List.find_map : ('a -> 'b option) -> 'a list -> 'b option (#8832)

Changes

@@ -125,6 +125,10 @@ Working version
 
 ### Standard library:
 
+- #8832: List.find_map : ('a -> 'b option) -> 'a list -> 'b option
+  (Gabriel Scherer, review by Jeremy Yallop, Nicolás Ojeda Bär
+   and Daniel Bünzli)
+
 - #8657: Optimization in [Array.make] when initializing with unboxed
    or young values.
    (Jacques-Henri Jourdan, review by Gabriel Scherer and Stephen Dolan)

stdlib/list.ml

@@ -228,6 +228,14 @@ let rec find_opt p = function
   | [] -> None
   | x :: l -> if p x then Some x else find_opt p l
 
+let rec find_map f = function
+  | [] -> None
+  | x :: l ->
+     begin match f x with
+       | Some _ as result -> result
+       | None -> find_map f l
+     end
+
 let find_all p =
   let rec find accu = function
   | [] -> rev accu

stdlib/list.mli

@@ -230,6 +230,13 @@ val find_opt: ('a -> bool) -> 'a list -> 'a option
     satisfies [p] in the list [l].
     @since 4.05 *)
 
+val find_map: ('a -> 'b option) -> 'a list -> 'b option
+(** [find_map f l] applies [f] to the elements of [l] in order,
+    and returns the first result of the form [Some v], or [None]
+    if none exist.
+    @since 4.10.0
+*)
+
 val filter : ('a -> bool) -> 'a list -> 'a list
 (** [filter p l] returns all the elements of the list [l]
    that satisfy the predicate [p].  The order of the elements

stdlib/listLabels.mli

@@ -235,6 +235,13 @@ val find_opt: f:('a -> bool) -> 'a list -> 'a option
    list [l].
    @since 4.05 *)
 
+val find_map: f:('a -> 'b option) -> 'a list -> 'b option
+(** [find_map f l] applies [f] to the elements of [l] in order,
+    and returns the first result of the form [Some v], or [None]
+    if none exist.
+    @since 4.10.0
+*)
+
 val filter : f:('a -> bool) -> 'a list -> 'a list
 (** [filter p l] returns all the elements of the list [l]
    that satisfy the predicate [p].  The order of the elements

testsuite/tests/lib-list/test.ml

@@ -26,6 +26,13 @@ let () =
   assert (not (List.exists (fun a -> a > 9) l));
   assert (List.exists (fun _ -> true) l);
 
+  begin
+    let f ~limit a = if a >= limit then Some (a, limit) else None in
+    assert (List.find_map (f ~limit:3) [] = None);
+    assert (List.find_map (f ~limit:3) l = Some (3, 3));
+    assert (List.find_map (f ~limit:30) l = None);
+  end;
+
   assert (List.compare_lengths [] [] = 0);
   assert (List.compare_lengths [1;2] ['a';'b'] = 0);
   assert (List.compare_lengths [] [1;2] < 0);