Package net.sf.saxon.query

Class QueryModule

  • All Implemented Interfaces:
    StaticContext
    Direct Known Subclasses:
    QueryLibrary
    public class QueryModule
extends java.lang.Object
implements StaticContext
    This class represents a query module, and includes information about the static context of the query module. The class is intended for internal Saxon use. User settings that affect the static context are made in the StaticQueryContext object, and those settings are copied to each QueryModule when the query module is compiled.

    • Constructor Detail

      • QueryModule

        public QueryModule​(StaticQueryContext sqc)
            throws XPathException
        Create a QueryModule for a main module, copying the data that has been set up in a StaticQueryContext object
        Parameters:
        sqc - the StaticQueryContext object from which this module is initialized
        Throws:
        XPathException - if information supplied is invalid

      • QueryModule

        public QueryModule​(Configuration config,
                   QueryModule importer)
        Create a QueryModule for a library module.
        Parameters:
        config - the Saxon configuration
        importer - the module that imported this module. This may be null, in the case where the library module is being imported into an XSLT stylesheet

    • Method Detail

      • makeQueryModule

        public static QueryModule makeQueryModule​(java.lang.String baseURI,
                                          Executable executable,
                                          QueryModule importer,
                                          java.lang.String query,
                                          NamespaceUri namespaceURI)
                                   throws XPathException
        Supporting method to load an imported library module. Used also by saxon:import-query in XSLT.

        This method is intended for internal use only.

        Parameters:
        baseURI - The base URI and location URI of the module. Must not be null.
        executable - The Executable
        importer - The importing query module (used to check for cycles). This is null when loading a query module from XSLT.
        query - The text of the query, after decoding and normalizing line endings
        namespaceURI - namespace of the query module to be loaded
        Returns:
        The StaticQueryContext representing the loaded query module
        Throws:
        XPathException - if an error occurs

      • initializeFunctionLibraries

        public void initializeFunctionLibraries()
        Reset function libraries

      • getPackageData

        public PackageData getPackageData()
        Get package data. This is a small data object containing information about the unit of compilation, which in the case of XQuery is a query module
        Specified by:
        getPackageData in interface StaticContext
        Returns:
        data about this query module

      • setPackageData

        public void setPackageData​(PackageData packageData)
        Set the package data. This method is used when we want the QueryModule to share the same package data as another module: notably when fn:load-query-module creates a "dummy" main module to go with the dynamic library module
        Parameters:
        packageData - the package information

      • isTopLevelModule

        public boolean isTopLevelModule()
        Test whether this is a "top-level" module. This is true for a main module and also for a module directly imported into an XSLT stylesheet. It may also be true in future for independently-compiled modules
        Returns:
        true if this is top-level module

      • setIsMainModule

        public void setIsMainModule​(boolean main)
        Set whether this is a "Main" module, in the sense of the XQuery language specification
        Parameters:
        main - true if this is a main module, false if it is a library module

      • isMainModule

        public boolean isMainModule()
        Ask whether this is a "main" module, in the sense of the XQuery language specification
        Returns:
        true if this is a main module, false if it is a library model

      • mayImportModule

        public boolean mayImportModule​(java.lang.String namespace)
        Check whether this module is allowed to import a module with namespace N. Note that before calling this we have already handled the exception case where a module imports another in the same namespace (this is the only case where cycles are allowed, though as a late change to the spec they are no longer useful, since they cannot depend on each other cyclically)
        Parameters:
        namespace - the namespace to be tested
        Returns:
        true if the import is permitted

      • isSchemaAware

        public boolean isSchemaAware()
        Ask whether expressions compiled under this static context are schema-aware. They must be schema-aware if the expression is to handle typed (validated) nodes
        Returns:
        true if expressions are schema-aware

      • makeRetainedStaticContext

        public RetainedStaticContext makeRetainedStaticContext()
        Construct a RetainedStaticContext, which extracts information from this StaticContext to provide the subset of static context information that is potentially needed during expression evaluation
        Specified by:
        makeRetainedStaticContext in interface StaticContext
        Returns:
        a RetainedStaticContext object: either a newly created one, or one that is reused from a previous invocation.

