FindDuplicate.fs

open System

type Foo = { Field : int }
let random = Random ()

let hasDuplicate keySelector =
   let adjacent = Seq.windowed 2 >> Seq.map (function | [| x; y |] -> x, y | _ -> failwith "invalid")
   Seq.scan (fun set value -> Set.add (keySelector value) set) Set.empty
   >> adjacent
   >> Seq.exists (fun (x, y) -> Set.count x = Set.count y)

Seq.initInfinite (fun _ -> { Field = random.Next (1000) })
//|> Seq.take 50  // 適当に小さな値を入れたら全部違う値になる気がする
|> hasDuplicate (fun foo -> foo.Field)
|> printfn "%b"