add session object and protocol with pinning

1 files changed, 23 insertions, 0 deletions

diff --git a/lib/protocols/Session.ml b/lib/protocols/Session.ml
new file mode 100644
index 0000000..f740c1a
--- /dev/null
+++ b/lib/protocols/Session.ml
@@ -0,0 +1,23 @@
+module Error = struct
+  open Concepts.Condition
+
+  let not_a_session () =
+    condition "not-a-session" "A handle was expected to carry the session protocol and did not"
+      empty
+end
+
+type state = {branch: string; snapshot: Concepts.Hash.hash}
+
+class type implementation = object
+  method state : state
+  method pin : state -> unit
+end
+
+type Handle.protocol += Session of implementation
+type t = implementation
+
+let make impl = Session (impl :> implementation)
+let from handle = Handle.into handle (function Session impl -> Some impl | _ -> None)
+let require handle = from handle |> Option.to_result ~none:(Error.not_a_session ())
+let state i = Handle.invoke i (fun o -> o#state)
+let pin i state = Handle.invoke i (fun o -> o#pin state)