lift retaining, membership and mode checks out of ephemeral relation

2 files changed, 12 insertions, 0 deletions

diff --git a/lib/protocols/Cursor.ml b/lib/protocols/Cursor.ml
index 36db97d..75dc77d 100644
--- a/lib/protocols/Cursor.ml
+++ b/lib/protocols/Cursor.ml
@@ -26,6 +26,13 @@ let rec next i =
   | Some (_, tuple) -> Ok (Some tuple)
   | None -> if batch.exhausted then Ok None else next i
 
+let rec exists i p =
+  let open Utilities.Result in
+  let* found = next i in
+  match found with
+  | None -> Ok false
+  | Some tuple -> if p tuple then Ok true else exists i p
+
 let drain i ?(limit = 256) () =
   let open Utilities.Result in
   let rec loop tuples =
diff --git a/lib/protocols/Cursor.mli b/lib/protocols/Cursor.mli
index 08c7628..c977fec 100644
--- a/lib/protocols/Cursor.mli
+++ b/lib/protocols/Cursor.mli
@@ -14,6 +14,11 @@ val fetch : t Handle.interface -> int -> (batch, Concepts.Condition.condition) r
 
 val next : t Handle.interface -> (Concepts.Tuple.t option, Concepts.Condition.condition) result
 
+val exists :
+  t Handle.interface ->
+  (Concepts.Tuple.t -> bool) ->
+  (bool, Concepts.Condition.condition) result
+
 val drain :
   t Handle.interface ->
   ?limit:int ->