CCList: add unfold

src/core/CCList.ml

@@ -276,6 +276,11 @@ let fold_flat_map_i f acc l =
   in
   aux f acc 0 [] l
 
+let rec unfold f seed =
+  match f seed with
+  | None -> []
+  | Some (v, next) -> v :: unfold f next
+
 [@@@iflt 5.1]
 
 (* keep this because it's tailrec for < 5.1 *)

src/core/CCList.mli

@@ -102,6 +102,16 @@ val fold_flat_map_i :
     list to a list of lists that is then [flatten]'d.
     @since 2.8 *)
 
+val unfold : ('seed -> ('b * 'seed) option) -> 'seed -> 'b list
+(** [unfold f init] builds up a list from a seed value.
+    When [f] produces [Some (next_seed, value)], [value] is added to the output list and
+    [next_seed] is used in the next call to [f]. However,
+    when [f] produces [None], list production ends.
+    {b NOTE} if [f] never produces [None], then a {b stack overflow will occur}. Therefore,
+    great care must be taken to ensure that [f] will produce [None].
+    @since 3.13
+*)
+
 val count : ('a -> bool) -> 'a list -> int
 (** [count p l] counts how many elements of [l] satisfy predicate [p].
     @since 1.5, but only

src/core/CCListLabels.mli

@@ -135,6 +135,16 @@ val fold_flat_map_i :
     list to a list of lists that is then [flatten]'d.
     @since 2.8 *)
 
+val unfold : f:('seed -> ('b * 'seed) option) -> init:'seed -> 'b list
+(** [unfold ~f ~init] builds up a list from a seed value.
+    When [f] produces [Some (next_seed, value)], [value] is added to the output list and
+    [next_seed] is used in the next call to [f]. However,
+    when [f] produces [None], list production ends.
+    {b NOTE} if [f] never produces [None], then a {b stack overflow will occur}. Therefore,
+    great care must be taken to ensure that [f] will produce [None].
+    @since 3.13
+*)
+
 val count : f:('a -> bool) -> 'a list -> int
 (** [count ~f l] counts how many elements of [l] satisfy predicate [f].
     @since 1.5, but only

tests/core/t_list.ml

@@ -131,6 +131,16 @@ q
     = (List.rev l, flat_map (fun x -> [ x; x + 10 ]) l))
 ;;
 
+t @@ fun () ->
+let f x =
+  if x <= 5 then
+    Some (2 * x, x + 1)
+  else
+    None
+in
+unfold f 0 = [ 0; 2; 4; 6; 8; 10 ]
+;;
+
 t @@ fun () -> init 0 (fun _ -> 0) = [];;
 t @@ fun () -> init 1 (fun x -> x) = [ 0 ];;
 t @@ fun () -> init 1000 (fun x -> x) = 0 -- 999;;