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<b>Common Lisp the Language, 2nd Edition</b>
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<H2><A NAME=SECTION003343000000000000000>29.4.3. Exhaustive Case Analysis</A></H2>
<P>
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
<A NAME=EXHAUSTIVECASEANALYSISCONDITIONS>The</A>
syntax for <tt>etypecase</tt> and <tt>ctypecase</tt> is the same as for <tt>typecase</tt>, except
that no <tt>otherwise</tt> clause is permitted. Similarly, the syntax for <tt>ecase</tt> and
<tt>ccase</tt> is the same as for <tt>case</tt> except for the <tt>otherwise</tt> clause.
<P>
<tt>etypecase</tt> and <tt>ecase</tt> are similar to <tt>typecase</tt> and <tt>case</tt>, respectively, but signal
a non-continuable error rather than returning <tt>nil</tt> if no clause is selected.
<P>
<tt>ctypecase</tt> and <tt>ccase</tt> are also similar to <tt>typecase</tt> and <tt>case</tt>, respectively,
but signal a
continuable error if no clause is selected.
<P>
<BR><b>[Macro]</b><BR>
<tt>etypecase <i>keyform</i> {(<i>type</i> {<i>form</i>}*)}*</tt><P> [This supersedes the description of <tt>etypecase</tt>
given in section <A HREF="node222.html#EXHAUSTIVECASEANALYSIS">24.3</A>.-GLS]
<P>
This control construct is similar to <tt>typecase</tt>, but no explicit <tt>otherwise</tt>
or <tt>t</tt> clause is permitted. If no clause is satisfied, <tt>etypecase</tt> signals
an error (of type <tt>type-error</tt>) with a message constructed from the clauses.
It is not permissible to continue from this error. To supply an error
message, the user should use <tt>typecase</tt> with an <tt>otherwise</tt> clause containing
a call to <tt>error</tt>. The name of this function stands for ``exhaustive type
case'' or ``error-checking type case.''
<P>
Example:
<P><pre>
Lisp&gt; (setq x 1/3)
=> 1/3
Lisp&gt; (etypecase x
(integer (* x 4))
(symbol (symbol-value x)))
Error: The value of X, 1/3, is neither an integer nor a symbol.
To continue, type :CONTINUE followed by an option number:
1: Return to Lisp Toplevel.
Debug&gt;
</pre><P>
<P>
<BR><b>[Macro]</b><BR>
<tt>ctypecase <i>keyplace</i> {(<i>type</i> {<i>form</i>}*)}*</tt><P> [This supersedes the description of <tt>ctypecase</tt>
given in section <A HREF="node222.html#EXHAUSTIVECASEANALYSIS">24.3</A>.-GLS]
<P>
This control construct is similar to <tt>typecase</tt>, but no explicit
<tt>otherwise</tt> or <tt>t</tt> clause is permitted.
<P>
The <i>keyplace</i> must be a generalized variable reference acceptable to <tt>setf</tt>.
If no clause is satisfied, <tt>ctypecase</tt> signals an error (of type <tt>type-error</tt>)
with a message constructed from the clauses. This error may be continued
using the <tt>store-value</tt> restart. The argument to <tt>store-value</tt> is stored in
<i>keyplace</i> and then <tt>ctypecase</tt> starts over, making the type tests again.
Subforms of <i>keyplace</i> may be evaluated multiple times. If the <tt>store-value</tt>
restart is invoked interactively, the user will be prompted for the value
to be used.
<P>
The name of this function is mnemonic for ``continuable (exhaustive)
type case.''
<P>
Example:
<P><pre>
Lisp&gt; (setq x 1/3)
=> 1/3
Lisp&gt; (ctypecase x
(integer (* x 4))
(symbol (symbol-value x)))
Error: The value of X, 1/3, is neither an integer nor a symbol.
To continue, type :CONTINUE followed by an option number:
1: Specify a value to use instead.
2: Return to Lisp Toplevel.
Debug&gt; :continue 1
Use value: 3.7
Error: The value of X, 3.7, is neither an integer nor a symbol.
To continue, type :CONTINUE followed by an option number:
1: Specify a value to use instead.
2: Return to Lisp Toplevel.
Debug&gt; :continue 1
Use value: 12
=> 48
</pre><P>
<P>
<BR><b>[Macro]</b><BR>
<tt>ecase <i>keyform</i> {({({<i>key</i>}*) | <i>key</i>} {<i>form</i>}*)}*</tt><P> [This supersedes the description of <tt>ecase</tt>
given in section <A HREF="node222.html#EXHAUSTIVECASEANALYSIS">24.3</A>.-GLS]
<P>
This control construct is similar to <tt>case</tt>, but no explicit <tt>otherwise</tt> or <tt>t</tt>
clause is permitted. If no clause is satisfied, <tt>ecase</tt> signals an error
(of type <tt>type-error</tt>) with a message constructed from the clauses. It is not
permissible to continue from this error. To supply an error message, the
user should use <tt>case</tt> with an <tt>otherwise</tt> clause containing a call to <tt>error</tt>.
The name of this function stands for ``exhaustive case'' or ``error-checking
case.''
<P>
Example:
<P><pre>
Lisp&gt; (setq x 1/3)
=> 1/3
Lisp&gt; (ecase x
(alpha (foo))
(omega (bar))
((zeta phi) (baz)))
Error: The value of X, 1/3, is not ALPHA, OMEGA, ZETA, or PHI.
To continue, type :CONTINUE followed by an option number:
1: Return to Lisp Toplevel.
Debug&gt;
</pre><P>
<P>
<BR><b>[Macro]</b><BR>
<tt>ccase <i>keyplace</i> {({({<i>key</i>}*) | <i>key</i>} {<i>form</i>}*)}*</tt><P> [This supersedes the description of <tt>ccase</tt>
given in section <A HREF="node222.html#EXHAUSTIVECASEANALYSIS">24.3</A>.-GLS]
<P>
This control construct is similar to <tt>case</tt>, but no explicit <tt>otherwise</tt> or
<tt>t</tt> clause is permitted.
<P>
The <i>keyplace</i> must be a generalized variable reference acceptable to <tt>setf</tt>.
If no clause is satisfied, <tt>ccase</tt> signals an error (of type <tt>type-error</tt>)
with a message constructed from the clauses. This error may be continued
using the <tt>store-value</tt> restart. The argument to <tt>store-value</tt> is stored in
<i>keyplace</i> and then <tt>ccase</tt> starts over, making the type tests again. Subforms
of <i>keyplace</i> may be evaluated multiple times. If the <tt>store-value</tt> restart is
invoked interactively, the user will be prompted for the value to be used.
<P>
The name of this function is mnemonic for ``continuable (exhaustive) case.''
<P>
<hr>
<b>Implementation note:</b> The <tt>type-error</tt> signaled by <tt>ccase</tt> and <tt>ecase</tt> is free to
choose any representation of the acceptable argument type that it wishes
for placement in the expected-type slot. It will always work to use type
<tt>(member . <i>keys</i>)</tt>, but in some cases it may be more efficient, for example,
to use a type that represents an integer subrange or a type composed using the
<tt>or</tt> type specifier.
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