Match with empty sequence

The suggestion that ildjarn gave in the comments is a good one: if you feel that using match would create more readable code, then make an active pattern to check for empty seqs:

let (|EmptySeq|_|) a = if Seq.isEmpty a then Some () else None

let s0 = Seq.empty<int>

match s0 with
| EmptySeq -> "empty"
| _ -> "not empty"

Run that in F# interactive, and the result will be "empty".

You can use a when guard to further qualify the case:

match span with 
| sequence when Seq.isEmpty sequence -> String.Empty
| span -> span
|> Seq.collect (fun (n: HtmlNode) ->
    n.Descendants()
    |> Seq.filter (fun m -> m.HasName("a")))
|> Seq.last
|> fun n -> n.AttributeValue("href")

ildjarn is correct in that in this case, an if...then...else may be the more readable alternative, though.