fix usage msg

day1.ml

@@ -41,7 +41,7 @@ let mains = [|main' 2; main' 3|]
 
 let main = function
   | [part; input] -> mains.(int_of_string part - 1) input
-  | _             -> Usage.exit "1 <part> <infile>"
+  | _             -> Usage.exit_default 1
 
 
 let%expect_test "part 1" =

day2.ml

@@ -30,7 +30,7 @@ let mains = [|main' ok1; main' ok2|]
 
 let main = function
   | [part; input] -> mains.(int_of_string part - 1) input
-  | _             -> Usage.exit "2 <part> <infile>"
+  | _             -> Usage.exit_default 2
 
 
 let%expect_test "part 1" =

day3.ml

@@ -48,7 +48,7 @@ let mains = [|main1; main2|]
 
 let main = function
   | [part; input] -> mains.(int_of_string part - 1) input
-  | _ -> failwith "a"
+  | _             -> Usage.exit_default 3
 
 let%expect_test _ =
   main1 "../../data/day3";

usage.ml

@@ -2,3 +2,5 @@ let exit args =
   let err = Format.sprintf "usage: %s %s" Sys.argv.(0) args in
   print_endline err;
   exit 1
+
+let exit_default day = exit (string_of_int day ^ " <part> <infile>")