在Erlang中传递参数并解析xml

Question

Howdy I'm still learning Erlang and I'm trying to pass more then one argument through functions but I'm hitting a wall.

To start I'm using xmerl.hrl to parse the xml. I call process_xml(Doc) through the command line with erlfile:process_xml("filename.xml").

process_xml(Doc) ->
makeBottom1(Doc,"yup got here").

From here the process calls makeBottom1 passing in the doc and a string to test output later on.

makeBottom1(E = #xmlElement{name='resource'}, Derp) -> "Got to resource";
makeBottom1(E, Derp) -> ["<h1>BURP</h1>", Derp, built_in_rules( fun makeBottom1/2 , [E, Derp] ), "-" ].

I expect it to return "Got to resource" loads of times but it never seems to trip that function call.

What it does return is this:

["<h1>BURP</h1>","yup got here",[],"-"]

Should I use a different xml parser for the functionality I want? Am I just making a rookie mistake?

Answer 1

The function built_in_rules expects 2 arguments: A function with arity 1 (one argument), and an XML node. Since your arguments do not fit the patterns described by the function clauses , you're triggering a catch-all condition which simply returns an empty list.

To work around this issue, you need to change the way you're using built_in_rules . Try the following:

built_in_rules(fun(Child) -> makeBottom1(Child, Derp) end, E)

Instead of trying to pass in a function with arity of two and expecting built_in_rules to know what to do with it, we instead pass in a function with arity 1, which built_in_rules knows exactly how to use, and use a closure to add our second argument. We also amend the second argument to the function call so that we're only passing in the current XML node.