Seq.unfold

Don’t know why this had escaped me for so long!

 

let PascalsTriangle =
        let nextLine aLine =
            let partialLine = 
                aLine
                |> Seq.pairwise
                |> Seq.map (fun (x,y) -> x+y)
                |> Seq.toList
            List.append (1::partialLine) [1]
        [1] |> Seq.unfold (fun s -> Some(s, nextLine s))

Compare to: Pascal’s Triangle in F#