RClassUtils             package:methods             R Documentation

Utilities for Managing Class Definitions

Description:

     These are various functions to support the definition and use of
     formal classes.  Most of them are rarely suitable to be called
     directly.

     Others are somewhat experimental and/or partially implemented
     only. Do refer to 'setClass' for normal code development.

Usage:

     testVirtual(properties, extends, prototype, where)

     makePrototypeFromClassDef(slots, ClassDef, extends, where)

     newEmptyObject()

     completeClassDefinition(Class, ClassDef, where, doExtends)

     getAllSuperClasses(ClassDef, simpleOnly = TRUE)

     superClassDepth(ClassDef, soFar, simpleOnly = TRUE)

     isVirtualClass(Class, where)

     newBasic(Class, ...)

     makeExtends(Class, to, coerce, test, replace, by, package, slots,
                      classDef1, classDef2)

     reconcilePropertiesAndPrototype(name, properties, prototype,
            superClasses, where)

     tryNew(Class, where)

     trySilent(expr)

     empty.dump()

     showClass(Class, complete=TRUE, propertiesAreCalled="Slots")

     showExtends(ext, printTo = stdout())

     possibleExtends(class1, class2, ClassDef1 = getClassDef(class1),
                      ClassDef2 = getClassDef(class2, where =
                      .classEnv(ClassDef1)))

     completeExtends(ClassDef, class2, extensionDef, where)

     classMetaName(name)

     methodsPackageMetaName(prefix, name, package = "")

     metaNameUndo(strings, prefix, searchForm = FALSE)

     requireMethods(functions, signature, message, where)

     checkSlotAssignment(obj, name, value)

     defaultPrototype()

     isClassDef(object)

     validSlotNames(names)

     getDataPart(object)
     setDataPart(object, value, check = TRUE)

Summary of Functions:


     'testVirtual': Test for a Virtual Class. Figures out, as well as
          possible, whether the class with these properties, extension,
          and prototype is a virtual class. Can be forced to be virtual
          by extending '"VIRTUAL"'.

          Otherwise, a class is virtual only if it has no slots,
          extends no non-virtual classes, and has a 'NULL' Prototype.


     'makePrototypeFromClassDef': Makes the prototype implied by the
          class definition.

          The following three rules are applied in this order.

             1.  If the class has slots, then the prototype for each
                slot is used by default, but a corresponding element in
                the explicitly supplied prototype in 'ClassDef', if
                there is one, is used instead (but it must be coercible
                to the class of the slot). This includes the data part
                ('".Data"' slot) if there is one.

             2.  If there are no slots but a non-null prototype was
                specified, this is returned.

             3.  If there is a non-virtual superclass (a class in the
                extends list), then its prototype is used.  The data
                part is extracted if needed (it is allowed to have two
                superclasses with a data part; the first is used and a
                warning issued on any others).

          If all three of the above fail, the prototype is 'NULL'.


     'newEmptyObject': Utility function to create an empty object into
          which slots can be set.

          Currently just creates an empty list with class '"NULL"'.

          Later version should create a special object reference that
          marks an object currently with no slots and no data.


     'completeClassDefinition': Completes the definition of 'Class',
          relative to the class definitions visible from environment
          'where'.  If 'doExtends' is 'TRUE', complete the super- and
          sub-class information.

          This function is called when a class is defined or
          re-defined.


     'getFromClassDef': Extracts one of the intrinsically defined class
          definition properties (".Properties", etc.)  Strictly a
          utility function.

     'getSlots': Returns a named character vector.  The names are the
          names of the slots, the values are the classes of the
          corresponding slots. The argument 'x' can either be the name
          of a class or the class definition object.


     'getAllSuperClasses', 'superClassDepth': Get the names of all the
          classes that this class definition extends.

          'getAllSuperClasses' is a utility function used to complete a
          class definition.  It returns all the superclasses reachable
          from this class, in breadth-first order (which is the order
          used for matching methods); that is, the first direct
          superclass followed by all its superclasses, then the next,
          etc.  (The order is relevant only in the case that some of
          the superclasses have multiple inheritance.)

          'superClassDepth', which is called from 'getAllSuperClasses',
          returns the same information, but as a list with components
          'label' and 'depth', the latter for the number of generations
          back each class is in the inheritance tree.  The argument
          'soFar' is used to avoid loops in the network of class
          relationships.


