1 files changed, 40 insertions, 0 deletions
diff --git a/lib/evaluators/FOL.ml b/lib/evaluators/FOL.ml
new file mode 100644
index 0000000..70cf9de
--- /dev/null
+++ b/lib/evaluators/FOL.ml
@@ -0,0 +1,40 @@
+module Error = struct
+  open Concepts.Condition
+
+  let unknown_relation name =
+    condition "unknown-relation" "A plan named a relation that does not exist under this snapshot"
+      ("name" |=| Concepts.Value.String name)
+
+  let cancelled () =
+    condition "evaluation-cancelled" "An evaluation was stopped before it produced its result" empty
+end
+
+type plan = Base of string
+
+module Program = Protocols.Program.Make (struct
+  type t = plan
+end)
+
+let execute context = function
+  | Base name ->
+     let open Utilities.Result in
+     match context.Protocols.Context.status () with
+     | `Cancelled | `Exhausted -> Error (Error.cancelled ())
+     | `Live ->
+        let* found = Runtime.Context.resolve context name in
+        let* handle = Option.to_result ~none:(Error.unknown_relation name) found in
+        Fun.protect
+          ~finally:(fun () -> Protocols.Handle.release handle)
+          (fun () ->
+            let* enumerable = Protocols.Enumerable.require handle in
+            Protocols.Enumerable.enumerate enumerable context)
+
+class evaluator = object (self)
+  inherit Kernel.Lifecycle.null
+  inherit Kernel.Identity.of_id
+
+  method invoke ~program context = execute context program
+  method protocols : Protocols.Handle.protocol list = [Program.make self]
+end
+
+let make () = new evaluator |> Protocols.Handle.make