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<H2><A NAME="sec:6.1"><SPAN class="sec-nr">6.1</SPAN> <SPAN class="sec-title">XPCE
is not Prolog!</SPAN></A></H2>
<A NAME="sec:notprolog"></A>
<P>Data managed by Prolog consists of logical variables, atoms,
integers, floats and compound terms (including lists). <font size=-1>XPCE</font>
has natural counterparts for atoms (a <A class="" href="summary.html#class:name">name</A>
object), integers (a <font size=-1>XPCE</font> int) and floating point
numbers (a <A class="" href="summary.html#class:real">real</A> object).
Prolog logical variables and compound terms however have no direct
counterpart in the
<font size=-1>XPCE</font> environment. <font size=-1>XPCE</font> has
variables (class <A class="" href="summary.html#class:var">var</A>), but
these obey totally different scoping and binding rules.
<P>Where Prolog uses a compound term to represent data that belongs
together (e.g. person(Name, Age, Address)), <font size=-1>XPCE</font>
uses objects for this purpose:<SUP class="fn">7<SPAN class="fn-text">This
example uses <font size=-1>XPCE</font> user-defined classes. The details
of this mechanism do not matter for the argument in this section.
User-defined classes are described in <A class="sec" href="udc.html">chapter
7</A>.</SPAN></SUP>
<PRE class="code">
:- pce_begin_class(person(name, age, address), object).
variable(name, name, both, "Name of the person").
variable(age, int, both, "Age in years").
variable(address, string, both, "Full address").
initialise(P, Name:name, Age:int, Address:string) :->
"Create from name, age and address"::
send(P, name, Name),
send(P, age, Age),
send(P, address, Address).
:- pce_end_class.
1 ?- new(P, person(fred, 30, 'Long Street 45')).
P = @3664437/person
</PRE>
<P>These two representations have very different properties:
<P>
<UL>
<LI><I>Equality</I><BR>
Prolog cannot distinguish between `<TT>person('Fred', 30, 'Long Street
45')</TT>' and a second instance of the same term. In <font size=-1>XPCE</font>
two instances of the same class having the same state are different
entities.
<LI><I>Attributes</I><BR>
Whereas an attribute (argument) of a Prolog term is either a logical
variable or instantiated to a Prolog data object, an attribute of an
object may be assigned to. The assignment is destructive.
<LI><I>Types</I><BR>
<font size=-1>XPCE</font> is a dynamically typed language and <font size=-1>XPCE</font>
object attributes may have types. Prolog is untyped.
</UL>
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