Session with stream control sublanguage (#55)

SCL adds cursor-based streaming to Sakura so clients can consume DRL query results incrementally in bounded batches rather than all at once. Three statements: Begin, Fetch, and Close. `open`, `advance`, and `release` cursors backed by a session registry that suspends the generator between calls. SCL is just an optional consumption layer above the existing algebra.

1 files changed, 19 insertions, 0 deletions

diff --git a/lib/scl/parser.ml b/lib/scl/parser.ml
new file mode 100644
index 0000000..3e9c4ed
--- /dev/null
+++ b/lib/scl/parser.ml
@@ -0,0 +1,19 @@
+type error = ParseError of string
+
+let validate = function
+  | (Ast.Begin { limit = Some limit; _ } | Ast.Fetch { limit = Some limit; _ })
+    when limit <= 0 ->
+      Error (ParseError "The provision of `limit` is expected to be positive.")
+  | stmt -> Ok stmt
+
+let of_sexp sexp =
+  match Ast.statement_of_sexp sexp with
+  | exception exn -> Error (ParseError (Printexc.to_string exn))
+  | stmt -> validate stmt
+
+let of_string s =
+  match Sexplib.Sexp.of_string s |> Ast.statement_of_sexp with
+  | exception exn -> Error (ParseError (Printexc.to_string exn))
+  | stmt -> validate stmt
+
+let to_string stmt = Ast.sexp_of_statement stmt |> Sexplib.Sexp.to_string_hum