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<H1>Tutorial on Type Analysis</H1>
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<H1><A NAME="SEC10" HREF="typetutorial_toc.html#SEC10">Type Definitions</A></H1>
<P>
We now introduce type definitions to our language.
A name can be defined for any <CODE>TypeDenoter</CODE>.
That name can be used to denote the type it stands for.
<P>
The following concrete productions are added:
<P>
<B>TypeDef.con</B>[40]==
<PRE>
<TT>
Declaration: 'type' TypeDenoter TypeDefIdent ';'.
TypeDefIdent: Ident.
</TT>
</PRE>
<FONT SIZE=1>
<PRE>
This macro is attached to a product file.
</PRE>
</FONT>
<P>
Here is an example program with type definitions. It makes
use of the facility that identifiers may be defined after
their uses:
<B>TypedefExamp</B>[41]==
<PRE>
<TT>begin
var tt rv;
type t tt;
type record int i, bool b, real r end t;
var int j, bool c, real s;
var t rt;
j = rv.i;
c = rv.b;
s = rv.r;
rt = rv;
end
</TT>
</PRE>
<FONT SIZE=1>
<PRE>
This macro is attached to a product file.
</PRE>
</FONT>
<P>
A type definition introduces a new name for a type given
by the <CODE>TypeDenoter</CODE>. In our language we specify that
a type definition does not introduce a new type, rather it introduces
another name for a type. Hence, there may be many different
names for the same type.
This view is supported by the roles of the <CODE>Typing</CODE> module:
They just establish a <CODE>Defer</CODE> relation from the
key of the <CODE>TypeDefIdent</CODE> to the type of the <CODE>TypeDenoter</CODE>.
<P>
Hence, in the following specification we only have to characterize
a defining occurrence of a type identifier by the corresponding
roles of the name analysis module and those for a defining
occurrence of a type identifier (<CODE>TypeDefDefId</CODE>,
<CODE>ChkTypeDefDefId</CODE>). In the <CODE>Declaration</CODE> context
the <CODE>Type</CODE> attribute is just passed from the <CODE>TypeDenoter</CODE>
to the <CODE>TypeDefIdent</CODE>.
<P>
<B>TypeDef.lido</B>[42]==
<PRE>
<TT>
SYMBOL TypeDefIdent INHERITS
ChkUnique, IdDefScope, IdentOcc,
TypeDefDefId, ChkTypeDefDefId
END;
RULE: Declaration ::= 'type' TypeDenoter TypeDefIdent ';' COMPUTE
TypeDefIdent.Type = TypeDenoter.Type;
END;
</TT>
</PRE>
<FONT SIZE=1>
<PRE>
This macro is attached to a product file.
</PRE>
</FONT>
<P>
If we wanted to deviate from the above rules for name equality
in case of certain types, we would use the same specification,
and would augment the function that checks for equality of
type by suitable comparisons, e. g. compare the structure of
two record types.
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