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<H1> 12.7 Formal Packages</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [<A NAME="I4316"></A> <A NAME="I4317"></A>Formal
packages can be used to pass packages to a generic unit. The <FONT FACE="Arial, Helvetica">formal_package_declaration</FONT>
declares that the formal package is an instance of a given generic package.
Upon instantiation, the actual package has to be an instance of that
generic package.] </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">formal_package_declaration<A NAME="I4318"></A>
::= </FONT><BR>
<B>with</B> <B>package</B> <A NAME="I4319"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT> <B>is</B> <B>new</B> <I>generic_package_</I><A NAME="I4320"></A><FONT FACE="Arial, Helvetica">name</FONT> <A NAME="I4321"></A><FONT FACE="Arial, Helvetica">formal_package_actual_part</FONT>;</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">formal_package_actual_part<A NAME="I4322"></A>
::= </FONT><BR>
(<>) | [<A NAME="I4323"></A><FONT FACE="Arial, Helvetica">generic_actual_part</FONT>]</DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4324"></A>The <I>generic_package_</I><FONT FACE="Arial, Helvetica">name</FONT>
shall denote a generic package (the <I>template</I> for the formal package);
the formal package is an instance of the template.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The actual shall
be an instance of the template. If the <FONT FACE="Arial, Helvetica">formal_package_actual_part</FONT>
is (<>), [then the actual may be any instance of the template];
otherwise, each actual parameter of the actual instance shall match the
corresponding actual parameter of the formal package [(whether the actual
parameter is given explicitly or by default)], as follows: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>For a formal object of mode <B>in</B> the actuals match
if they are static expressions with the same value, or if they statically
denote the same constant, or if they are both the literal <B>null</B>.
</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>We can't simply
require full conformance between the two actual parameter expressions,
because the two expressions are being evaluated at different times. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>For a formal subtype, the actuals match if they denote
statically matching subtypes. <A NAME="I4325"></A></LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>For other kinds of formals, the actuals match if they statically
denote the same entity. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>8.1/1</FONT></DIV>
<DIV Class="Normal"> {<I><A HREF="defect1.html#8652/0039">8652/0039</A></I>}
<U>For the purposes of matching, any actual parameter that is the name
of a formal object of mode <B>in</B> is replaced by the formal object's
actual expression (recursively).</U> </DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal"> A <FONT FACE="Arial, Helvetica">formal_package_declaration</FONT>
declares a generic formal package.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4326"></A>The visible part of a formal
package includes the first list of <FONT FACE="Arial, Helvetica">basic_declarative_item</FONT>s
of the <FONT FACE="Arial, Helvetica">package_specification</FONT>. In
addition, if the <FONT FACE="Arial, Helvetica">formal_package_actual_part</FONT>
is (<>), it also includes the <FONT FACE="Arial, Helvetica">generic_formal_part</FONT>
of the template for the formal package. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>If the <FONT FACE="Arial, Helvetica">formal_package_actual_part</FONT>
is (<>), then the declarations that occur immediately within the
<FONT FACE="Arial, Helvetica">generic_formal_part</FONT> of the template
for the formal package are visible outside the formal package, and can
be denoted by expanded names outside the formal package. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>We always want
either the actuals or the formals of an instance to be namable from outside,
but never both. If both were namable, one would get some funny anomalies
since they denote the same entity, but, in the case of types at least,
they might have different and inconsistent sets of primitive operators
due to predefined operator ``reemergence.'' Formal derived types exacerbate
the difference. We want the implicit declarations of the <FONT FACE="Arial, Helvetica">generic_formal_part</FONT>
as well as the explicit declarations, so we get operations on the formal
types. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>A generic
formal package is a package, and is an instance. Hence, it is possible
to pass a generic formal package as an actual to another generic formal
package. </FONT></DIV>
<H4 ALIGN=CENTER>Extensions to Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>10.d</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I4327"></A>Formal packages
are new to Ada 95. </FONT></DIV>
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