MESSAGE...IS

 Synopsis

         Define alternate name for inherited message

 Syntax

         MESSAGE <message name> IS <original name>

 Arguments

         <message name> is the name of the message being declared.

         <original name> is the name of an inherited message (a message
         defined in an ancestor class) which the declared message will be
         mapped to.

 Description

         This command allows an alternative name to be defined for an
         existing message. The existing message must be defined in an
         ancestor class of the class being created. The command allows a
         class to be tailored so that its message names match its behavior.

         One common way in which this command is used is to declare
         messages as deferred or null using a statement of the
         following form:

           MESSAGE <name> IS [DEFERRED | NULL]

         This has the effect of mapping the specified message onto a
         predefined method in the Class(y) system class, Object. See the
         sections on Deferred Methods and Null Methods for more
         information.

 Example

         Assume that a Vehicle class defines the message width. We
         would like to define a subclass of Vehicle, called Boat. However,
         the width of a boat at its widest point is usually referred to as
         its beam. We could handle this by writing a method in the Boat
         class:

           METHOD beam
           RETURN ::width

         The MESSAGE...IS command provides us with a more efficient
         shorthand for doing this in the class specification, as follows:

           CREATE CLASS Boat FROM Vehicle
              // ...
              MESSAGE beam IS width

         This achieves the same effect, without the need for a method, and
         will provide better performance.

 Note

         The MESSAGE...IS command can only be used to define an
         alternative name for a message in an ancestor class. To define an
         alternative name for a method in the current class, use the
         MESSAGE...METHOD command. Currently, an alternative name for a
         variable in the current class cannot be defined directly.

 See Also

         VAR...IS, Deferred Methods, Null Methods