      • setInheritNamespaces

        public void setInheritNamespaces​(boolean inherit)
        Set the namespace inheritance mode
        Parameters:
        inherit - true if namespaces are inherited, false if not
        Since:
        8.4

      • isInheritNamespaces

        public boolean isInheritNamespaces()
        Get the namespace inheritance mode
        Returns:
        true if namespaces are inherited, false if not
        Since:
        8.4

      • setPreserveNamespaces

        public void setPreserveNamespaces​(boolean inherit)
        Set the namespace copy mode
        Parameters:
        inherit - true if namespaces are preserved, false if not

      • isPreserveNamespaces

        public boolean isPreserveNamespaces()
        Get the namespace copy mode
        Returns:
        true if namespaces are preserved, false if not

      • setConstructionMode

        public void setConstructionMode​(int mode)
        Set the construction mode for this module
        Parameters:
        mode - one of Validation.STRIP, Validation.PRESERVE

      • setPreserveBoundarySpace

        public void setPreserveBoundarySpace​(boolean preserve)
        Set the policy for preserving boundary space
        Parameters:
        preserve - true if boundary space is to be preserved, false if it is to be stripped

      • isPreserveBoundarySpace

        public boolean isPreserveBoundarySpace()
        Ask whether the policy for boundary space is "preserve" or "strip"
        Returns:
        true if the policy is to preserve boundary space, false if it is to strip it

      • setEmptyLeast

        public void setEmptyLeast​(boolean least)
        Set the option for where an empty sequence appears in the collation order, if not otherwise specified in the "order by" clause
        Parameters:
        least - true if the empty sequence is considered less than any other value (the default), false if it is considered greater than any other value

      • isEmptyLeast

        public boolean isEmptyLeast()
        Ask what is the option for where an empty sequence appears in the collation order, if not otherwise specified in the "order by" clause
        Returns:
        true if the empty sequence is considered less than any other value (the default), false if it is considered greater than any other value

      • getGlobalFunctionLibrary

        public XQueryFunctionLibrary getGlobalFunctionLibrary()
        Get the function library object that holds details of global functions
        Returns:
        the library of global functions

      • getImportedFunctionLibrary

        public ImportedFunctionLibrary getImportedFunctionLibrary()
        Get the function library object that holds details of imported functions
        Returns:
        the library of imported functions

      • addImportedNamespace

        public void addImportedNamespace​(NamespaceUri uri)
        Register that this module imports a particular module namespace

        This method is intended for internal use.

        Parameters:
        uri - the URI of the imported namespace.

      • importsNamespace

        public boolean importsNamespace​(NamespaceUri uri)
        Ask whether this module directly imports a particular namespace

        This method is intended for internal use.

        Parameters:
        uri - the URI of the possibly-imported namespace.
        Returns:
        true if the schema for the namespace has been imported

      • getTopLevelModule

        public QueryModule getTopLevelModule()
        Get the QueryModule for the top-level module. This will normally be a main module, but in the case of saxon:import-query it will be the library module that is imported into the stylesheet
        Returns:
        the StaticQueryContext object associated with the top level module

      • getExecutable

        public Executable getExecutable()
        Get the Executable, an object representing the compiled query and its environment.

        This method is intended for internal use only.

        Returns:
        the Executable

      • setExecutable

        public void setExecutable​(Executable executable)
        Set the executable.

        This method is intended for internal use only.

        Parameters:
        executable - the Executable

      • getUserQueryContext

        public StaticQueryContext getUserQueryContext()
        Get the StaticQueryContext object containing options set up by the user
        Returns:
        the user-created StaticQueryContext object

      • getContainingLocation

        public Location getContainingLocation()
        Get the LocationMap, an data structure used to identify the location of compiled expressions within the query source text.

        This method is intended for internal use only.

        Specified by:
        getContainingLocation in interface StaticContext
        Returns:
        the LocationMap

      • setModuleNamespace

        public void setModuleNamespace​(NamespaceUri uri)
        Set the namespace for a library module.

        This method is for internal use only.

        Parameters:
        uri - the module namespace URI of the library module. Null is allowed only for a main module, not for a library module.

      • getModuleNamespace

        public NamespaceUri getModuleNamespace()
        Get the namespace of the current library module.

        This method is intended primarily for internal use.

