The opaque type of zippers.

make ~left ~right constructs a zipper with left list left and right list right.

These lists go directly into the zipper itself, so left, if non-empty, should be in the reverse order to how it should appear when fully rewound.

of_list xs converts a list xs to a fully-rewound zipper.

It is equivalent to make with an empty left.

to_list zipper returns the list of _all_ items in the zipper, including those in the left list.

All items appear in the same order that they would take in the right list if the zipper was fully rewound. In other words, the left list appears first (in reverse order), followed by the right list (in forwards order).

To get only the items in the right list, use right_list; to get only the items in the left list (reversed), use left_list.

left_list zipper gets the raw left list of the zipper: all of the already-processed items in reverse order.

right_list zipper gets the right list of the zipper: all of the not-yet-processed items in forwards order.

to_two_lists zipper is (left_list zipper, right_list zipper).

left_length zipper gets the length of zipper's left list.

right_length zipper gets the length of zipper's right list.

is_at_start zipper tests whether zipper's left list is empty.

is_at_end zipper tests whether zipper's right list is empty.

push zipper ~value pushes value into zipper at the cursor. The current cursor becomes the second item in the right list, and so on.

push_left zipper ~value pushes value into zipper just to the left of the cursor.

peek_opt ?steps zipper retrieves the cursor value without popping it from the zipper. If the cursor is empty, None is returned.

If steps is given, it shifts the effective cursor steps places forwards.

pop zipper returns an error if zipper has no cursor, or Ok (a, zipper') where a is zipper's cursor and zipper' is the new zipper formed by removing a.

pop_opt zipper behaves as pop, but returns None if zipper has no cursor and Some (a, zipper') otherwise.

map_head zipper ~f maps f across the cursor of zipper, if it exists, and replaces the cursor with the result (or drops it if f returns None).

step ?steps zipper ~on_empty takes one or more steps across zipper. The number of steps defaults to 1 (forwards), but can be given by steps; negative numbers step backwards through the zipper. If the number of steps exceeds the bounds of the zipper, an error is returned.