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## net.sf.saxon.expr
Class TypeChecker

    java.lang.Object
      net.sf.saxon.expr.TypeChecker

public final class TypeChecker
extends java.lang.Object

This class provides Saxon's type checking capability. It contains a static method, staticTypeCheck, which is called at compile time to perform type checking of an expression. This class is never instantiated.

Method Summary
`static XPathException` | `ebvError(Expression exp, TypeHierarchy th)` Test whether a given expression is capable of returning a value that has an effective boolean value.
`static Expression` | `staticTypeCheck(Expression supplied, SequenceType req, boolean backwardsCompatible, RoleLocator role, StaticContext env)` Check an expression against a required type, modifying it if necessary.
`static Expression` | `strictTypeCheck(Expression supplied, SequenceType req, RoleLocator role, StaticContext env)` Check an expression against a required type, modifying it if necessary.
`static DynamicError` | `testConformance(Value val, SequenceType requiredType, Configuration config)` Test whether a given value conforms to a given type

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Method Detail

### staticTypeCheck

    public static Expression staticTypeCheck(Expression supplied,
                                             SequenceType req,
                                             boolean backwardsCompatible,
                                             RoleLocator role,
                                             StaticContext env)
                                      throws StaticError

  Check an expression against a required type, modifying it if necessary.

  This method takes the supplied expression and checks to see whether it is known statically to conform to the specified type. There are three possible outcomes. If the static type of the expression is a subtype of the required type, the method returns the expression unchanged. If the static type of the expression is incompatible with the required type (for example, if the supplied type is integer and the required type is string) the method throws an exception (this results in a compile-time type error being reported). If the static type is a supertype of the required type, then a new expression is constructed that evaluates the original expression and checks the dynamic type of the result; this new expression is returned as the result of the method.

  The rules applied are those for function calling in XPath, that is, the rules that the argument of a function call must obey in relation to the signature of the function. Some contexts require slightly different rules (for example, operands of polymorphic operators such as "+"). In such cases this method cannot be used.

  Note that this method does not do recursive type-checking of the sub-expressions.

  Parameters:
    `supplied` - The expression to be type-checked
    `req` - The required type for the context in which the expression is used
    `backwardsCompatible` - True if XPath 1.0 backwards compatibility mode is applicable
    `role` - Information about the role of the subexpression within the containing expression, used to provide useful error messages
    `env` - The static context containing the types being checked. At present this is used only to locate a NamePool
  Returns:
    The original expression if it is type-safe, or the expression wrapped in a run-time type checking expression if not.
  Throws:
    `StaticError` - if the supplied type is statically inconsistent with the required type (that is, if they have no common subtype)

### strictTypeCheck

    public static Expression strictTypeCheck(Expression supplied,
                                             SequenceType req,
                                             RoleLocator role,
                                             StaticContext env)
                                      throws StaticError

  Check an expression against a required type, modifying it if necessary. This is a variant of the method `staticTypeCheck(net.sf.saxon.expr.Expression, net.sf.saxon.value.SequenceType, boolean, net.sf.saxon.expr.RoleLocator, net.sf.saxon.expr.StaticContext)` used for expressions that declare variables in XQuery. In these contexts, conversions such as numeric type promotion and atomization are not allowed.

  Parameters:
    `supplied` - The expression to be type-checked
    `req` - The required type for the context in which the expression is used
    `role` - Information about the role of the subexpression within the containing expression, used to provide useful error messages
    `env` - The static context containing the types being checked. At present this is used only to locate a NamePool
  Returns:
    The original expression if it is type-safe, or the expression wrapped in a run-time type checking expression if not.
  Throws:
    `StaticError` - if the supplied type is statically inconsistent with the required type (that is, if they have no common subtype)

### testConformance

    public static DynamicError testConformance(Value val,
                                               SequenceType requiredType,
                                               Configuration config)

  Test whether a given value conforms to a given type

  Parameters:
    `val` - the value
    `requiredType` - the required type
    `config` -
  Returns:
    a DynamicError describing the error condition if the value doesn't conform; or null if it does.

### ebvError

    public static XPathException ebvError(Expression exp,
                                          TypeHierarchy th)

  Test whether a given expression is capable of returning a value that has an effective boolean value.

  Returns:
    null if the expression is OK (optimistically), an exception object if not

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