     'isVirtualClass': Is the named class a virtual class?

          A class is virtual if explicitly declared to be, and also if
          the class is not formally defined.

     'assignClassDef': assign the definition of the class to the
          specially named object


     'newBasic': the implementation of the function 'new' for basic
          classes that don't have a formal definition.

          Any of these could have a formal definition, except for
          'Class="NULL"' (disallowed because 'NULL' can't have
          attributes).  For all cases except '"NULL"', the class of the
          result will be set to 'Class'.

          See 'new' for the interpretation of the arguments.


     'makeExtends': convert the argument to a list defining the
          extension mechanism.


     'reconcilePropertiesAndPrototype': makes a list or a structure
          look like a prototype for the given class.

          Specifically, returns a structure with attributes
          corresponding to the slot names in properties and values
          taken from prototype if they exist there, from 'new(classi)'
          for the class, 'classi' of the slot if that succeeds, and
          'NULL' otherwise.

          The prototype may imply slots not in the properties list,
          since properties does not include inherited slots (these are
          left unresolved until the class is used in a session).


     'tryNew': Tries to generate a new element from this class, but if
          the attempt fails (as, e.g., when the class is undefined or
          virtual) just returns 'NULL'.

          This is inefficient and also not a good idea when actually
          generating objects, but is useful in the initial definition
          of classes.


     'showClass': Print the information about a class definition.

          If 'complete' is 'TRUE', include the indirect information
          about extensions.

          It is the utility called from 'show(getClass(.))', and the
          user should typically use 'getClass(.)' for looking at class
          definitions.


     'showExtends': Print the elements of the list of extensions; for
          'printTo = FALSE', returns a list with components 'what' and
          'how'; this is used e.g., by 'promptClass()'.


     'possibleExtends': Find the information that says whether class1
          extends class2, directly or indirectly.

          This can be either a logical value or an object of class
          'SClassExtension' containing various functions to test and/or
          coerce the relationship.


     'completeExtends': complete the extends information in the class
          definition, by following transitive chains.

          If 'class2' and 'extensionDef' are included, this class
          relation is to be added.  Otherwise just use the current
          'ClassDef'.

          Both the 'contains' and 'subclasses' slots are completed with
          any indirect relations visible.


     'classMetaName': a name for the object storing this class's
          definition


     'methodsPackageMetaName': a name mangling device to hide metadata
          defining method and class information.

     'metaNameUndo' As its name implies, this function undoes the
          name-mangling used to produce meta-data object names, and
          returns a object of class 'ObjectsWithPackage'.


     'requireMethods': Require a subclass to implement methods for the
          generic functions, for this signature.

          For each generic, 'setMethod' will be called to define a
          method that throws an error, with the supplied message.

          The 'requireMethods' function allows virtual classes to
          require actual classes that extend them to implement methods
          for certain functions, in effect creating an API for the
          virtual class.

          Otherwise, default methods for the corresponding function
          would be called, resulting in less helpful error messages or
          (worse still) silently incorrect results.


     'checkSlotAssignment': Check that the value provided is allowed
          for this slot, by consulting the definition of the class. 
          Called from the C code that assigns slots.

          For privileged slots (those that can only be set by accessor
          functions defined along with the class itself), the class
          designer may choose to improve efficiency by validating the
          value to be assigned in the accessor function and then
          calling 'slot<-' with the argument 'check=FALSE', to prevent
          the call to 'checkSlotAssignment'.


     'defaultPrototype': The prototype for a class which will have
          slots, is not a virtual class, and does not extend one of the
          basic classes. Both its 'class' and its (R internal) type,
          'typeof()', are '"S4"'.


     '.InitBasicClasses', '.InitMethodsListClass', '.setCoerceGeneric': 
          These functions perform part of the initialization of classes
          and methods, and are called (only!) from '.First.lib'.


     'isClassDef': Is 'object' a representation of a class?


     'validSlotNames': Returns 'names' unless one of the names is
          reserved, in which case there is an error.  (As of writing,
          '"class"' is the only reserved slot name.)

     'getDataPart', 'setDataPart': Utilities called to implement
          'object@.Data'.  Calls to 'setDataPart' are also used to
          merge the data part of a superclass prototype.


Examples:

     typeof(defaultPrototype()) #-> "S4"

