Changes

Jump to navigation Jump to search
3,798 bytes added ,  13:03, 6 May 2019
This article is part of the [[XQuery|XQuery Portal]]. It lists extensions and optimizations that are specific to the BaseX XQuery processor.
 
=Expressions=
 
Some of the extensions that have been added to BaseX may also be made available in other XQuery processors in the near future.
 
==Ternary If==
 
The [https://en.wikipedia.org/wiki/%3F: ternary if] operator provides a short syntax for conditions. It is also called '''conditional operator''' or '''ternary operator'''. In most languages, the syntax is <code>a ? b : c</code>. As <code>?</code> and <code>:</code> have already been taken in XQuery, the syntax of Perl 6 is used:
 
<pre class="brush:xquery">
$test ?? 'ok' !! 'fails'
</pre>
 
The expression returns <code>ok</code> if the effective boolean value of <code>$test</code> is true, and it returns <code>fails</code> otherwise.
 
==Elvis Operator==
 
The Elvis operator is also available in other languages. It is sometimes called [https://en.wikipedia.org/wiki/Null_coalescing_operator null-coalescing operator]. In XQuery, the value of the first operand will be returned if it is a non-empty sequence. Otherwise, the value of the second operand will be returned.
 
<pre class="brush:xquery">
let $number := 123
return (
(: if/then/else :)
if (exists($number)) then $number else 0,
(: elvis operator :)
$number ?: 0
)
</pre>
 
The behavior of the operator is equivalent to the {{Function|Utility|util:or}} function.
 
==If Without Else==
 
In XQuery 3.1, both branches of the <code>if</code> expression need to be specified. In many cases, only one branch is required, so the <code>else</code> branch was made optional in BaseX. If the second branch is omitted, an empty sequence will be returned if the effective boolean value of the test expression is false. Some examples:
 
<pre class="brush:xquery">
if (doc-available($doc)) then doc($doc),
if (file:exists($file)) then file:delete($file),
if (permissions:valid($user)) then <html>Welcome!</html>
</pre>
 
If conditions are nested, a trailing else branch will be associated with the innermost <code>if</code>:
 
<pre class="brush:xquery">
if ($a) then if($b) then '$a and $b is true' else 'only $b is true'
</pre>
 
In general, if you have multiple or nested if expressions, additional parentheses can improve the readibility of your code:
 
<pre class="brush:xquery">
if ($a) then (
if($b) then '$a and $b is true' else 'only $b is true'
)
</pre>
 
The behavior of the if expression is equivalent to the {{Function|Utility|util:if}} function.
 
=Functions=
 
==Regular Expressions==
 
In analogy with Saxon, you can specify the flag {{Code|j}} to revert to Java’s default regex parser. For example, this allows you to use the word boundary option {{Code|\b}}, which has not been included in the XQuery grammar for regular expressions:
 
'''Example:'''
<pre class="brush:xquery">
(: yields "!Hi! !there!" :)
replace('Hi there', '\b', '!', 'j')
</pre>
 
=Serialization=
 
* <code>basex</code>is used as the default serialization method: nodes are serialized as XML, atomic values are serialized as string, and items of binary type are output in their native byte representation. Function items (including maps and arrays) are output just like with the [[XQuery 3.1#Adaptive Serialization|adaptive]] method.
* With {{Code|csv}}, you can output XML nodes as CSV data (see the [[CSV Module]] for more details).
* With {{Code|json}}, items are output as JSON as described in the [https://www.w3.org/TR/xslt-xquery-serialization-31/#json-output official specification]. If the root node is of type {{Code|element(json)}}, items are serialized as described for the {{Code|direct}} format in the [[JSON Module]].
 
For more information and some additional BaseX-specific parameters, see the article on [[Serialization]].
=Option Declarations=
This pragma can be helpful when debugging your code.
 