        Returns:
        the module namespace, or null if this is a main module

      • setLocationURI

        public void setLocationURI​(java.net.URI uri)
        Set the location URI for a module
        Parameters:
        uri - the location URI

      • getLocationURI

        public java.net.URI getLocationURI()
        Get the location URI for a module
        Returns:
        the location URI

      • getSystemId

        public java.lang.String getSystemId()
        Get the System ID for a module
        Specified by:
        getSystemId in interface StaticContext
        Returns:
        the location URI

      • setBaseURI

        public void setBaseURI​(java.lang.String uri)
        Set the base URI for a module
        Parameters:
        uri - the base URI

      • getStaticBaseURI

        public java.lang.String getStaticBaseURI()
        Get the base URI for a module
        Specified by:
        getStaticBaseURI in interface StaticContext
        Returns:
        the base URI

      • getGlobalStackFrameMap

        public SlotManager getGlobalStackFrameMap()
        Get the stack frame map for global variables.

        This method is intended for internal use.

        Returns:
        the stack frame map (a SlotManager) for global variables.

      • declareVariable

        public void declareVariable​(GlobalVariable var)
                     throws XPathException
        Declare a global variable. A variable must normally be declared before an expression referring to it is compiled, but there are exceptions where a set of modules in the same namespace import each other cyclically. Global variables are normally declared in the Query Prolog, but they can also be predeclared using the Java API. All global variables are held in the QueryModule for the main module. The fact that a global variable is present therefore does not mean that it is visible: there are two additional conditions (a) the module namespace must be imported into the module where the reference appears, and (b) the declaration must not be in the same module and textually after the reference.

        Note that the same VariableDeclaration object cannot be used with more than one query. This is because the VariableDeclaration is modified internally to hold a list of references to all the places where the variable is used.

        Parameters:
        var - the Variable declaration being declared
        Throws:
        XPathException - if a static error is detected

      • getImportedGlobalVariables

        public java.lang.Iterable<GlobalVariable> getImportedGlobalVariables()
        Get all global variables imported into this module
        Returns:
        a collection of global variables. In the case of a main module, this includes only variables imported into this module, it does not include variables declared within this module. In the case of a library module, it includes both locally declared and imported variables. Blame history.

      • getAllGlobalVariables

        public java.lang.Iterable<GlobalVariable> getAllGlobalVariables()
        Get all global variables declared anywhere in the query
        Returns:
        a collection of global variables.

      • fixupGlobalVariables

        public java.util.List<GlobalVariable> fixupGlobalVariables​(SlotManager globalVariableMap)
                                                    throws XPathException
        Fixup all references to global variables.

        This method is for internal use by the Query Parser only.

        Parameters:
        globalVariableMap - a SlotManager that holds details of the assignment of slots to global variables.
        Returns:
        a list containing the global variable definitions.
        Throws:
        XPathException - if compiling a global variable definition fails

      • getModuleVariables

        public java.util.Iterator<GlobalVariable> getModuleVariables()
        Get global variables declared in this module
        Returns:
        an Iterator whose items are GlobalVariable objects

      • checkForCircularities

        public void checkForCircularities​(java.util.List<GlobalVariable> compiledVars,
                                  XQueryFunctionLibrary globalFunctionLibrary)
                           throws XPathException
        Check for circular definitions of global variables.

        This method is intended for internal use

        Parameters:
        compiledVars - a list of GlobalVariable objects to be checked
        globalFunctionLibrary - the library of global functions
        Throws:
        XPathException - if a circularity is found

      • typeCheckGlobalVariables

        public void typeCheckGlobalVariables​(java.util.List<GlobalVariable> compiledVars)
                              throws XPathException
        Perform type checking on global variables.

        This method is intended for internal use

        Parameters:
        compiledVars - a list of GlobalVariable objects to be checked
        Throws:
        XPathException - if a type error occurs

      • bindVariable

        public Expression bindVariable​(StructuredQName qName)
                        throws XPathException
        Bind a variable used in a query to the expression in which it is declared.

        This method is provided for use by the XQuery parser, and it should not be called by the user of the API, or overridden, unless variables are to be declared using a mechanism other than the declareVariable method of this class.

        Specified by:
        bindVariable in interface StaticContext
        Parameters:
        qName - the name of the variable to be bound
        Returns:
        a VariableReference object representing a reference to a variable on the abstract syntac rtee of the query.
        Throws:
        XPathException - if the variable cannot be bound (has not been declared)

      • getFunctionLibrary

        public FunctionLibrary getFunctionLibrary()
        Get the function library containing all the in-scope functions available in this static context (that is, the functions available in this query module).

