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xpath.pl -- Select nodes in an XML DOM
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The library xpath.pl provides predicates to select nodes from an XML DOM tree as produced by library(sgml) based on descriptions inspired by the XPath language.

The predicate xpath/3 selects a sub-structure of the DOM non-deterministically based on an XPath-like specification. Not all selectors of XPath are implemented, but the ability to mix xpath/3 calls with arbitrary Prolog code provides a powerful tool for extracting information from XML parse-trees.

See also
- http://www.w3.org/TR/xpath
Source xpath_chk(+DOM, +Spec, ?Content) is semidet
Semi-deterministic version of xpath/3.
Source xpath(+DOM, +Spec, ?Content) is nondet
Match an element in a DOM structure. The syntax is inspired by XPath, using () rather than [] to select inside an element. First we can construct paths using / and //:
Select any node in the DOM matching term.
Match the root against Term.
Select the immediate children of the root matching Term.

The Terms above are of type callable. The functor specifies the element name. The element name '*' refers to any element. The name self refers to the top-element itself and is often used for processing matches of an earlier xpath/3 query. A term NS:Term refers to an XML name in the namespace NS. Optional arguments specify additional constraints and functions. The arguments are processed from left to right. Defined conditional argument values are:

True if the element is the Index-th child of its parent, where 1 denotes the first child. Index can be one of:
Var is unified with the index of the matched element.
True for the last element.
last - IntExpr
True for the last-minus-nth element. For example, last-1 is the element directly preceding the last one.
True for the element whose index equals IntExpr.
The N-th element with the given name, with 1 denoting the first element. Same as index(Integer).
The last element with the given name. Same as index(last).
last - IntExpr
The IntExpr-th element before the last. Same as index(last-IntExpr).

Defined function argument values are:

Evaluate to the entire element
Evaluate to the content of the element (a list)
Evaluates to all text from the sub-tree as an atom
Evaluates to all text from the sub-tree according to As, which is either atom or string.
As text, but uses normalize_space/2 to normalise white-space in the output
Extract an integer or float from the value. Ignores leading and trailing white-space
Evaluates to the value of the given attribute. Attribute can be a compound term. In this case the functor name denotes the attribute and arguments perform transformations on the attribute value. Defined transformations are:
Translate the value into a number using xsd_number_string/2 from library(sgml).
As number, but subsequently transform the value into an integer using the round/1 function.
As number, but subsequently transform the value into a float using the float/1 function.
Translate the value into a Prolog atom. Note that an atom is normally the default, so @href and @href(atom) are equivalent. The SGML parser can return attributes as strings using the attribute_value(string) option.
Translate the value into a Prolog string.
Translate the value to lower case, preserving the type.
Translate the value to upper case, preserving the type.

In addition, the argument-list can be conditions:

Left = Right
Succeeds if the left-hand unifies with the right-hand. If the left-hand side is a function, this is evaluated. The right-hand side is never evaluated, and thus the condition content = content defines that the content of the element is the atom content. The functions lower_case and upper_case can be applied to Right (see example below).
contains(Haystack, Needle)
Succeeds if Needle is a sub-string of Haystack.
Succeeds if XPath matches in the currently selected sub-DOM. For example, the following expression finds an h3 element inside a div element, where the div element itself contains an h2 child with a strong child.

This is equivalent to the conjunction of XPath goals below.

   xpath(DOM, //(div), Div),
   xpath(Div, h2/strong, _),
   xpath(Div, h3, Result)


Match each table-row in DOM:

xpath(DOM, //tr, TR)

Match the last cell of each tablerow in DOM. This example illustrates that a result can be the input of subsequent xpath/3 queries. Using multiple queries on the intermediate TR term guarantee that all results come from the same table-row:

xpath(DOM, //tr, TR),
xpath(TR,  /td(last), TD)

Match each href attribute in an <a> element

xpath(DOM, //a(@href), HREF)

Suppose we have a table containing rows where each first column is the name of a product with a link to details and the second is the price (a number). The following predicate matches the name, URL and price:

product(DOM, Name, URL, Price) :-
    xpath(DOM, //tr, TR),
    xpath(TR, td(1), C1),
    xpath(C1, /self(normalize_space), Name),
    xpath(C1, a(@href), URL),
    xpath(TR, td(2, number), Price).

Suppose we want to select books with genre="thriller" from a tree containing elements <book genre=...>

thriller(DOM, Book) :-
    xpath(DOM, //book(@genre=thiller), Book).

Match the elements <table align="center"> and <table align="CENTER">:

    //table(@align(lower) = center)

Get the width and height of a div element as a number, and the div node itself:

    xpath(DOM, //div(@width(number)=W, @height(number)=H), Div)

Note that div is an infix operator, so parentheses must be used in cases like the following:

    xpath(DOM, //(div), Div)