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190 lines
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>24.3. Special Forms for Exhaustive Case Analysis</TITLE>
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<meta name="description" value=" Special Forms for Exhaustive Case Analysis">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html4323 HREF="node223.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4321 HREF="node219.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4317 HREF="node221.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4325 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4326 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4324 HREF="node223.html"> Miscellaneous Features</A>
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<B>Up:</B> <A NAME=tex2html4322 HREF="node219.html"> Errors</A>
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<B> Previous:</B> <A NAME=tex2html4318 HREF="node221.html"> Specialized Error-Signaling Forms </A>
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<HR> <P>
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<H1><A NAME=SECTION002830000000000000000>24.3. Special Forms for Exhaustive Case Analysis</A></H1>
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<P>
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<A NAME=EXHAUSTIVECASEANALYSIS>The</A>
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syntax for <tt>etypecase</tt> and <tt>ctypecase</tt> is the same as for
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<tt>typecase</tt>, except that no <tt>otherwise</tt> clause is permitted.
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Similarly, the syntax for <tt>ecase</tt> and <tt>ccase</tt> is the same as for
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<tt>case</tt> except for the <tt>otherwise</tt> clause.
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<P>
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<tt>etypecase</tt> and <tt>ecase</tt> are similar to <tt>typecase</tt> and <tt>case</tt>,
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respectively, but signal a non-continuable error rather than returning
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<tt>nil</tt> if no clause is selected.
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<P>
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<tt>ctypecase</tt> and <tt>ccase</tt> are also similar to <tt>typecase</tt> and <tt>case</tt>,
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but signal a continuable error if no clause is selected.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>etypecase <i>keyform</i> {(<i>type</i> {<i>form</i>}*)}*</tt><P>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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This control construct is similar to <tt>typecase</tt>,
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but no explicit <tt>otherwise</tt> or <tt>t</tt> clause is permitted.
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If no clause is satisfied, <tt>etypecase</tt> signals an error with
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a message constructed from the clauses. It is not permissible to
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continue from this error. To supply an application-specific error message, the
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user should use <tt>typecase</tt> with an <tt>otherwise</tt> clause containing a call
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to <tt>error</tt>. The name of this function stands for ``exhaustive
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type case'' or ``error-checking type case.''
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For example:
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<P><pre>
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(setq x 1/3)
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(etypecase x
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(integer x)
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(symbol (symbol-value x)))
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Error: The value of X, 1/3, is neither
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an integer nor a symbol.
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>
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</pre><P>
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1988
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(CONDITION-SYSTEM) <A NAME=28167> </A>
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to adopt a proposal for a Common Lisp Condition System.
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This proposal modifies the definition of <tt>etypecase</tt> to specify its
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interaction with the condition system.
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See section <A HREF="node337.html#EXHAUSTIVECASEANALYSISCONDITIONS">29.4.3</A>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>ctypecase <i>keyplace</i> {(<i>type</i> {<i>form</i>}*)}*</tt><P>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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This control construct is similar to <tt>typecase</tt>,
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but no explicit <tt>otherwise</tt> or <tt>t</tt> clause is permitted.
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The <i>keyplace</i> must be a generalized variable reference
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acceptable to <tt>setf</tt>. If no clause is satisfied, <tt>ctypecase</tt> signals an
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error with a message constructed from the clauses. Continuing from this
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error causes <tt>ctypecase</tt> to accept a new value from the user, store
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it into <i>keyplace</i>, and start over, making the type tests again.
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Subforms of <i>keyplace</i> may be evaluated multiple times. The name
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of this function stands for ``continuable exhaustive type case.''
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1988
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(CONDITION-SYSTEM) <A NAME=28186> </A>
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to adopt a proposal for a Common Lisp Condition System.
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This proposal modifies the definition of <tt>ctypecase</tt> to specify its
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interaction with the condition system.
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See section <A HREF="node337.html#EXHAUSTIVECASEANALYSISCONDITIONS">29.4.3</A>.
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<P>
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X3J13 voted in March 1988 (PUSH-EVALUATION-ORDER) <A NAME=28191> </A>
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to clarify order of evaluation (see section <A HREF="node80.html#SETFSECTION">7.2</A>).
