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<H1> 3.2.2 Subtype Declarations</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> A <FONT FACE="Arial, Helvetica">subtype_declaration</FONT>
declares a subtype of some previously declared type, as defined by a
<FONT FACE="Arial, Helvetica">subtype_indication</FONT>. </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">subtype_declaration<A NAME="I1393"></A>
::= </FONT><BR>
<B>subtype</B> <A NAME="I1394"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT> <B>is</B> <A NAME="I1395"></A><FONT FACE="Arial, Helvetica">subtype_indication</FONT>;</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">subtype_indication<A NAME="I1396"></A>
::= </FONT> <A NAME="I1397"></A><FONT FACE="Arial, Helvetica">subtype_mark</FONT> [<A NAME="I1398"></A><FONT FACE="Arial, Helvetica">constraint</FONT>]</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">subtype_mark<A NAME="I1399"></A>
::= </FONT><I>subtype_</I><A NAME="I1400"></A><FONT FACE="Arial, Helvetica">name</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Note that
<FONT FACE="Arial, Helvetica">name</FONT> includes <FONT FACE="Arial, Helvetica">attribute_reference</FONT>;
thus, S'Base can be used as a <FONT FACE="Arial, Helvetica">subtype_mark</FONT>.
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>We considered changing
<FONT FACE="Arial, Helvetica">subtype_mark</FONT> to <FONT FACE="Arial, Helvetica">subtype_name</FONT>.
However, existing users are used to the word "mark," so we're
keeping it. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">constraint<A NAME="I1401"></A>
::= </FONT><A NAME="I1402"></A><FONT FACE="Arial, Helvetica">scalar_constraint</FONT> | <A NAME="I1403"></A><FONT FACE="Arial, Helvetica">composite_constraint</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">scalar_constraint<A NAME="I1404"></A>
::= </FONT><BR>
<A NAME="I1405"></A><FONT FACE="Arial, Helvetica">range_constraint</FONT> | <A NAME="I1406"></A><FONT FACE="Arial, Helvetica">digits_constraint</FONT> | <A NAME="I1407"></A><FONT FACE="Arial, Helvetica">delta_constraint</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">composite_constraint<A NAME="I1408"></A>
::= </FONT><BR>
<A NAME="I1409"></A><FONT FACE="Arial, Helvetica">index_constraint</FONT> | <A NAME="I1410"></A><FONT FACE="Arial, Helvetica">discriminant_constraint</FONT></DIV>
<H4 ALIGN=CENTER>Name Resolution Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> A <FONT FACE="Arial, Helvetica">subtype_mark</FONT>
shall resolve to denote a subtype. <A NAME="I1411"></A>The type <I>determined
by</I> a <FONT FACE="Arial, Helvetica">subtype_mark</FONT> is the type
of the subtype denoted by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>.
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Types are
never directly named; all <FONT FACE="Arial, Helvetica">subtype_mark</FONT>s
denote subtypes -- possibly an unconstrained (base) subtype, but never
the type. When we use the term <I>anonymous type</I> we really mean a
type with no namable subtypes. </FONT></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <A NAME="I1412"></A>The
elaboration of a <FONT FACE="Arial, Helvetica">subtype_declaration</FONT>
consists of the elaboration of the <FONT FACE="Arial, Helvetica">subtype_indication</FONT>.
<A NAME="I1413"></A>The elaboration of a <FONT FACE="Arial, Helvetica">subtype_indication</FONT>
creates a new subtype. If the <FONT FACE="Arial, Helvetica">subtype_indication</FONT>
does not include a <FONT FACE="Arial, Helvetica">constraint</FONT>, the
new subtype has the same (possibly null) constraint as that denoted by
the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>. The elaboration
of a <FONT FACE="Arial, Helvetica">subtype_indication</FONT> that includes
a <FONT FACE="Arial, Helvetica">constraint</FONT> proceeds as follows:
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>The <FONT FACE="Arial, Helvetica">constraint</FONT> is
first elaborated.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I1414"></A><A NAME="I1415"></A>A check is then
made that the <FONT FACE="Arial, Helvetica">constraint</FONT> is <I>compatible</I>
with the subtype denoted by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>.
