VECTOR_ELT() can only be applied to a 'list', not a 'logical'

[MichaelChirico]

Found this by mistake, probably a friendlier error is warranted:

xml_find_all(read_xml("<a line1='1'></a>"), "@line1 = 1")
# Error in xml_nodeset(nodes) :
# VECTOR_ELT() can only be applied to a 'list', not a 'logical'

[MichaelChirico]

Well, the issue is like so:

xml_find_all() evaluates the XPath search to a logical vector on which xml_nodeset() is attempted:

xml2/R/xml_find.R

Lines 87 to 90 in ab20789

	xml_find_all.xml_node <- function(x, xpath, ns = xml_ns(x), ...) {
	nodes <- .Call(xpath_search, x$node, x$doc, xpath, ns, Inf)
	xml_nodeset(nodes)
	}

xml_nodeset() attempts the node_duplicated routine:

xml2/R/classes.R

Line 76 in 2f3da48

nodes <- nodes[!.Call(nodes_duplicated, nodes)]

Which attempts the VECTOR_ELT() call:

xml2/src/xml2_node.cpp

Line 560 in 2f3da48

SEXP cur = VECTOR_ELT(nodes, i);

It's easy enough to throw a stopifnot(is.list(.)) somewhere, but I do pause to wonder the expected behavior in this case. What should xml_find_all() do when the search result is a logical vector?

On the one hand, the XPath search executed successfully, so it seems like a waste to trigger an error. OTOH, it may be peculiar to allow xml_find_all() to return a variety of types.

[hadley]

You probably wanted to use xml_find_lgl() here, but in general I'm not sure how xpath types should interact with these functions. I think the ideal would probably be to throw an informative error saying that you should use xml_find_lgl() etc instead.