ensure unfold is tailrec

src/core/CCList.ml

@@ -281,11 +281,6 @@ 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 *)
@@ -314,6 +309,32 @@ let init len f =
   else
     direct_ 0
 
+let rec unfold_kont f seed k =
+  match f seed with
+  | None -> k []
+  | Some (v, next) ->
+    let k' tl = v :: tl in
+    unfold_kont f next k'
+
+let[@inline] unfold f seed =
+  let rec direct i f seed =
+    if i = 0 then
+      unfold_kont f seed (fun x -> x)
+    else (
+      match f seed with
+      | None -> []
+      | Some (v, next) -> v :: direct (i - 1) f next
+    )
+  in
+  direct 100 f seed
+
+[@@@else_]
+
+let[@tail_mod_cons] rec unfold f seed =
+  match f seed with
+  | None -> []
+  | Some (v, next) -> v :: unfold f next
+
 [@@@endif]
 
 let rec compare f l1 l2 =

tests/core/t_list.ml

@@ -141,6 +141,20 @@ in
 unfold f 0 = [ 0; 2; 4; 6; 8; 10 ]
 ;;
 
+t @@ fun () ->
+let l =
+  unfold
+    (fun n ->
+      if n < 1_000_000 then
+        Some (n, n + 1)
+      else
+        None)
+    0
+in
+assert_equal ~printer:Q.Print.int (List.length l) 1_000_000;
+true
+;;
+
 t @@ fun () -> init 0 (fun _ -> 0) = [];;
 t @@ fun () -> init 1 (fun x -> x) = [ 0 ];;
 t @@ fun () -> init 1000 (fun x -> x) = 0 -- 999;;