Add a generic main function.

lib/aoc.ml

@@ -3,3 +3,20 @@ let distance1 a b = abs (a - b)
 
 let strings_from_file fname =
   In_channel.with_open_text fname In_channel.input_lines
+
+let main prep parts =
+  try
+    match Sys.argv with
+    | [| _; fname |] ->
+        let lines = prep fname in
+        let do_part i (fmt, fn) =
+          Printf.printf "Part %d = %s\n" i (fmt (fn lines))
+        in
+        List.iteri do_part parts;
+        exit 0
+    | _ ->
+        Printf.printf "Usage: %s <fname>\n" Sys.executable_name;
+        exit 2
+  with e ->
+    Printf.printf "An error occured: %s\n" (Printexc.to_string e);
+    exit 1

lib/aoc.mli

@@ -8,3 +8,11 @@ val distance1 : int -> int -> int
 val strings_from_file : string -> string list
 (** [strings_from_file fname] returns a list of strings from the file 
     [fname].  Each string represents a line from the file. *)
+
+val main : (string -> 'a) -> (('b -> string) * ('a -> 'b)) list -> unit
+(** [main prep parts] executes an advent of code problem.  [prep fname] should
+    be a function that returns the input from [fname].  Each elemet of
+    [parts] is a pair of functions.  The first converts the output to a string
+    (for example [string_of_int]).  The second executes the given part.
+    Output is given as if done by: 
+    [print_string ( prep fname |> snd |> fst )] *)