Converted 1st of December into using batteries as well.

dec1.ml

@@ -10,13 +10,13 @@
     (*91212129 produces 9 because the only digit that matches the next one is the*)
     (*last digit, 9.*)
 
-open Core
+open Batteries
 
-let char_to_int x = int_of_string (Char.to_string x)
+let char_to_int x = int_of_string (Char.escaped x)
 
 let rec algo' (chars : char list) (first : int) (acc : int list) =
     match chars with
-    | [] -> List.fold_left ~f:(+) ~init:0 acc
+    | [] -> List.fold_left (+) 0 acc
     | c1 :: (c2 :: rest) ->
             if Char.equal c1 c2
             then algo' (c2 :: rest) first (char_to_int c1 :: acc)
@@ -29,9 +29,7 @@ let rec algo' (chars : char list) (first : int) (acc : int list) =
 
 let algo (chars : char list) =
     let
-        first = match List.hd chars with
-        | Some x -> x
-        | None -> '0'
+        first = List.hd chars
     in
         algo' chars (char_to_int first) []
 
@@ -40,7 +38,7 @@ let solve_captcha number =
     number
     |> String.to_list
     |> algo
-    |> printf "Passcode is: %d\n"
+    |> print_int
 
 
 let () =