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# Project Euler Problem 19

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

 ```1: 2: 3: 4: 5: 6: 7: 8: 9: ``` ``````open System Seq.unfold (fun x -> if x > DateTime (2000,12,31) then None else Some (x, x.AddMonths(1)))(DateTime(1901,1,1)) |> Seq.filter (fun x -> x.DayOfWeek = DayOfWeek.Sunday) |> Seq.length |> printfn "Problem 19 answer: %d" ``````
namespace System
module Seq

from Microsoft.FSharp.Collections
val unfold : generator:('State -> ('T * 'State) option) -> state:'State -> seq<'T>

Full name: Microsoft.FSharp.Collections.Seq.unfold
val x : DateTime
Multiple items
type DateTime =
struct
new : ticks:int64 -> DateTime + 10 overloads
member Add : value:TimeSpan -> DateTime
member AddDays : value:float -> DateTime
member AddHours : value:float -> DateTime
member AddMilliseconds : value:float -> DateTime
member AddMinutes : value:float -> DateTime
member AddMonths : months:int -> DateTime
member AddSeconds : value:float -> DateTime
member AddTicks : value:int64 -> DateTime
member AddYears : value:int -> DateTime
...
end

Full name: System.DateTime

--------------------
DateTime()
DateTime(ticks: int64) : unit
DateTime(ticks: int64, kind: DateTimeKind) : unit
DateTime(year: int, month: int, day: int) : unit
DateTime(year: int, month: int, day: int, calendar: Globalization.Calendar) : unit
DateTime(year: int, month: int, day: int, hour: int, minute: int, second: int) : unit
DateTime(year: int, month: int, day: int, hour: int, minute: int, second: int, kind: DateTimeKind) : unit
DateTime(year: int, month: int, day: int, hour: int, minute: int, second: int, calendar: Globalization.Calendar) : unit
DateTime(year: int, month: int, day: int, hour: int, minute: int, second: int, millisecond: int) : unit
DateTime(year: int, month: int, day: int, hour: int, minute: int, second: int, millisecond: int, kind: DateTimeKind) : unit
union case Option.None: Option<'T>
union case Option.Some: Value: 'T -> Option<'T>
val filter : predicate:('T -> bool) -> source:seq<'T> -> seq<'T>

Full name: Microsoft.FSharp.Collections.Seq.filter
property DateTime.DayOfWeek: DayOfWeek
type DayOfWeek =
| Sunday = 0
| Monday = 1
| Tuesday = 2
| Wednesday = 3
| Thursday = 4
| Friday = 5
| Saturday = 6

Full name: System.DayOfWeek
field DayOfWeek.Sunday = 0
val length : source:seq<'T> -> int

Full name: Microsoft.FSharp.Collections.Seq.length
val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn