diff --git a/src/core/CCString.cppo.ml b/src/core/CCString.cppo.ml
index 2fd5639d..097cd078 100644
--- a/src/core/CCString.cppo.ml
+++ b/src/core/CCString.cppo.ml
@@ -38,6 +38,8 @@ module type S = sig
   val blit : t -> int -> t -> int -> int -> unit
   (** See {!String.blit} *)
 
+  val fold : ('a -> char -> 'a) -> 'a -> t -> 'a
+
   (** {2 Conversions} *)
 
   val to_gen : t -> char gen
@@ -195,6 +197,12 @@ let suffix ~suf s =
 
 let blit = String.blit
 
+let fold f acc s =
+  let rec fold_rec f acc s i =
+    if i = String.length s then acc
+    else fold_rec f (f acc s.[i]) s (i+1)
+  in fold_rec f acc s 0
+
 let _to_gen s i0 len =
   let i = ref i0 in
   fun () ->
@@ -286,6 +294,12 @@ module Sub = struct
     if o1+len>len1 || o2+len>len2 then invalid_arg "CCString.Sub.blit";
     String.blit a1 (i1+o1) a2 (i2+o2) len
 
+  let fold f acc (s,i,len) =
+    let rec fold_rec f acc s i j =
+      if i = j then acc
+      else fold_rec f (f acc s.[i]) s (i+1) j
+    in fold_rec f acc s i (i+len)
+
   let to_gen (s,i,len) = _to_gen s i len
   let to_seq (s,i,len) k =
     for i=i to i+len-1 do k s.[i] done
diff --git a/src/core/CCString.mli b/src/core/CCString.mli
index 9e6f7e98..6a0e4e63 100644
--- a/src/core/CCString.mli
+++ b/src/core/CCString.mli
@@ -43,6 +43,10 @@ module type S = sig
   val blit : t -> int -> t -> int -> int -> unit
   (** See {!String.blit} *)
 
+  val fold : ('a -> char -> 'a) -> 'a -> t -> 'a
+  (** Fold on chars by increasing index.
+      @since NEXT_RELEASE *)
+
   (** {2 Conversions} *)
 
   val to_gen : t -> char gen
@@ -169,4 +173,11 @@ module Sub : sig
   (** Sub-slice *)
 
   include S with type t := t
+
+  (*$T
+    let s = Sub.make "abcde" 1 3 in \
+      Sub.fold (fun acc x -> x::acc) [] s = ['d'; 'c'; 'b']
+    Sub.make "abcde" 1 3 |> Sub.copy = "bcd"
+    Sub.full "abcde" |> Sub.copy = "abcde"
+  *)
 end