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<H1> 11.2 Exception Handlers</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [The response to one or more exceptions is specified
by an <FONT FACE="Arial, Helvetica">exception_handler</FONT>.] </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">handled_sequence_of_statements<A NAME="I4036"></A>
::= </FONT><BR>
<A NAME="I4037"></A><FONT FACE="Arial, Helvetica">sequence_of_statements</FONT><BR>
[<B>exception</B><BR>
<A NAME="I4038"></A><FONT FACE="Arial, Helvetica">exception_handler</FONT><BR>
{<A NAME="I4039"></A><FONT FACE="Arial, Helvetica">exception_handler</FONT>}]</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">exception_handler<A NAME="I4040"></A>
::= </FONT><BR>
<B>when</B> [<A NAME="I4041"></A><FONT FACE="Arial, Helvetica">choice_parameter_specification</FONT>:] <A NAME="I4042"></A><FONT FACE="Arial, Helvetica">exception_choice</FONT> {| <A NAME="I4043"></A><FONT FACE="Arial, Helvetica">exception_choice</FONT>} =><BR>
<A NAME="I4044"></A><FONT FACE="Arial, Helvetica">sequence_of_statements</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">choice_parameter_specification<A NAME="I4045"></A>
::= </FONT><A NAME="I4046"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">exception_choice<A NAME="I4047"></A>
::= </FONT><I>exception_</I><A NAME="I4048"></A><FONT FACE="Arial, Helvetica">name</FONT> | <B>others</B></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>To be honest: </B><A NAME="I4049"></A>``<I>Handler</I>''
is an abbreviation for ``<FONT FACE="Arial, Helvetica">exception_handler</FONT>.''</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I4050"></A>Within this
section, we sometimes abbreviate ``<FONT FACE="Arial, Helvetica">exception_choice</FONT>''
to ``<I>choice</I>.'' </FONT></DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4051"></A>A choice with an <I>exception_</I><FONT FACE="Arial, Helvetica">name</FONT>
<I>covers</I> the named exception. A choice with <B>others</B> covers
all exceptions not named by previous choices of the same <FONT FACE="Arial, Helvetica">handled_sequence_of_statements</FONT>.
Two choices in different <FONT FACE="Arial, Helvetica">exception_handler</FONT>s
of the same <FONT FACE="Arial, Helvetica">handled_sequence_of_statements</FONT>
shall not cover the same exception. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Two <FONT FACE="Arial, Helvetica">choice</FONT>s
of the same <FONT FACE="Arial, Helvetica">exception_handler</FONT> may
cover the same exception. For example, given two renaming declarations
in separate packages for the same exception, one may nevertheless write,
for example, ``<B>when</B> Ada.Text_IO.Data_Error | My_Seq_IO.Data_Error
=>''.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>An <B>others</B> choice even covers
exceptions that are not visible at the place of the handler. Since exception
raising is a dynamic activity, it is entirely possible for an <B>others</B>
handler to handle an exception that it could not have named. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> A choice with <B>others</B> is allowed only for
the last handler of a <FONT FACE="Arial, Helvetica">handled_sequence_of_statements</FONT>
and as the only choice of that handler.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> An <I>exception_</I><FONT FACE="Arial, Helvetica">name</FONT>
of a choice shall not denote an exception declared in a generic formal
package. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>This is because
the compiler doesn't know the identity of such an exception, and thus
can't enforce the coverage rules. </FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4052"></A>A <FONT FACE="Arial, Helvetica">choice_parameter_specification</FONT>
declares a <I>choice parameter</I>, which is a constant object of type
Exception_Occurrence (see <A HREF="AA-11-4-1.html">11.4.1</A>). During
the handling of an exception occurrence, the choice parameter, if any,
of the handler represents the exception occurrence that is being handled.</DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4053"></A>The execution of a <FONT FACE="Arial, Helvetica">handled_sequence_of_statements</FONT>
consists of the execution of the <FONT FACE="Arial, Helvetica">sequence_of_statements</FONT>.
[The optional handlers are used to handle any exceptions that are propagated
by the <FONT FACE="Arial, Helvetica">sequence_of_statement</FONT>s.]
</DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Example of an
exception handler:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Examples"><TT><B>begin</B><BR>
Open(File, In_File, "input.txt"); <I>-- see <A HREF="AA-A-8-2.html">A.8.2</A></I><BR>
<B>exception</B><BR>
<B>when</B> E : Name_Error =><BR>
Put("Cannot open input file : ");<BR>
Put_Line(Exception_Message(E)); <I>-- see <A HREF="AA-11-4-1.html">11.4.1</A></I><BR>
<B>raise</B>;<BR>
<B>end</B>;</TT></DIV>
<H4 ALIGN=CENTER>Extensions to Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>12.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I4054"></A>The syntax
rule for <FONT FACE="Arial, Helvetica">exception_handler</FONT> is modified
to allow a <FONT FACE="Arial, Helvetica">choice_parameter_specification</FONT>.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>Different <FONT FACE="Arial, Helvetica">choice</FONT>s
of the same <FONT FACE="Arial, Helvetica">exception_handler</FONT> may
cover the same exception. This allows for ``when Numeric_Error | Constraint_Error
=>'' even though Numeric_Error is a rename of Constraint_Error. This
also allows one to ``with'' two different I/O packages, and then write,
for example, ``when Ada.Text_IO.Data_Error | My_Seq_IO.Data_Error =>''
even though these might both be renames of the same exception. </FONT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>12.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The syntax rule for <FONT FACE="Arial, Helvetica">handled_sequence_of_statements</FONT>
is new. These are now used in all the places where handlers are allowed.
This obviates the need to explain (in Sections 5, 6, 7, and 9) what portions
of the program are handled by the handlers. Note that there are more
such cases in Ada 95.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12.d</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The syntax rule for <FONT FACE="Arial, Helvetica">choice_parameter_specification</FONT>
is new. </FONT></DIV>
<HR>
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