Add partition_map to ListLabels.

stdlib/.depend

@@ -409,7 +409,8 @@ stdlib__listLabels.cmx : \
     stdlib__list.cmx \
     stdlib__listLabels.cmi
 stdlib__listLabels.cmi : \
-    stdlib__seq.cmi
+    stdlib__seq.cmi \
+    stdlib__either.cmi
 stdlib__map.cmo : \
     stdlib__seq.cmi \
     stdlib__map.cmi

stdlib/listLabels.mli

@@ -318,6 +318,21 @@ val partition : f:('a -> bool) -> 'a list -> 'a list * 'a list
    The order of the elements in the input list is preserved.
  *)
 
+val partition_map : f:('a -> ('b, 'c) Either.t) -> 'a list -> 'b list * 'c list
+(** [partition_map f l] returns a pair of lists [(l1, l2)] such that,
+    for each element [x] of the input list [l]:
+    - if [f x] is [Left y1], then [y1] is in [l1], and
+    - if [f x] is [Right y2], then [y2] is in [l2].
+
+    The output elements are included in [l1] and [l2] in the same
+    relative order as the corresponding input elements in [l].
+
+    In particular, [partition_map (fun x -> if p x then Left x else Right x) l]
+    is equivalent to [partition p l].
+
+    @since 4.12.0
+*)
+
 
 (** {1 Association lists} *)