Scryer Prolog documentation

Module xpath

:- use_module(library(xpath)).

Select nodes in an XML DOM

The library 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.

xpath_chk(+DOM, +Spec, ?Content) is semidet.

Semi-deterministic version of xpath/3.

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 //:

  • //Term Select any node in the DOM matching term.

  • /Term Match the root against Term.

  • 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:

  • index(?Index) True if the element is the Index-th child of its parent, where 1 denotes the first child. Index can be one of:

    • Var Var is unified with the index of the matched element.
    • last 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.
    • IntExpr True for the element whose index equals IntExpr.
  • Integer The N-th element with the given name, with 1 denoting the first element. Same as index(Integer).

  • last 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:

  • self Evaluate to the entire element

  • content Evaluate to the content of the element (a list)

  • text Evaluates to all text from the sub-tree, represented as a list of characters.

  • text(atom) Evaluates to all text from the sub-tree as an atom.

  • normalize_space As text, but uses normalize_space/2 to normalise white-space in the output

  • number Extract an integer or float from the value. Ignores leading and trailing white-space

  • @Attribute Evaluates to the value of the given attribute. Attribute can be a compound term. In this case the functor name denotes the element and arguments perform transformations on the attribute value. Defined transformations are:

    • number Translate the value into a number using xsd_number_chars/2.
    • integer As number, but subsequently transform the value into an integer using the round/1 function.
    • float As number, but subsequently transform the value into a float using the float/1 function.
    • lower Translate the value to lower case, preserving the type.
    • upper 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-list of Haystack.

  • XPath 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 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

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

Match the elements


//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)