Hello fine code4libbers, I have a technical question about metadata 
vocab reuse, and the best way to do something I'm doing.

I'm working on an API for returning a list of scholarly articles.

I am trying to do as much as I can with already existing technical 
metadata devices.

In general, I am going to do this as an Atom XML response, with some 
'third party' XML namespaces in use too for full expression of what I 
want to express.  Using already existing vocabularies, identified by URI.

In general, this is working fine -- especially using the PRISM 
vocabulary for some scholarly citation-specific metadata elements. Also 
some things that were already part of Atom, and may be a bit of DC here 
or there.

I am generally happy with this approach, and plan to stick to it.

But there are a few places where I am not sure what to do. In general, 
there's a common pattern where I need to express a certain 'element' 
using _multiple_ vocabularies simultaneously (and/or no vocabulary at 
all, free text).

For instance, let's take the (semantically vague, yes) concept of 
type/genre.  I have a schema.org type URI that expresses the 'type'.  I 
can _also_ express the 'type' using the dcterms 'type' vocabulary. I 
could theoretically have a couple more format/type vocabularies I'd like 
to expose, but let's stop there as an example. And on top of this, I 
_also_ have a free text 'type' string (which may or may not be derivable 
from the controlled vocabs), which I'd like to make available to API 
consumer too.

Any individual item may have some, all, or none of these data associated 
with it.

Now, the dcterms 'type' element is capable of holding any or all of 
these. http://dublincore.org/documents/dcmi-terms/#terms-type

"Recommended best practice is to use a controlled vocabulary such as the 
DCMI Type Vocabulary [DCMITYPE]. To describe the file format, physical 
medium, or dimensions of the resource, use the Format element."

See, _recommended_ is to use a controlled vocab _such as_ DCMI Type 
Vocab, but this makes it clear you can also use the 'type' element for 
another controlled vocab, or no controlled vocab at all.

So it's _legal_ to simply do something like this:

<!-- schema.org: -->
<dcterms:type>http://schema.org/ScholarlyArticle</dcterms:type>

<!-- dcterms type vocab: -->
<dcterms:type>http://purl.org/dc/dcmitype/Text</dcterms:type>

<!-- free text not from a controlled vocab: -->
<dcterms:type>Scholarly Book Review</dcterms:type>



And I've been the _consumer_ of API's which do something like that: Just 
throw a grab bag of differnet things into repeated dcterms:type 
elements, including URIs representing values from different vocabs, and 
free text.  They figure, hey, it's legal to use dcterms:type that way 
according to the docs for the dcterms vocab.

And as a consumer of services that do that... I do not want to do it. It 
is too difficult to work with as a consumer, when you don't know what 
the contents of a dcterms:type element might be, from any vocab, or none 
at all. It kind of ruins the utility of the controlled vocabs in the 
first place, or requires unreasonably complex logic on the client side.

So. Another idea that occurs is just to add some custom attributes to 
the dcterms:type element.

<dcterms:type 
vocab="schema.org">http://schema.org/ScholarlyArticle</dcterms:type>
<dcterms:type 
vocab="dcterms">http://purl.org/dc/dcmitype/Text</dcterms:type>
<dcterms:type>Scholarly Book Review</dcterms:type>

Now at least the client can a lot more easily write logic for "Is there 
a dcterms value? If so what is it.

But I can't really tell if this is legal or not -- attributes are 
handled kind of inconsistently by various XML validators. Maybe I'd need 
to namespace the attribute with a custom namespace too:

... xmlns:mine=http://example.org/vocab ...

<dcterms:type 
mine:vocab="schema.org">http://schema.org/ScholarlyArticle</dcterms:type>

But namespaces on attributes are handled _very_ inconsistently and 
buggily by various standard XML parsing libraries I've used, so I don't 
really want to do that, it's going to make things too hard on the client 
to use namespaced attributes.

But I kind of like the elegancy of that 'add attributes to dcterms:type' 
approach. I suppose you could even use full URIs instead of random terms 
to identify the vocab, for the elegance of it:

<dcterms:type 
vocab="http://schema.org">http://schema.org/ScholarlyArticle</dcterms:type>
<dcterms:type 
vocab="http://dublincore.org/documents/dcmi-terms">http://purl.org/dc/dcmitype/Text</dcterms:type>

But another option, especially if that isn't legal,  is to give up 
dcterms entirely and use only my own custom namespace/vocab for 'type' 
elements:

<mine:schema-type>http://schema.org/ScholarlyArticle</dcterms:type>
<mine:dcterms-type>http://purl.org/dc/dcmitype/Text</dcterms:type>
<mine:uncontrolled-type>Scholarly Book Review</dcterms:type>


Which is kind of 'inelegant', but would probably work fine too. 
Realistically, any consumer of my response is going to be custom written 
for my response, it can be written to deal with mine:schema-type just as 
well as dcterms:type with attribute vocab=something.  Standardizing here 
isn't really _neccesary_ at all for primary use cases, although there 
are a variety of anxilary hypothetical benefits.

Or maybe there's some other solution entirely I'm not thinking of.

So, any feedback?  What solution makes sense, balancing standards, 
clarity, parsimony, ease of development, ease of client development, etc.?

The 'type' example is a good example, but this comes up in some other 
places too. Another example is for 'language', I may have either or both 
of an ISO code (two letter or three letter variety, such as "en" or 
"eng"), and an English-language free text description of the language 
"English", and want to provide either one or both, unambiguously and 
easy to consume for the client.