</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The checks
associated with constraint compatibility are all Range_Checks. Discriminant_Checks
and Index_Checks are associated only with checks that a value satisfies
a constraint. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal"> The condition imposed by a <FONT FACE="Arial, Helvetica">constraint</FONT>
is the condition obtained after elaboration of the <FONT FACE="Arial, Helvetica">constraint</FONT>.
<A NAME="I1416"></A>The rules defining compatibility are given for each
form of <FONT FACE="Arial, Helvetica">constraint</FONT> in the appropriate
subclause. These rules are such that if a <FONT FACE="Arial, Helvetica">constraint</FONT>
is <I>compatible</I> with a subtype, then the condition imposed by the
<FONT FACE="Arial, Helvetica">constraint</FONT> cannot contradict any
condition already imposed by the subtype on its values. <A NAME="I1417"></A>The
exception Constraint_Error is raised if any check of compatibility fails.
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>To be honest: </B>The condition
imposed by a <FONT FACE="Arial, Helvetica">constraint</FONT> is named
after it -- a <FONT FACE="Arial, Helvetica">range_constraint</FONT> imposes
a range constraint, etc. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>A <FONT FACE="Arial, Helvetica">range_constraint</FONT>
causes freezing of its type. Other <FONT FACE="Arial, Helvetica">constraint</FONT>s
do not. </FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>4 A <FONT FACE="Arial, Helvetica">scalar_constraint</FONT>
may be applied to a subtype of an appropriate scalar type (see <A HREF="AA-3-5.html">3.5</A>,
<A HREF="AA-3-5-9.html">3.5.9</A>, and <A HREF="AA-J-3.html">J.3</A>),
even if the subtype is already constrained. On the other hand, a <FONT FACE="Arial, Helvetica">composite_constraint</FONT>
may be applied to a composite subtype (or an access-to-composite subtype)
only if the composite subtype is unconstrained (see <A HREF="AA-3-6-1.html">3.6.1</A>
and <A HREF="AA-3-7-1.html">3.7.1</A>). </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Examples of
subtype declarations:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Examples"><TT><B>subtype</B> Rainbow <B>is</B> Color <B>range</B> Red .. Blue; <I>-- see <A HREF="AA-3-2-1.html">3.2.1</A></I><BR>
<B>subtype</B> Red_Blue <B>is</B> Rainbow;<BR>
<B>subtype</B> Int <B>is</B> Integer;<BR>
<B>subtype</B> Small_Int <B>is</B> Integer <B>range</B> -10 .. 10;<BR>
<B>subtype</B> Up_To_K <B>is</B> Column <B>range</B> 1 .. K; <I>-- see <A HREF="AA-3-2-1.html">3.2.1</A></I><BR>
<B>subtype</B> Square <B>is</B> Matrix(1 .. 10, 1 .. 10); <I>-- see <A HREF="AA-3-6.html">3.6</A></I><BR>
<B>subtype</B> Male <B>is</B> Person(Sex => M); <I>-- see <A HREF="AA-3-10-1.html">3.10.1</A></I></TT></DIV>
<H4 ALIGN=CENTER>Incompatibilities With Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>15.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I1418"></A>In Ada 95,
all <FONT FACE="Arial, Helvetica">range_constraint</FONT>s cause freezing
of their type. Hence, a type-related representation item for a scalar
type has to precede any <FONT FACE="Arial, Helvetica">range_constraint</FONT>s
whose type is the scalar type. </FONT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>15.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><FONT FACE="Arial, Helvetica">Subtype_mark</FONT>s
allow only subtype names now, since types are never directly named. There
is no need for RM83-3.3.2(3), which says a <FONT FACE="Arial, Helvetica">subtype_mark</FONT>
can denote both the type and the subtype; in Ada 95, you denote an unconstrained
(base) subtype if you want, but never the type.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The syntactic category <FONT FACE="Arial, Helvetica">type_mark</FONT>
is now called <FONT FACE="Arial, Helvetica">subtype_mark</FONT>, since
it always denotes a subtype. </FONT></DIV>
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