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>ecase <i>keyform</i> {({({<i>key</i>}*) | <i>key</i>} {<i>form</i>}*)}*</tt><P>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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This control construct is similar to <tt>case</tt>,
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but no explicit <tt>otherwise</tt> or <tt>t</tt> clause is permitted.
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If no clause is satisfied, <tt>ecase</tt> signals an error with a
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message constructed from the clauses. It is not permissible to continue
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from this error. To supply an error message, the user should use
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<tt>case</tt> with an <tt>otherwise</tt> clause containing a call to <tt>error</tt>.
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The name of this function stands for ``exhaustive case'' or
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``error-checking case.''
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For example:
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<P><pre>
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(setq x 1/3)
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(ecase x
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(alpha (foo))
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(omega (bar))
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((zeta phi) (baz)))
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Error: The value of X, 1/3, is not
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ALPHA, OMEGA, ZETA, or PHI.
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</pre><P>
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1988
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(CONDITION-SYSTEM) <A NAME=28210> </A>
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to adopt a proposal for a Common Lisp Condition System.
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This proposal modifies the definition of <tt>ecase</tt> to specify its
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interaction with the condition system.
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See section <A HREF="node337.html#EXHAUSTIVECASEANALYSISCONDITIONS">29.4.3</A>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>ccase <i>keyplace</i> {({({<i>key</i>}*) | <i>key</i>} {<i>form</i>}*)}*</tt><P>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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This control construct is similar to <tt>case</tt>,
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but no explicit <tt>otherwise</tt> or <tt>t</tt> clause is permitted.
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The <i>keyplace</i> must be a generalized variable reference
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acceptable to <tt>setf</tt>. If no clause is satisfied, <tt>ccase</tt> signals an error
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with a message constructed from the clauses. Continuing from this error
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causes <tt>ccase</tt> to accept a new value from the user, store it into
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<i>keyplace</i>, and start over, making the clause tests again. Subforms of
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<i>keyplace</i> may be evaluated multiple times. The name of this function
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stands for ``continuable exhaustive case.''
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1988
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(CONDITION-SYSTEM) <A NAME=28230> </A>
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to adopt a proposal for a Common Lisp Condition System.
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This proposal modifies the definition of <tt>ccase</tt> to specify its
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interaction with the condition system.
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See section <A HREF="node337.html#EXHAUSTIVECASEANALYSISCONDITIONS">29.4.3</A>.
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<P>
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X3J13 voted in March 1988 (PUSH-EVALUATION-ORDER) <A NAME=28235> </A>
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to clarify order of evaluation (see section <A HREF="node80.html#SETFSECTION">7.2</A>).
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<hr>
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<b>Rationale:</b> The special forms
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<tt>etypecase</tt>, <tt>ctypecase</tt>, <tt>ecase</tt>, and <tt>ccase</tt>
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are included in Common Lisp, even though a user
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could write them himself using the other standard facilities provided,
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because it is likely that many users will want these.
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Common Lisp therefore provides
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a standard consistent set rather than allowing
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a variety of incompatible dialects to develop.
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<P>
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In addition, experience has shown that
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some Lisp programmers are too lazy to put an appropriate
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<tt>otherwise</tt> clause into every <tt>case</tt> statement to
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check for cases they
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didn't anticipate, even if they would agree that it will probably
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hurt them later. If an <tt>otherwise</tt> clause can be included
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very easily by adding one character to the name of the construct,
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it is perhaps more likely that programmers will take the trouble to do it.
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<P>
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The <tt>e</tt> versions do nothing more than supply
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automatically generated <tt>otherwise</tt> clauses, but correct
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implementation of the <tt>c</tt> versions
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requires some care. It is therefore especially
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important that the <tt>c</tt> versions be provided
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by the system so users don't have to puzzle them out on
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their own. Individual implementations may be able to do a better job
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of supporting these special forms,
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using their own idiosyncratic facilities, than can be done
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using the error-signaling facilities defined by Common Lisp.
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<hr>
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<P>
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<BR> <HR><A NAME=tex2html4323 HREF="node223.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4321 HREF="node219.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4317 HREF="node221.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4325 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4326 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4324 HREF="node223.html"> Miscellaneous Features</A>
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<B>Up:</B> <A NAME=tex2html4322 HREF="node219.html"> Errors</A>
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<B> Previous:</B> <A NAME=tex2html4318 HREF="node221.html"> Specialized Error-Signaling Forms </A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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