feat(zipper): add option to not move past last element when moving right

1 files changed, 15 insertions, 9 deletions

diff --git a/lib/zipper.ml b/lib/zipper.ml
index 5f3b37a..6077f21 100644
--- a/lib/zipper.ml
+++ b/lib/zipper.ml
@@ -1,4 +1,4 @@
-(* Module [Zipper]: functional zippers *)
+(* Module [Zipper]: functional linear zippers *)
 
 open Base
 
@@ -11,7 +11,10 @@ let after z = z.after
 let focus z = after z |> Sequence.next |> Option.map ~f:fst
 let focus_or ~default z = Option.value ~default (focus z)
 let is_far_left z = before z |> Sequence.is_empty
-let is_far_right z = after z |> Sequence.is_empty
+
+let is_far_right ?(by_one = false) z =
+  after z |> Sequence.length_is_bounded_by ~max:(if by_one then 1 else 0)
+
 let is_empty z = is_far_left z && is_far_right z
 let left_length z = z.pos
 let right_length z = after z |> Sequence.length
@@ -30,23 +33,26 @@ let rec left_while f z =
 
 let rec far_left z = if is_far_left z then z else z |> left |> far_left
 
-let right z =
+let right ?(by_one = false) z =
   match Sequence.next z.after with
   | None -> z
+  | Some (_, t) when by_one && Sequence.is_empty t -> z
   | Some (h, t) ->
       { pos = z.pos + 1; before = Sequence.shift_right z.before h; after = t }
 
-let rec right_while f z =
+let rec right_while ?(by_one = false) f z =
   if
-    (not (is_far_right z)) && Option.(focus z |> map ~f |> value ~default:false)
-  then right z |> right_while f
+    (not (is_far_right ~by_one z))
+    && Option.(focus z |> map ~f |> value ~default:false)
+  then right z |> right_while ~by_one f
   else z
 
-let rec far_right z = if is_far_right z then z else z |> right |> far_right
+let rec far_right ?(by_one = false) z =
+  if is_far_right ~by_one z then z else z |> right |> far_right ~by_one
 
-let goto n z =
+let goto ?(by_one = false) n z =
   let n = n - z.pos in
-  let step = if n < 0 then left else right in
+  let step = if n < 0 then left else right ~by_one in
   Fn.apply_n_times ~n:(abs n) step z
 
 let pop ?(n = 1) z =