XQuery在谓词中使用函数

Question

I am learning XQuery and am curious about functions such as last() . As I understand it from the documentation , I am allowed something like:

doc('http://www.functx.com/input/catalog.xml')/catalog/product[last()]

From my understanding, this passes the sequence of product elements:

doc('http://www.functx.com/input/catalog.xml')/catalog/product

... to last() , which then returns the last member. However, last() isn't supposedly taking any arguments, so how is the data-structure being passed to the function?

Thanks!

Answer 1

XQuery is an extension to XPath, but the answer to this question entirely related to XPath.

XPath expressions have a dynamic context; here's the details in the XPath spec: http://www.w3.org/TR/xpath-30/#id-xp-evaluation-context-components . The context item can be references as . :

fn:doc('http://www.functx.com/input/catalog.xml')/catalog/product[. eq "my product name"]

The predicate [. eq "my product name"] [. eq "my product name"] is re-evaluated for each product element, and . is a reference to the context item, ie, the element specific to that evaluation.

Several XPath functions only accept the context, while others default to the context, with optional args.

Here's the complete list of functions that only accept the context in XPath 3: http://www.w3.org/TR/xpath-functions-30/#context .

Some examples:

• fn:last() takes zero arguments and returns the size of the context item

If there are 10 <product/> elements, that following expressions are equivalent:

fn:doc('http://www.functx.com/input/catalog.xml')/catalog/product[fn:last()]

fn:doc('http://www.functx.com/input/catalog.xml')/catalog/product[10]

• fn:position() takes zero arguments and returns the context position

fn:doc('http://www.functx.com/input/catalog.xml')/catalog/product[fn:position() > 3]

---

There are many function that take zero-or-1 arguments, where the zero-argument form accesses the context. I see 15 of them, searching the spec for this phrase: "The zero-argument form of this function is ·deterministic·, ·context-dependent·, and ·focus-dependent·."

One example:

• fn:string() takes zero-or-1 arguments and returns the argument or the context item as a string

fn:doc('http://www.functx.com/input/catalog.xml')/catalog/product/fn:string()

returns the string value of each <product/> element. It's equivalent to

fn:doc('http://www.functx.com/input/catalog.xml')/catalog/product/fn:string(.)

where the context item is explicitly passed as an argument. fn:string() can also be used for type conversions: fn:string(1) returns "1" .

Answer 2

Quoting from Walmsley , p. 49:

The position and last functions are... useful when writing predicates based on position... [Omitted here is a discussion of position .] The last function returns the number of nodes in the current sequence. It takes no arguments and returns an integer representing the number of items. The last function is useful for testing whether an item is the last one in the sequence. For example, catalog/product[last()] returns the last product child of catalog.

So, basically, these functions operate on their context sequence, rather than on parameters/arguments passed to them. position returns the position of the context item within the context sequence, while last returns the number of nodes in the current sequence.