create angstrom wrapper in Utils for int parser

and other common parsers for the future

src/day13.ml

@@ -7,13 +7,8 @@ module Packet = struct
   [@@deriving show { with_path = false }]
 
   let of_str line =
-    let open Angstrom in
-    let int =
-      take_while1 (function
-        | '0' .. '9' -> true
-        | _ -> false)
-      >>= fun s -> return @@ I (int_of_string s) <?> "int"
-    in
+    let open Utils.Parse in
+    let int = int >>= fun i -> return @@ I i in
     let packet =
       fix (fun packet ->
         let lb, delimited, rb = char '[', sep_by (char ','), char ']' in

src/utils.ml

@@ -9,3 +9,15 @@ let lines_of_input day =
   let path = Printf.sprintf "%s/day%d.tt" base_path day in
   IO.(with_in path read_lines_l)
 ;;
+
+module Parse = struct
+  include Angstrom
+
+  let int =
+    take_while1 (function
+      | '0' .. '9' -> true
+      | _ -> false)
+    >>| int_of_string
+    <?> "int"
+  ;;
+end