List.concat_map : ('a -> 'b list) -> 'a list -> 'b list (#8760)

Changes

@@ -184,6 +184,9 @@ Working version
 - #8716: Optimize [Array.fill] and [Hashtbl.clear] with a new runtime primitive
   (Alain Frisch, review by David Allsopp, Stephen Dolan and Damien Doligez)
 
+- #8760: List.concat_map : ('a -> 'b list) -> 'a list -> 'b list
+  (Gabriel Scherer, review by Daniel Bünzli and Thomas Refis)
+
 - #8530: List.sort: avoid duplicate work by chop
   (Guillaume Munch-Maccagnoni, review by David Allsopp, Damien Doligez and
    Gabriel Scherer)

stdlib/list.ml

@@ -254,6 +254,14 @@ let filter_map f =
   in
   aux []
 
+let concat_map f l =
+  let rec aux f acc = function
+    | [] -> rev acc
+    | x :: l ->
+       let xs = f x in
+       aux f (rev_append xs acc) l
+  in aux f [] l
+
 let partition p l =
   let rec part yes no = function
   | [] -> (rev yes, rev no)

stdlib/list.mli

@@ -141,6 +141,13 @@ val filter_map : ('a -> 'b option) -> 'a list -> 'b list
     @since 4.08.0
 *)
 
+val concat_map : ('a -> 'b list) -> 'a list -> 'b list
+(** [List.concat_map f l] gives the same result as
+    {!List.concat}[ (]{!List.map}[ f l)]. Tail-recursive.
+
+    @since 4.10.0
+*)
+
 val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a
 (** [List.fold_left f a [b1; ...; bn]] is
    [f (... (f (f a b1) b2) ...) bn]. *)

stdlib/listLabels.mli

@@ -143,6 +143,13 @@ val filter_map : f:('a -> 'b option) -> 'a list -> 'b list
     @since 4.08.0
 *)
 
+val concat_map : f:('a -> 'b list) -> 'a list -> 'b list
+(** [List.concat_map f l] gives the same result as
+    {!List.concat}[ (]{!List.map}[ f l)]. Tail-recursive.
+
+    @since 4.10.0
+*)
+
 val fold_left : f:('a -> 'b -> 'a) -> init:'a -> 'b list -> 'a
 (** [List.fold_left f a [b1; ...; bn]] is
    [f (... (f (f a b1) b2) ...) bn]. *)

testsuite/tests/lib-list/test.ml

@@ -49,6 +49,10 @@ let () =
   assert (List.compare_length_with ['1'] 1 = 0);
   assert (List.compare_length_with ['1'] 2 < 0);
   assert (List.filter_map string_of_even_opt l = ["0";"2";"4";"6";"8"]);
+  assert (List.concat_map (fun i -> [i; i+1]) [1; 5] = [1; 2; 5; 6]);
+  assert (
+    let count = ref 0 in
+    List.concat_map (fun i -> incr count; [i; !count]) [1; 5] = [1; 1; 5; 2]);
   ()
 ;;