This is an early access version, made available for early testing and evaluation.
The Content Selection feature provides a mechanism with which publishers can
define alternate renditions of content fragments, and a mechanism for defining conditional
expressions that allows runtime selection of these renditions.
One core use-case for this feature is to vary the information set of output documents, based on criteria such as output content type, or target consumer group. As examples,
In the Z39.86 Authoring and Interchange context, the feature may also be used to provide
feature fallbacks. While a publisher would normally
be in a situation where the feature support in used Processing Agents is known, this is not
always the case, especially in a content interchange scenario. As an example, not knowing
whether a processing chain has full support for the MathML feature, a publisher may choose to
use the Content Selection feature to provide a fallback for MathML content.
The Content Selection feature employs a conditional switch mechanism,
represented by 1-n when elements appearing as children to the parent
select element. There is an always-present default choice within the switch,
represented by the otherwise element.
This feature is maintained by the ANSI/NISO Z39.86 advisory committee under the auspices of NISO.
An expression used at runtime to test whether a content segment should be selected or
not. The expression appears as the value of the expr attribute on the
when element. A conditional expression is syntactically a combination of
one or more functions, and zero or more operators. The result of the evaluation of a conditional expression must always
be a boolean.
The syntax of a conditional expression is normatively defined in EBNF Definition of the Conditional Expression.
A function is the core component of a conditional expression. The return value of a function must always be boolean. A function can take zero or more arguments.
A function must not take another function as an argument, with the exception
of the not() function, which always takes another
function as its argument.
The literal reference to a function always uses a prefix. This prefix defines which function set the function belongs to.
The operators and and or are used when
referencing more than one function in a conditional expression. These operators have the
following meaning:
and: the value of a conditional expression is true if, and only if,
all functions in the expression evaluate to true.
or: the value of a conditional expression is true if, and only if, at
least one of the functions in the expression evaluates to true.
The and and or operators must not occur simultaneously in the
same conditional expression. There is no limitation on the number of functions to be
connected through logical operators.
This feature also defines the function not(), with one, and only one,
argument, which must be another function. The purpose of this function is to return the
value false if the argument function evaluates to true, and vice versa. Note that, contrary
to all other functions, the not() function must not be
prefixed.
A Function Set is a collection of 1-n functions. The set is defined by a canonical URI, where human-readable prose is available defining the name and nature of each function included in the set.
When a function is used in a conditional expression, is uses a name prefix as a token for that URI.
The Content Selection feature provides a set of predefined function sets. Additional function sets may be defined.
The namespace of all members of the Content Selection feature element set
is http://www.daisy.org/z3986/2010/auth/features/select/#.
The otherwise element must not contain descendants in the
http://www.daisy.org/z3986/2010/auth/features/select/# namespace.
The prefix of each function set that is used in a document instance must be declared in the
document instance head using meta elements with the prefix
property of the Resource Directory
Vocabulary.
The declared prefix must resolve to a URI which must contain a human-readable document where the processing behavior of included functions are defined. This URI serves as the canonical identifier of the function set.
In the example below, a target prefix is bound to the URI
http://www.daisy.org/z3986/2010/auth/features/select/functionSets/#targetFunctionSet:
<head xmlns:res="http://www.daisy.org/z3986/2010/vocab/resourcedirectory/#">
<meta about="http://www.daisy.org/z3986/2010/auth/features/select/functionSets/#targetFunctionSet"
property="res:prefix" content="target" />
</head>
Note that function set prefixes, such as target in the example above, are
static throughout the document scope.
[Validity constraint] Several prefixes for the same function set URI must not be declared within the document scope.
[Validity constraint] With the exception of the not() function, which is unique in that it
takes no prefix, all functions in Conditional Expressions must be literally referenced using prefixes,
and these prefixes must be bound to a URI using the mechanism defined above.
Note - the prefix declaration mechanism for function sets is subject to change, since changes to the prefix decl mechanism for CURIEs is currently being investigated.
Conditional Expression := function, (operator, function)*
function := ('not','(' .*')') | (qname, ('()' | '(' .*')'))
operator := 'and' | 'or'
qname := prefix ':' localname
prefix := ncname
localname := ncname
ncname := [A-Za-z][A-Za-z0-9_]*
A set of predefined Function Sets are available. These Function Sets do not differ behaviorally from externally defined function sets in terms of processing requirements.
TODO describe process
select element
From a document information set perspective, the result of the processing of a
select element is that the select element is replaced by
the content of exactly one of the select element's when or
otherwise children.
Processing Agents must process select elements in the following way:
when child, starting with
the first when child in document order.
when
element results in true, select the content of the current
when element, and return.
when
element results in false, continue processing the next sibling in document
order (which is a when or otherwise element).
when or otherwise element). Processing Agents should
issue a warning when encountering unrecognized or unsupported functions or parameters.
when tests evaluate to true, select the content of the
otherwise child, and return.
TODO fix the above to reflect nested when's
Note that the mechanism for supply of runtime parameters to test the conditional expressions against is implementation dependant.
Processing Agents must process conditional expressions (the value of the
expr attribute on the when element) in the following way:
The canonical identity URI of this version of this
feature is http://www.daisy.org/z3986/2010/auth/features/select/0.2/.
This feature is compliant with the Z3986-2010 Part A Specification.
The normative schema of this feature is z3986-feature-select.rng.
Note - this feature schema does not represent an entire document model; it is intended for inclusion in host profiles.
The normative schema includes a number of modules and/or subschemas, which are listed in Appendix 1.
The latest version of the normative schema is available at the canonical URI http://www.daisy.org/z3986/2010/schema/mod/z3986-feature-select.rng.
A listing of all public versions of this feature is available.
This section will contain references to transformation, validation etc software, once provisioning schemes are in place.
Note: the canonical base URI of these modules is http://www.daisy.org/z3986/2010/schema/mod/ , where the modules are available in their latest version.
The below list represents the modules at the time of version 0.2 of this feature.