From: Robert Klemme <shortcutter@...>
Date: 2007-02-11T21:15:08+09:00
Subject: Re: Hash into Two Arguments (for REXML)

On 10.02.2007 17:48, Keith Fahlgren wrote:
> Hey,
> 
> I'm storing my XML namespaces for REXML as Hashes (for XPath.first [1]) 
> like so:
> 
> RDF_NS = {"rdf" => "http://www.w3.org/1999/02/22-rdf-syntax-ns#"}
> 
> When I need to do an XPath lookup, these work great (especially as I
> can merge() more than one together), but when I have to create new
> Elements and add their namespaces, the Hash isn't what's expected[2]
> (it needs two distinct args).
> 
> Is there a cooler way to get from a Hash to two args other than my
> *Hash.to_a.flatten ?
> 
> require 'rexml/document'
> # => true
> RDF_NS = {"rdf" => "http://www.w3.org/1999/02/22-rdf-syntax-ns#"}
> # => {"rdf"=>"http://www.w3.org/1999/02/22-rdf-syntax-ns#"}
> a = REXML::Element.new "rdf:RDF"
> # => <rdf:RDF/>
> a.add_namespace(*RDF_NS.to_a.flatten)
> # => <rdf:RDF xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'/>

Here's a variant, but this is not very nice either:

a.add_namespace(*RDF_NS..each {|k,v| break k,v}}

> 1. namespaces:    If supplied, a Hash which defines a namespace mapping:
> XPath.first( node, "a/x:b", { "x"=>"http://doofus" } )
> http://www.germane-software.com/software/rexml/doc/classes/REXML/XPath.html#M000452 
> 
> 
> 2.   add_namespace( prefix, uri=nil )
> 
> Adds a namespace to this element.
>  prefix:    the prefix string, or the namespace URI if uri is not supplied
>  uri:    the namespace URI. May be nil, in which prefix is used as the URI
> 
>  a.add_namespace("foo", "bar")
> http://www.germane-software.com/software/rexml/doc/classes/REXML/Element.html#M000490 

I get the feeling that you're probably not using Hashes properly, 
especially since you mention you have a lot of them with one key value 
pair and merge them together.

As far as I can see the most reasonable approach would be to have a 
single Hash that records all namespaces.  That would be the Hash that 
you pass to XPath.first.

When you want to add a specific namespace to an element, you would then 
rather do something like this:

a.add_namespace("foo", namespaces["foo"])

If you want to avoid double typing you could do

def namespaces.pair(key)
   (val = self[key]) ? [key, val] : nil
end

or

def namespaces.pair(key)
   val = self[key] and [key, val]
end

and then

a.add_namespace(*namespaces.pair("foo"))

Or you reverse the logic and do

def namespaces.add_ns(elem, ns)
   val = self[ns] and elem.add_namespace(ns, val)
end

Kind regards

	robert