add CCString.set for updating immutable strings

src/core/CCString.cppo.ml

@@ -281,6 +281,16 @@ let unlines l = String.concat "\n" l
 
 let unlines_gen g = concat_gen ~sep:"\n" g
 
+let set s i c =
+  if i<0 || i>= String.length s then invalid_arg "CCString.set";
+  init (String.length s) (fun j -> if i=j then c else s.[j])
+
+(*$T
+  set "abcd" 1 '_' = "a_cd"
+  set "abcd" 0 '-' = "-bcd"
+  (try set "abc" 5 '_'; false with Invalid_argument _ -> true)
+*)
+
 let pp buf s =
   Buffer.add_char buf '"';
   Buffer.add_string buf s;

src/core/CCString.mli

@@ -137,6 +137,12 @@ val unlines_gen : string gen -> string
   Q.printable_string (fun s -> unlines (lines s) = s)
 *)
 
+val set : string -> int -> char -> string
+(** [set s i c] creates a new string which is a copy of [s], except
+    for index [i], which becomes [c].
+    @raise Invalid_argument if [i] is an invalid index
+    @since NEXT_RELEASE *)
+
 include S with type t := string
 
 (** {2 Splitting} *)