        This method is provided for use by advanced applications. The details of the interface are subject to change.

        Specified by:
        getFunctionLibrary in interface StaticContext
        Returns:
        the FunctionLibrary used. For XQuery, this will always be a FunctionLibraryList.
        See Also:
        FunctionLibraryList

      • getLocalFunctionLibrary

        public XQueryFunctionLibrary getLocalFunctionLibrary()
        Get the functions declared locally within this module
        Returns:
        a FunctionLibrary object containing the function declarations

      • declareFunction

        public void declareFunction​(XQueryFunction function)
                     throws XPathException
        Register a user-defined XQuery function.

        This method is intended for internal use only.

        Parameters:
        function - the function being declared
        Throws:
        XPathException - if an error occurs, for example a duplicate function name

      • bindUnboundFunctionCalls

        public void bindUnboundFunctionCalls()
                              throws XPathException
        Bind function calls that could not be bound when first encountered. These will either be forwards references to functions declared later in the same query module, or in modules that are being imported recursively, or errors.

        This method is for internal use only.

        Throws:
        XPathException - if a function call refers to a function that has not been declared

      • fixupGlobalFunctions

        public void fixupGlobalFunctions()
                          throws XPathException
        Fixup all references to global functions. This method is called on completion of query parsing. Each XQueryFunction is required to bind all references to that function to the object representing the run-time executable code of the function.

        This method is for internal use only. It is called only on the StaticQueryContext for the main query body (not for library modules).

        Throws:
        XPathException - if an error occurs

      • optimizeGlobalFunctions

        public void optimizeGlobalFunctions()
                             throws XPathException
        Optimize the body of all global functions.

        This method is for internal use only. It is called only on the StaticQueryContext for the main query body (not for library modules).

        Throws:
        XPathException - if an error occurs during optimization

      • explainGlobalFunctions

        public void explainGlobalFunctions​(ExpressionPresenter out)
                            throws XPathException
        Output "explain" information about each declared function.

        This method is intended primarily for internal use.

        Parameters:
        out - the expression presenter used to display the output
        Throws:
        XPathException - if things go wrong, for example and I/O error

      • getUserDefinedFunction

        public UserFunction getUserDefinedFunction​(NamespaceUri uri,
                                           java.lang.String localName,
                                           int arity)
        Get the function with a given name and arity. This method is provided so that XQuery functions can be called directly from a Java application. Note that there is no type checking or conversion of arguments when this is done: the arguments must be provided in exactly the form that the function signature declares them.
        Parameters:
        uri - the uri of the function name
        localName - the local part of the function name
        arity - the number of arguments.
        Returns:
        the user-defined function, or null if no function with the given name and arity can be located
        Since:
        8.4

      • bindUnboundVariables

        public void bindUnboundVariables()
                          throws XPathException
        Bind unbound variables (these are typically variables that reference another module participating in a same-namespace cycle, since local forwards references are not allowed)
        Throws:
        XPathException - if an error occurs, for example if the variable reference cannot be resolved or if the variable is private

      • addImportedSchema

        public void addImportedSchema​(NamespaceUri targetNamespace,
                              java.lang.String baseURI,
                              java.util.List<java.lang.String> locationURIs)
        Add an imported schema to this static context. A query module can reference types in a schema provided two conditions are satisfied: the schema containing those types has been loaded into the Configuration, and the target namespace has been imported by this query module. This method achieves the second of these conditions. It does not cause the schema to be loaded.
        Parameters:
        targetNamespace - The target namespace of the schema to be added
        baseURI - The base URI against which the locationURIs are to be absolutized
        locationURIs - a list of strings containing the absolutized URIs of the "location hints" supplied for this schema
        Since:
        8.4

      • isImportedSchema

        public boolean isImportedSchema​(NamespaceUri namespace)
        Ask whether a given schema target namespace has been imported
        Specified by:
        isImportedSchema in interface StaticContext
        Parameters:
        namespace - The namespace of the required schema. Supply "" for a no-namespace schema.
        Returns:
        The schema if found, or null if not found.
        Since:
        8.4

