TEACH SLOT_DEFAULTS                                 Steve Knight, April 92

[See REF * OBJECTCLASS/Field Specifications for more detail]

Associated with each slot there is a default-value and a default-procedure.
The ordinary "=" syntax syntax specifies a default-procedure which is executed
at run-time.  e.g.

    define :class timestamp;
        slot stamp = sysdaytime();
    enddefine;

When the timestamp objects are built, the default is calculated at the moment
of creation.

    newtimestamp() =>
    ** <timestamp stamp:Sat Apr 25 19:29:39 BST 1992>

    ;;; A few moments later
    newtimestamp() =>
    ** <timestamp stamp:Sat Apr 25 19:29:56 BST 1992>

Unfortunately, there is inevitably a small efficiency penalty associated with
running procedures at creation-time.  Almost always this penalty is too small
to worry about.  But for applications in which speed matters another way of
specifying defaults is avaiable.  This uses the "==" keyword.

    define :class sillystamp;
        slot stamp == sysdaytime();     ;;; this expression is calculated
    enddefine;                          ;;; at compile-time!

    newsillystamp() =>
    ** <sillystamp stamp:Sat Apr 25 19:32:12 BST 1992>

    ;;; It doesn't matter when this is run.
    newsillystamp() =>
    ** <sillystamp stamp:Sat Apr 25 19:32:12 BST 1992>

The "==" specifies that the expression is to be used to calculate a value
at the point of compilation.  This is then used as the default value.  Since
the computation only ever happens once, the construction of objects can avoid
running a procedure to initialise that slot.
--- C.all/lib/objectclass/teach/slot_defaults
--- Copyright University of Sussex 1993. All rights reserved.