#light

let answer =
    {1901 .. 2000} |>
    Seq.map (fun y -> {1 .. 12} |> Seq.map (fun m -> (y,m,1))) |>
    Seq.concat |>
    Seq.map (fun (y,m,d) -> new System.DateTime(y,m,d)) |>
    Seq.map (fun dt -> dt.DayOfWeek) |>
    Seq.filter (fun dow -> dow = System.DayOfWeek.Sunday) |>
    Seq.length

This function can be of any type but has to return an option type, which is a union type whose members are None and Some(x).

When the function returns None, unfold stops iterating and generating new items, but if the output is Some(x, y) then x is added to the lazy list as its next element while y is passed to the function for the next iteration.

In the code above, for each iteration we take the current date and add one month to generate to next value to be checked.