bug in casting values to integer or float

[ckruse]

Hi,

I'm trying to parse a XML feed with sweet_xml. Due to the nature of the web ;-) there may be feeds that are invalid. In this case I had a feed with a missing element, and thus the XPath expression could not be resolved;

However, this clause matches for an integer cast in this case:

defp to_cast(value, :integer, _is_opt?), do: String.to_integer(to_string(value))

This leads to an argument error because String.to_integer gets called with an empty string:

** (ArgumentError) argument error
    :erlang.binary_to_integer("")
    (sweet_xml) lib/sweet_xml.ex:665: SweetXml.to_cast/3
    (sweet_xml) lib/sweet_xml.ex:441: SweetXml.xpath/2
    (sweet_xml) lib/sweet_xml.ex:531: anonymous fn/3 in SweetXml.xmap/3
    (elixir) lib/map.ex:791: Map.get_and_update/3
    (sweet_xml) lib/sweet_xml.ex:531: SweetXml.xmap/3
    (sweet_xml) lib/sweet_xml.ex:530: SweetXml.xmap/3

Is this a thing you will want to fix? All other XPath expressions don't lead to a crash, so it seems like a bug to me. On the other hand, the input is invalid; what's the right thing to do here?

To fix this one could add another function clause for empty strings in casts to float or integer; but then it would fail if the input string is not a number.

That's why I didn't create a pull request, I'm really not sure what your thoughts are about this.