ b/lib/protocols/Indexed.ml
@@ -0,0 +1,20 @@
+module Error = struct
+  open Concepts.Condition
+
+  let not_indexed () =
+    condition "not-indexed" "A handle was expected to carry the indexed protocol and did not" empty
+end
+
+class type implementation = object
+  method indexes : string list
+  method lookup : string -> Concepts.Value.value -> (Handle.t, Concepts.Condition.condition) result
+end
+
+type Handle.protocol += Indexed of implementation
+type t = implementation
+
+let make impl = Indexed (impl :> implementation)
+let from handle = Handle.into handle (function Indexed impl -> Some impl | _ -> None)
+let require handle = from handle |> Option.to_result ~none:(Error.not_indexed ())
+let indexes i = Handle.invoke i (fun o -> o#indexes)
+let lookup i attribute value = Handle.invoke i (fun o -> o#lookup attribute value)