forgot to add a file

src/core/editor/Tab.re

@@ -0,0 +1,58 @@
+open Stds;
+
+let complete_face = (site: Zipper.Site.t, ctx: Ctx.t) =>
+  switch (site) {
+  | Within(tok) =>
+    Token.complete(tok)
+    |> Option.map(tok => ctx |> Ctx.push(~onto=L, Token.clear_marks(tok)))
+  | Between =>
+    open Options.Syntax;
+    let complete = side => {
+      let (face, ctx) = Ctx.pull(~from=side, ctx);
+      let* tok = Delim.to_opt(face);
+      let+ tok = Token.complete(tok);
+      Ctx.push(~onto=L, tok, ctx);
+    };
+    // assuming this is only called when tabbing forward
+    let/ () = complete(L);
+    complete(R);
+  };
+
+let perform = (d: Dir.t, z: Zipper.t): option(Zipper.t) =>
+  switch (Zipper.cursor_site(z)) {
+  | (Select(_), _) => Move.hstep(d, z)
+  | (Point(site), ctx) =>
+    open Options.Syntax;
+    // first try completing or expanding a neighbor
+    let/ () =
+      switch (d) {
+      | L => None
+      | R =>
+        let/ () = Option.map(Zipper.mk, complete_face(site, ctx));
+        Modify.try_expand(" ", z);
+      };
+    // otherwise jump to next obligation
+    let c = Zipper.zip(~save_cursor=true, z);
+    let normal = Zipper.normalize(~cell=c);
+    let car =
+      c.marks.cursor
+      |> Options.get_exn(Zipper.Bug__lost_cursor)
+      |> Path.Cursor.get_point
+      |> Option.get;
+    let path =
+      c.marks.obligs
+      |> Path.Map.filter((_, mtrl: Mtrl.T.t) => mtrl != Space(Unmolded))
+      |> Dir.pick(
+           d,
+           (
+             Path.Map.find_last_opt(p => Path.lt(normal(p), car.path)),
+             Path.Map.find_first_opt(p => Path.gt(normal(p), car.path)),
+           ),
+         )
+      |> Option.map(fst)
+      |> Options.get(() => Cell.end_path(~side=d, c));
+    c
+    |> Cell.put_cursor(Point(Caret.focus(path)))
+    |> Zipper.unzip_exn
+    |> Option.some;
+  };