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I'm learning F# and I've started to play around with both sequences and match expressions.
I'm writing a web scraper that's looking through HTML similar to the following and taking the last URL in a parent <span> with the paging class.
<html>
<body>
<span class="paging">
<a href="http://google.com">Link to Google</a>
<a href="http://TheLinkIWant.com">The Link I want</a>
</span>
</body>
</html>
My attempt to get the last URL is as follows:
type AnHtmlPage = FSharp.Data.HtmlProvider<"http://somesite.com">
let findMaxPageNumber (page:AnHtmlPage)=
page.Html.Descendants()
|> Seq.filter(fun n -> n.HasClass("paging"))
|> Seq.collect(fun n -> n.Descendants() |> Seq.filter(fun m -> m.HasName("a")))
|> Seq.last
|> fun n -> n.AttributeValue("href")
However I'm running into issues when the class I'm searching for is absent from the page. In particular I get ArgumentExceptions with the message: Additional information: The input sequence was empty.
My first thought was to build another function that matched empty sequences and returned an empty string when the paging class wasn't found on a page.
let findUrlOrReturnEmptyString (span:seq<HtmlNode>) =
match span with
| Seq.empty -> String.Empty // <----- This is invalid
| span -> span
|> Seq.collect(fun (n:HtmlNode) -> n.Descendants() |> Seq.filter(fun m -> m.HasName("a")))
|> Seq.last
|> fun n -> n.AttributeValue("href")
let findMaxPageNumber (page:AnHtmlPage)=
page.Html.Descendants()
|> Seq.filter(fun n -> n.HasClass("paging"))
|> findUrlOrReturnEmptyStrin
My issue is now that Seq.Empty is not a literal and cannot be used in a pattern. Most examples with pattern matching specify empty lists [] in their patterns so I'm wondering: How can I use a similar approach and match empty sequences?
630
Use a guard clause
match myseq with
| s when Seq.isEmpty s -> "empty"
| _ -> "not 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.
31
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