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<TITLE>Tutorial on Type Analysis - Properties of Predefined Objects</TITLE>
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<H1>Tutorial on Type Analysis</H1>
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<H1><A NAME="SEC4" HREF="typetutorial_toc.html#SEC4">Properties of Predefined Objects</A></H1>
<P>
In our language the type names <CODE>int</CODE>, <CODE>real</CODE>, 
<CODE>bool</CODE>, and <CODE>void</CODE>, and the names <CODE>true</CODE> and
<CODE>false</CODE> for boolean values are predefined.
They are introduced using the following library modules:
<P>
<B>Predef.specs</B>[10]==
<PRE>
<TT>$/Name/PreDefine.gnrc+referto=Ident:inst
$/Name/PreDefId.gnrc+referto=(Predef.d):inst
</TT>
</PRE>
<FONT SIZE=1>
<PRE>
This macro is attached to a product file.
</PRE>
</FONT>
<P>
The following description defines a key for each of the
predefined identifiers:
<P>
<B>Predef.d</B>[11]==
<PRE>
<TT>PreDefKey ("int", intKey)
PreDefKey ("real", realKey)
PreDefKey ("bool", boolKey)
PreDefKey ("void", voidKey)
PreDefKey ("true", trueKey)
PreDefKey ("false", falseKey)
</TT>
</PRE>
<FONT SIZE=1>
<PRE>
This macro is attached to a product file.
</PRE>
</FONT>
<P>
As we are going to use the library module <CODE>Typing</CODE> to support
type analysis we have to adopt its strategy for representing types:
<P>
Types are represented by <CODE>DefTableKeys</CODE>.
Such a key is created for each program construct which
denotes a particular type, and for each predefined type, i. e.
the keys <CODE>intType</CODE>, etc. below. The property <CODE>IsType</CODE>
distinguishes them from non-type objects.
These type keys have to be different from the keys associated
to the predefined type names, <CODE>intKey</CODE>, etc.
The former are associated to the latter via the property
<CODE>Defer</CODE>. This technique allows the module roles to
also deal with type definitions, and allows different
type names stand for the same type.
<P>
<B>PredefTypes.pdl</B>[12]==
<PRE>
<TT>
intType -&#62; IsType = {1};
realType -&#62; IsType = {1};
boolType -&#62; IsType = {1};
voidType -&#62; IsType = {1};

intKey -&#62; Defer = {intType};
realKey -&#62; Defer = {realType};
boolKey -&#62; Defer = {boolType};
voidKey -&#62; Defer = {voidType};
</TT>
</PRE>
<FONT SIZE=1>
<PRE>
This macro is attached to a product file.
</PRE>
</FONT>
<P>
Typed objects, like variables,
have the property <CODE>TypeOf</CODE> that represents their type.
Hence, we associate that property with the predefined names
for boolean values:
<P>
<B>PredefConsts.pdl</B>[13]==
<PRE>
<TT>
trueKey -&#62; TypeOf = {boolType};
falseKey-&#62; TypeOf = {boolType};
</TT>
</PRE>
<FONT SIZE=1>
<PRE>
This macro is attached to a product file.
</PRE>
</FONT>
<P>
Usually the properties <CODE>TypeOf</CODE>, <CODE>Defer</CODE>, and <CODE>IsType</CODE>
are not set or accessed explicitly. Module roles ensure that 
they are properly used.
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