      • getImportedSchemaNamespaces

        public java.util.Set<NamespaceUri> getImportedSchemaNamespaces()
        Get the set of imported schemas
        Specified by:
        getImportedSchemaNamespaces in interface StaticContext
        Returns:
        a Set, the set of URIs representing the names of imported schemas

      • reportStaticError

        public void reportStaticError​(XPathException err)
        Report a static error in the query (via the registered ErrorListener)
        Parameters:
        err - the error to be signalled

      • reportStaticError

        public void reportStaticError​(XmlProcessingError err)
        Report a static error in the query (via the registered ErrorListener)
        Parameters:
        err - the error to be signalled

      • makeEarlyEvaluationContext

        public XPathContext makeEarlyEvaluationContext()
        Construct a dynamic context for early evaluation of constant subexpressions
        Specified by:
        makeEarlyEvaluationContext in interface StaticContext
        Returns:
        a dynamic context object

      • getDefaultCollationName

        public java.lang.String getDefaultCollationName()
        Get the name of the default collation.
        Specified by:
        getDefaultCollationName in interface StaticContext
        Returns:
        the name of the default collation; or the name of the codepoint collation if no default collation has been defined

      • setDefaultCollationName

        public void setDefaultCollationName​(java.lang.String collation)
        Set the name of the default collation
        Parameters:
        collation - the URI of the default collation

      • declarePrologNamespace

        public void declarePrologNamespace​(java.lang.String prefix,
                                   NamespaceUri uri)
                            throws XPathException
        Register a namespace that is explicitly declared in the prolog of the query module.
        Parameters:
        prefix - The namespace prefix. Must not be null.
        uri - The namespace URI. Must not be null. The value "" (zero-length string) is used to undeclare a namespace; it is not an error if there is no existing binding for the namespace prefix.
        Throws:
        XPathException - if the declaration is invalid

      • declareActiveNamespace

        public void declareActiveNamespace​(java.lang.String prefix,
                                   NamespaceUri uri)
        Declare an active namespace, that is, a namespace which as well as affecting the static context of the query, will also be copied to the result tree when element constructors are evaluated. When searching for a prefix-URI binding, active namespaces are searched first, then passive namespaces. Active namespaces are later undeclared (in reverse sequence) using undeclareNamespace().

        This method is intended for internal use only.

        Parameters:
        prefix - the namespace prefix
        uri - the namespace URI

      • undeclareNamespace

        public void undeclareNamespace()
        Undeclare the most recently-declared active namespace. This method is called when a namespace declaration goes out of scope (while processing an element end tag). It is NOT called when an XML 1.1-style namespace undeclaration is encountered.

        This method is intended for internal use only.

        See Also:
        declareActiveNamespace(String, NamespaceUri)

      • getLiveNamespaceResolver

        public NamespaceResolver getLiveNamespaceResolver()
        Return a NamespaceResolver which is "live" in the sense that, as the parse proceeds, it always uses the namespaces declarations in scope at the relevant time
        Returns:
        a live NamespaceResolver

      • checkURIForPrefix

        public NamespaceUri checkURIForPrefix​(java.lang.String prefix)
        Get the URI for a prefix if there is one, return null if not. This method is used by the XQuery parser to resolve namespace prefixes.

        This method is intended primarily for internal use.

        Parameters:
        prefix - The prefix. Supply "" to obtain the default namespace for elements and types.
        Returns:
        the corresponding namespace URI, or null if the prefix has not been declared. If the prefix is "" and the default namespace is the non-namespace, return "".

      • getDefaultElementNamespace

        public NamespaceUri getDefaultElementNamespace()
        Get the default XPath namespace for elements and types. Note that this is not necessarily the default namespace declared in the query prolog; within an expression, it may change in response to namespace declarations on element constructors.
        Specified by:
        getDefaultElementNamespace in interface StaticContext
        Returns:
        the default namespace, or NamespaceConstant.NULL for the non-namespace

      • setDefaultElementNamespace

        public void setDefaultElementNamespace​(NamespaceUri uri,
                                       boolean isFixedDefault)
        Set the default element namespace as declared in the query prolog
        Parameters:
        uri - the default namespace for elements and types

      • setDefaultFunctionNamespace

        public void setDefaultFunctionNamespace​(NamespaceUri uri)
        Set the default function namespace
        Parameters:
        uri - the default namespace for functions

