F# で Project Euler Problem 19

> let weekday year month d =
-     let mutable y = year
-     let mutable m = month
-
-     if (m = 1 || m = 2) then
-         y <- y - 1
-         m <- m + 12
-     (y + y / 4 - y / 100 + y / 400 + (13 * m + 8) / 5 + d) % 7
- ;;

val weekday : int -> int -> int -> int

> weekday 2013 2 1
- ;;
val it : int = 5

> let mutable i = 0
- for y in [1901..2000] do
-     for m in [1..12] do
-         if (weekday y m 1) = 0 then i <- i + 1
- ;;

val mutable i : int = 171