From 73e68dae7c60ba05ac70a634c05db6630a487f35 Mon Sep 17 00:00:00 2001
From: Ben Bellick <benbellick@protonmail.com>
Date: Mon, 4 Dec 2023 23:01:06 -0600
Subject: [PATCH] CCList: add unfold

---
 src/core/CCList.ml        |  5 +++++
 src/core/CCList.mli       | 10 ++++++++++
 src/core/CCListLabels.mli | 10 ++++++++++
 tests/core/t_list.ml      | 10 ++++++++++
 4 files changed, 35 insertions(+)

diff --git a/src/core/CCList.ml b/src/core/CCList.ml
index c3c73cda..e00e1e32 100644
--- a/src/core/CCList.ml
+++ b/src/core/CCList.ml
@@ -281,6 +281,11 @@ 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 *)
diff --git a/src/core/CCList.mli b/src/core/CCList.mli
index 0e159cb6..865dca79 100644
--- a/src/core/CCList.mli
+++ b/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
diff --git a/src/core/CCListLabels.mli b/src/core/CCListLabels.mli
index c0f99696..fe5d2176 100644
--- a/src/core/CCListLabels.mli
+++ b/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
diff --git a/tests/core/t_list.ml b/tests/core/t_list.ml
index 4bc6ffb1..4d2026ba 100644
--- a/tests/core/t_list.ml
+++ b/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;;