cursor and context

1 files changed, 48 insertions, 0 deletions

diff --git a/lib/protocols/Cursor.ml b/lib/protocols/Cursor.ml
new file mode 100644
index 0000000..7c976b9
--- /dev/null
+++ b/lib/protocols/Cursor.ml
@@ -0,0 +1,48 @@
+module Error = struct
+  open Concepts.Condition
+
+  let not_a_cursor () =
+    condition "not-a-cursor" "A handle was expected to carry the cursor protocol and did not" empty
+end
+
+type batch = {tuples: Concepts.Tuple.t BatFingerTree.t; exhausted: bool}
+
+class type implementation = object
+  method fetch : int -> (batch, Concepts.Condition.condition) result
+end
+
+type Handle.protocol += Cursor of implementation
+type t = implementation
+
+let make impl = Cursor (impl :> implementation)
+let from handle = Handle.into handle (function Cursor impl -> Some impl | _ -> None)
+let require handle = from handle |> Option.to_result ~none:(Error.not_a_cursor ())
+let fetch i limit = Handle.invoke i (fun o -> o#fetch limit)
+
+(* A scalar loop over a batching protocol. The buffer is per-call rather than per-cursor state, so
+   [next] stays a derived convenience and an implementation still writes exactly one method: the
+   cost is that a caller alternating [next] and [fetch] on one cursor sees the buffered tuples
+   dropped. Callers pick one or the other. *)
+let rec next i =
+  let open Utilities.Result in
+  (* TODO: fetch 1 and hand back its head. Because an empty non-exhausted batch means "ask again",
+     this loops until it either has a tuple or sees [exhausted]. That loop is unbounded by
+     construction, which is precisely the bounded-work property [fetch] has and [next] gives up --
+     which is why [next] is documented for small results and scalar loops, not for scans a host
+     needs to be able to cancel. *)
+  let* batch = fetch i 1 in
+  match BatFingerTree.front batch.tuples with
+  | Some (_, tuple) -> Ok (Some tuple)
+  | None -> if batch.exhausted then Ok None else next i
+
+let drain i ?(limit = 256) () =
+  let open Utilities.Result in
+  (* TODO: fetch [limit] at a time, appending each batch, until [exhausted]. Appending is
+     [BatFingerTree.append], which is logarithmic, so accumulating a large result does not degrade
+     the way repeated list concatenation would. *)
+  let rec loop tuples =
+    let* batch = fetch i limit in
+    let tuples = BatFingerTree.append tuples batch.tuples in
+    if batch.exhausted then Ok tuples else loop tuples
+  in
+  loop BatFingerTree.empty