add List.filteri to the stdlib (#9059)

3 files changed, 21 insertions, 0 deletions

diff --git a/stdlib/list.ml b/stdlib/list.ml
index 2b9e545b88..ab43fee436 100644
--- a/stdlib/list.ml
+++ b/stdlib/list.ml
@@ -244,6 +244,13 @@ let find_all p =
 
 let filter = find_all
 
+let filteri p l =
+  let rec aux i acc = function
+  | [] -> rev acc
+  | x::l -> aux (i + 1) (if p i x then x::acc else acc) l
+  in
+  aux 0 [] l
+
 let filter_map f =
   let rec aux accu = function
     | [] -> rev accu
diff --git a/stdlib/list.mli b/stdlib/list.mli
index b7b6a89b6a..e60c9bd0a2 100644
--- a/stdlib/list.mli
+++ b/stdlib/list.mli
@@ -252,6 +252,13 @@ val filter : ('a -> bool) -> 'a list -> 'a list
 val find_all : ('a -> bool) -> 'a list -> 'a list
 (** [find_all] is another name for {!List.filter}. *)
 
+val filteri : (int -> 'a -> bool) -> 'a list -> 'a list
+(** Same as {!List.filter}, but the predicate is applied to the index of
+   the element as first argument (counting from 0), and the element
+   itself as second argument.
+   @since 4.11.0
+*)
+
 val partition : ('a -> bool) -> 'a list -> 'a list * 'a list
 (** [partition p l] returns a pair of lists [(l1, l2)], where
    [l1] is the list of all the elements of [l] that
diff --git a/stdlib/listLabels.mli b/stdlib/listLabels.mli
index 7004d78909..c19588a147 100644
--- a/stdlib/listLabels.mli
+++ b/stdlib/listLabels.mli
@@ -266,6 +266,13 @@ val filter : f:('a -> bool) -> 'a list -> 'a list
 val find_all : f:('a -> bool) -> 'a list -> 'a list
 (** [find_all] is another name for {!List.filter}. *)
 
+val filteri : f:(int -> 'a -> bool) -> 'a list -> 'a list
+(** Same as {!List.filter}, but the predicate is applied to the index of
+   the element as first argument (counting from 0), and the element
+   itself as second argument.
+   @since 4.11.0
+*)
+
 val partition : f:('a -> bool) -> 'a list -> 'a list * 'a list
 (** [partition p l] returns a pair of lists [(l1, l2)], where
    [l1] is the list of all the elements of [l] that