# let even_digit n =
match n with
| 0 -> true
| 2 -> true
| 4 -> true
| 6 -> true
| 8 -> true
| _ -> false;;
val even_digit : int -> bool = <fun>
The patterns here, save the last one, are just integer literals. If n is the same as any of these integers, the value true is produced. The last case uses a "wildcard," which matches anything and produces true false.
Fix a very minor typo