2 files changed, 21 insertions, 28 deletions

diff --git a/lib/evaluators/FOL.ml b/lib/evaluators/FOL.ml
index e28222b..a7685bd 100644
--- a/lib/evaluators/FOL.ml
+++ b/lib/evaluators/FOL.ml
@@ -187,32 +187,23 @@ let rec modes =
            index )
 
 let project description relation =
-  let open Utilities.Result in
-  let* cursor = Protocols.Enumerable.cursor relation in
-  let produce ~yield =
-    let* scan = Protocols.Cursor.require cursor in
-    let rec distinct seen =
-      let* next = Protocols.Cursor.next scan in
-      match next with
-      | None -> Ok ()
-      | Some tuple ->
-          let tuple =
-            { tuple with
-              attributes=
-                BatMap.String.filter
-                  (fun name _ -> BatMap.String.mem name description)
-                  tuple.Concepts.Tuple.attributes }
-          in
-          let key = Concepts.Tuple.hash tuple |> Concepts.Hash.to_raw_string in
-          if BatSet.String.mem key seen then distinct seen
-          else begin
-            yield tuple;
-            distinct (BatSet.String.add key seen)
-          end
-    in
-    distinct BatSet.String.empty
-  in
-  Ok (Kernel.Generator.cursor_of ~finally:(fun () -> Protocols.Handle.release cursor) produce)
+  let seen = ref BatSet.String.empty in
+  Protocols.Enumerable.cursor relation
+  |> Result.map
+       (Kernel.Generator.filter_map (fun tuple ->
+            let tuple =
+              { tuple with
+                attributes=
+                  BatMap.String.filter
+                    (fun name _ -> BatMap.String.mem name description)
+                    tuple.Concepts.Tuple.attributes }
+            in
+            let key = Concepts.Tuple.hash tuple |> Concepts.Hash.to_raw_string in
+            if BatSet.String.mem key !seen then None
+            else begin
+              seen := BatSet.String.add key !seen;
+              Some tuple
+            end ) )
 
 let agrees binding {Concepts.Tuple.attributes; _} =
   BatMap.String.for_all
diff --git a/lib/kernel/Generator.ml b/lib/kernel/Generator.ml
index a037b3d..4fd1ce0 100644
--- a/lib/kernel/Generator.ml
+++ b/lib/kernel/Generator.ml
@@ -54,7 +54,7 @@ class producer_cursor produce finally =
 
 let cursor_of ?finally produce = new producer_cursor produce finally |> Protocols.Handle.make
 
-let filter keep cursor =
+let filter_map f cursor =
   cursor_of
     ~finally:(fun () -> Protocols.Handle.release cursor)
     (fun ~yield ->
@@ -65,7 +65,9 @@ let filter keep cursor =
         match next with
         | None -> Ok ()
         | Some tuple ->
-            if keep tuple then yield tuple;
+            Option.iter yield (f tuple);
             pass ()
       in
       pass () )
+
+let filter keep = filter_map (fun tuple -> if keep tuple then Some tuple else None)