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2.2. Ambiguities Between CURIEs and URIs In some cases language designers will want to use both URIs and CURIEs as the value of an attribute. For example, in XHTML+RDFa [XHTMLRDFa] the about attribute allows a URI to be specified that some metadata is "about", but it is also be useful to abbreviate this URI, using the compact syntax. However, the problem is that it is not possible for the language parser to be completely sure whether it has located a CURIE or a URI. For example, a resource could be specified as follows: <p rel="foaf:homePage" about="http://www.example.org/home.html";>home</p> There is no way to be sure that this is a normal URI, or a CURIE. Therefore the syntax for carrying a CURIE when there is any possibility of ambiguity is to enclose the CURIE in square brackets, as in the following example: <html xmlns:ex="http://www.example.org/";> <head>...</head> <body> <p rel="foaf:homePage" about="[ex:home.html]">home</p> </body> </html>
the second example fails to explain how the first /should/ have been cast in order to avoid the ambiguity referred to. The two examples should use exactly the same code fragment, modulo the addition of square brackets, in order to clarify the point being made. Philip TAYLOR