{{Mark|Introduced with Version 9.1:}}
In analogy with option declarations and function annotations, [[Transactions#XQuery_Locks|XQuery Locks]] can also set via pragmas:
</pre>
==Lazy evaluationEvaluation==
{{Code|%basex:lazy}} enforces lazy evaluation of a global variable. An example:
==XQuery Locks==
 
{{Mark|Introduced with Version 9.1:}}
In analogy with option declarations and pragmas, [[Transactions#XQuery_Locks|XQuery Locks]] can also set via annotations:
</pre>
=Functions= ==Regular expressions== {{Mark|Introduced with Version 9.1:}} In analogy with Saxon, you can specify the flag {{Code|j}} to revert to Java’s default regex parser. For example, this allows you to use the word boundary option {{Code|\b}}, which has not been included in the XQuery grammar for regular expressions: '''Example:''' <pre class="brush:xquery">(: yields "!Hi! !there!" :)replace('Hi there', '\b', '!', 'j')</pre> =Serialization= * <code>basex</code> is used as the default serialization method: nodes are serialized as XML, atomic values are serialized as string, and items of binary type are output in their native byte representation. Function items (including maps and arrays) are output just like with the [[XQuery 3.1#Adaptive Serialization|adaptive]] method.* {{Code|csv}} allows you to output XML nodes as CSV data (see the [[CSV Module]] for more details). For more information and some additional BaseX-specific parameters, see the article on [[Serialization]]. =Non-determinismDeterminism=
In [http://www.w3.org/TR/xpath-functions-31/#dt-deterministic XQuery], ''deterministic'' functions are “guaranteed to produce ·identical· results from repeated calls within a single ·execution scope· if the explicit and implicit arguments are identical”. In BaseX, many extension functions are non-deterministic or side-effecting. If an expression is internally flagged as non-deterministic, various optimizations that might change their execution order will not be applied.
Two non-deterministic functions will be bound to <code>$read</code>, and the result of the function call will be bound to <code>$ignored</code>. As the variable is not referenced in the subsequent code, the let clause would usually be discarded by the compiler. In the given query, however, execution will be enforced because of the BaseX-specific {{Code|non-deterministic}} keyword.
 
=Namespaces=
 
In XQuery, some namespaces are statically bound to prefixes. The following query requires no additional namespaces declarations in the query prolog:
 
<pre class="brush:xquery">
<xml:abc xmlns:prefix='uri' local:fn='x'/>,
fn:exists(1)
</pre>
 
In BaseX, various other namespaces are predefined. Apart from the namespaces that are listed on the [[Module Library]] page, the following namespaces are statically bound:
 
{| class="wikitable sortable"
|-
! Description
! Prefix
! Namespace URI
|-
| [[#Annotations|BaseX Annotations]], [[#Pragmas|Pragmas]], …
| <code>basex</code>
| <code><nowiki>http://basex.org</nowiki></code>
|-
| [[RESTXQ#Input Options|RESTXQ: Input Options]]
| <code>input</code>
| <code><nowiki>http://basex.org/modules/input</nowiki></code>
|-
| [[Repository#EXPath_Packaging|EXPath Packages]]
| <code>pkg</code>
| <code><nowiki>http://expath.org/ns/pkg</nowiki></code>
|-
| [[XQuery Errors]]
| <code>err</code>
| <code><nowiki>http://www.w3.org/2005/xqt-errors</nowiki></code>
|-
| [[Serialization]]
| <code>output</code>
| <code><nowiki>http://www.w3.org/2010/xslt-xquery-serialization</nowiki></code>
|}
=Suffixes=
;Version 9.1:
* Added: New [[#Expressions|Expressions]]: Ternary if, elvis Operator, if without else
* Added: XQuery Locks via pragmas and function annotations.
* Added: [[#Regular expressions|Regular Expressions]], {{Code|j}} flag for using Java’s default regex parser.
Bureaucrats, editor, reviewer, Administrators
11,903

edits

Navigation menu