source: trunk/src/xmlpatterns/expr/qexpression_p.h@ 214

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//
//  W A R N I N G
//  -------------
//
// This file is not part of the Qt API.  It exists purely as an
// implementation detail.  This header file may change from version to
// version without notice, or even be removed.
//
// We mean it.

#ifndef Patternist_Expression_H
#define Patternist_Expression_H

#include <QFlags>
#include <QSharedData>

#include "qcppcastinghelper_p.h"
#include "qdebug_p.h"
#include "qdynamiccontext_p.h"
#include "qexpressiondispatch_p.h"
#include "qitem_p.h"
#include "qsequencetype_p.h"
#include "qsourcelocationreflection_p.h"
#include "qstaticcontext_p.h"

QT_BEGIN_HEADER

QT_BEGIN_NAMESPACE

template<typename T> class QList;
template<typename T> class QVector;

namespace QPatternist
{
    template<typename T, typename ListType> class ListIterator;
    class OptimizationPass;

    /**
     * @short Base class for all AST nodes in an XPath/XQuery/XSL-T expression.
     *
     * @section ExpressionCreation Expression Compilation
     *
     * @subsection ExpressionCreationParser The process of creating an Expression
     *
     * The initial step of creating an internal representation(in some circles
     * called an IR tree) of the XPath string follows classic compiler design: a scanner
     * is invoked, resulting in tokens, which sub-sequently are consumed by a parser
     * which groups the tokens into rules, resulting in the creation of
     * Abstract Syntax Tree(AST) nodes that are arranged in a hierarchical structure
     * similar to the EBNF.
     *
     * More specifically, ExpressionFactory::createExpression() is called with a
     * pointer to a static context, and the string for the expression. This is subsequently
     * tokenized by a Flex scanner. Mistakes detected at this stage is syntax
     * errors, as well as a few semantical errors. Syntax errors can be divided
     * in two types:
     *
     *     - The scanner detects it. An example is the expression "23Eb3" which
     *     is not a valid number literal, or "1prefix:my-element" which is not a
     *     valid QName.
     *     - The parser detects it. This means a syntax error at a
     *     higher level, that a group of tokens couldn't be reduced to a
     *     rule(expression). An example is the expression "if(a = b) 'match' else
     *     'no match'"; the tokenizer would handle it fine, but the parser would
     *     fail because the tokens could not be reduced to a rule due to the token
     *     for the "then" word was missing.
     *
     * Apart from the syntax errors, the actions in the parser also detects
     * errors when creating the corresponding expressions. This is for example
     * that no namespace binding for a prefix could be found, or that a function
     * call was used which no function implementation could be found for.
     *
     * When the parser has finished, the result is an AST. That is, a
     * hierarchical structure consisting of Expression sub-classes. The
     * individual expressions haven't at this point done anything beyond
     * receiving their child-expressions(if any), and hence reminds of a
     * "construction scaffold". In other words, a tree for the expression
     * <tt>'string' + 1 and xs:date('2001-03-13')</tt> could have been created, even if
     * that expression contains errors(one can't add a xs:integer to a xs:string,
     * and the Effective %Boolean Value cannot be extracted for date types).
     *
     * @subsection ExpressionCreationTypeChecking Type Checking
     *
     * After the AST creation, ExpressionFactory::createExpression continues with
     * calling the AST node(which is an Expression instance)'s typeCheck()
     * function. This step ensures that the static types of the operands matches
     * the operators, and in the cases where it doesn't, modifies the AST such
     * that the necessary conversions are done -- if possible, otherwise the
     * result is a type error.
     *
     *
     * This step corresponds roughly to what <a
     * href="http://www.w3.org/TR/xpath20/#id-static-analysis">2.2.3.1 Static Analysis Phase</a>
     * labels operation tree normalization; step SQ5.
     *
     * @subsection ExpressionCreationCompression Compressing -- Optimization and Fixup
     *
     * The last step is calling compress(). This function is not called
     * 'optimize', 'simplify' or the like, because although it performs all
     * optimization, it also involves mandatory stages.
     *
     * One such is const folding, which while being an efficient optimization,
     * also is a necessity for many XSL-T constructs. Another important step is
     * that functions which had an evaluation dependency on the static context(as
     * opposed to the dynamic) performs their "fixup".
     *
     * In other words, this stage potentially performs AST re-writes. For example,
     * the expression <tt>3 + 3, concat('foo', '-', 'bar'), true() and false()</tt> would
     * result in an AST corresponding to <tt>6, 'foo-bar', false()</tt>. This process
     * is done backwards; each expression asks its operands to compress before it
     * performs its own compression(and so forth, until the root expression's call
     * returns to the caller).
     *
     * @see <a href="http://www.w3.org/TR/xpath20/#id-errors-and-opt">XML Path Language
     * (XPath) 2.0, 2.3.4 Errors and Optimization</a>
     * @see <a href="http://www.w3.org/TR/xpath20/#id-expression-processing">XML Path
     * Language (XPath) 2.0, 2.2.3 Expression Processing</a>
     * @see <a href="http://www.w3.org/TR/xquery-xpath-parsing/">Building a Tokenizer
     * for XPath or XQuery</a>
     * @see ExpressionFactory
     * @author Frans Englich <[email protected]>
     * @ingroup Patternist_expressions