Hi

I tested the lazyness of F#, and found something interesting, which i can not understand.
First declare an infinite seq:
let infinite_seq = Seq.init_infinite (fun i -> i+1)

After create a span, and groupBy function's in Haskell's style.
let rec span p l =
(seq { if (Seq.is_empty l) then
()
else
let x = Seq.hd l
let xs = Seq.skip 1 l
let ys,zs = span p xs
if (p x) then
yield x
else
()
}, l)

let rec groupBy eq l =
(seq{
if Seq.is_empty l then
()
else
let x = Seq.hd l
let xs = Seq.skip 1 l
let (ys,zs) = (span (eq x) xs)
yield! seq{yield (Seq.append (Seq.singleton x) ys) }
yield! (groupBy eq zs)
})

And iter the grouped infinite seq.
Seq.iter (fun a -> printfn "%A" a) (groupBy (fun a b -> (a-b)%2=0) infinite_seq)

It is lazy, but to reach the nth element of the seq is not linear, and looks more worse like exponential.

Why? I messed up something in gropuby, or the F# seq is messed up?