      • setRevalidationMode

        public void setRevalidationMode​(int mode)
        Set the revalidation mode. This is used only if XQuery Updates are in use, in other cases the value is ignored.
        Parameters:
        mode - the revalidation mode. This must be one of Validation.STRICT, Validation.LAX, or Validation.SKIP

      • getRevalidationMode

        public int getRevalidationMode()
        Get the revalidation mode. This is used only if XQuery Updates are in use, in other cases the value is ignored.
        Returns:
        the revalidation mode. This will be one of Validation.STRICT, Validation.LAX, or Validation.SKIP

      • getNamespaceResolver

        public NamespaceResolver getNamespaceResolver()
        Get a copy of the Namespace Context. This method is used internally by the query parser when a construct is encountered that needs to save the namespace context for use at run-time. Note that unlike other implementations of StaticContext, the state of the QueryModule changes as the query is parsed, with different namespaces in scope at different times. It's therefore necessary to compute the whole namespace context each time.

        This method is for internal use only.

        Specified by:
        getNamespaceResolver in interface StaticContext
        Returns:
        a namespace resolver.

      • getRequiredContextItemType

        public ItemType getRequiredContextItemType()
        Get the required type of the context item. If no type has been explicitly declared for the context item, an instance of AnyItemType (representing the type item()) is returned.
        Specified by:
        getRequiredContextItemType in interface StaticContext
        Returns:
        the required type of the context item. Note that this is the same for all modules.
        Since:
        9.3

      • getDecimalFormatManager

        public DecimalFormatManager getDecimalFormatManager()
        Get a DecimalFormatManager to resolve the names of decimal formats used in calls to the format-number() function.
        Specified by:
        getDecimalFormatManager in interface StaticContext
        Returns:
        the decimal format manager for this static context, or null if named decimal formats are not supported in this environment.

      • issueWarning

        public void issueWarning​(java.lang.String s,
                         java.lang.String errorCode,
                         Location locator)
        Issue a compile-time warning. This method is used during XQuery expression compilation to output warning conditions.

        This method is intended for internal use only.

        Specified by:
        issueWarning in interface StaticContext
        Parameters:
        s - The warning message. This should not contain any prefix such as "Warning".
        errorCode - Error code identifying the warning condition
        locator - the location of the construct in question. May be null.

      • isInBackwardsCompatibleMode

        public boolean isInBackwardsCompatibleMode()
        Determine whether Backwards Compatible Mode is used
        Specified by:
        isInBackwardsCompatibleMode in interface StaticContext
        Returns:
        false; XPath 1.0 compatibility mode is not supported in XQuery
        Since:
        8.4

      • isUpdating

        public boolean isUpdating()
        Ask whether the query module is allowed to be updating
        Returns:
        true if the query module is allowed to use the XQuery Update facility
        Since:
        9.1

      • setXPathVersion

        public void setXPathVersion​(int languageLevel)
        Set the XPath/XQuery language level
        Parameters:
        languageLevel - 31 for 3.1, 40 for 4.0.

      • getXPathVersion

        public int getXPathVersion()
        Get the XPath language level supported, as an integer (being the actual version number times ten).
        Specified by:
        getXPathVersion in interface StaticContext
        Returns:
        the XPath language level; the return value will be 31 for XPath 3.1 and 40 for XPath 4.0
        Since:
        9.7. In Saxon 9.9 the only value supported for XQuery is 3.1. Saxon 11 also supports 4.0 (identified as integer 40).

      • getCodeInjector

        public CodeInjector getCodeInjector()
        Get the CodeInjector if one is in use
        Returns:
        the code injector if there is one

      • getKeyManager

        public KeyManager getKeyManager()
        Get the KeyManager, containing definitions of keys available for use.
        Specified by:
        getKeyManager in interface StaticContext
        Returns:
        the KeyManager. This is used to resolve key names, both explicit calls on key() used in XSLT, and system-generated calls on key() which may also appear in XQuery and XPath

      • resolveTypeAlias

        public ItemType resolveTypeAlias​(StructuredQName typeName)
        Get type alias. This is a Saxon extension. A type alias is a QName which can be used as a shorthand for an itemtype, using the syntax ~typename anywhere that an item type is permitted.
        Specified by:
        resolveTypeAlias in interface StaticContext
        Parameters:
        typeName - the name of the type alias
        Returns:
        the corresponding item type, if the name is recognised; otherwise null.