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>error | Common Lisp Nova Spec</TITLE
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>Common Lisp Nova Spec</A
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="9_Conditions.html"
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>9. Conditions</A
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><INPUT ID="apropos" AUTOFOCUS="AUTOFOCUS" PLACEHOLDER="Type here to search" ONINPUT="aproposInput(this);" ONKEYUP="aproposKeyup(event);" ONCHANGE="aproposChange(this);" ONFOCUS="aproposFocus(this);" ONFOCUSOUT="aproposFocusout(this);"
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><DIV CLASS="com"
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><HR
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><TABLE WIDTH="100%" CELLSPACING="0" CELLPADDING="0"
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><TR
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><TD ALIGN="LEFT" VALIGN="BASELINE" WIDTH="100%" CLASS="name"
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><SPAN CLASS="idx" DATA-KIND="idxref" DATA-TERM="error"
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></SPAN
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><B
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>error</B
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></TD
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><TD ALIGN="RIGHT" VALIGN="BASELINE" WIDTH="0" NOWRAP="NOWRAP" CLASS="ftype"
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><I
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>Function</I
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></TD
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></TR
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></TABLE
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><HR
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></DIV
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><UL CLASS="subtoc"
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></UL
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><DL
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><DT
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><B
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>Syntax</B
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></DT
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><DD
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><P CLASS="j"
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><B
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>error</B
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> <SPAN CLASS="cmssi"
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>datum</SPAN
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> <SPAN CLASS="cmtt"
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>&rest</SPAN
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> <SPAN CLASS="cmssi"
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>arguments</SPAN
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> <SPAN CLASS="arrow"
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>→</SPAN
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>|</P
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></DD
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><DT
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><B
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>Arguments and Values</B
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></DT
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><DD
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><P CLASS="j"
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><VAR CLASS="param"
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>datum</VAR
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>, <VAR CLASS="param"
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>arguments</VAR
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> — <A HREF="26_1_Glossary.html#designator"
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><EM CLASS="term"
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>designators</EM
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></A
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> for a <A HREF="26_1_Glossary.html#condition"
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><EM CLASS="term"
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>condition</EM
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></A
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> of default type <A HREF="t_simple-error.html" CLASS="typeref"
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><B
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>simple-error</B
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></A
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>.</P
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></DD
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><DT
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><B
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>Description</B
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></DT
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><DD
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><P CLASS="j"
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><A HREF="f_error.html" CLASS="funref"
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><B
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>error</B
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></A
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> effectively invokes <A HREF="f_signal.html" CLASS="funref"
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><B
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>signal</B
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></A
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> on the denoted <A HREF="26_1_Glossary.html#condition"
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><EM CLASS="term"
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>condition</EM
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></A
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>. </P
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><P CLASS="j"
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>If the <A HREF="26_1_Glossary.html#condition"
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><EM CLASS="term"
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>condition</EM
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></A
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> is not handled, <CODE CLASS="f"
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>(invoke-debugger <I CLASS="i"
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><I
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>condition</I
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></I
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>)</CODE
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> is done. As a consequence of calling <A HREF="f_invoke-debugger.html" CLASS="funref"
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><B
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>invoke-debugger</B
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></A
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>, <A HREF="f_error.html" CLASS="funref"
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><B
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>error</B
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></A
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> cannot directly return; the only exit from <A HREF="f_error.html" CLASS="funref"
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><B
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>error</B
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></A
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> can come by non-local transfer of control in a handler or by use of an interactive debugging command.</P
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></DD
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><DT
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><B
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>Examples</B
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></DT
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><DD
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><PRE CLASS="screen"
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> (defun factorial (x)
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(cond ((or (not (typep x 'integer)) (minusp x))
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(error "~S is not a valid argument to FACTORIAL." x))
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((zerop x) 1)
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(t (* x (factorial (- x 1))))))
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<SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> FACTORIAL
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(factorial 20)
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<SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 2432902008176640000
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(factorial -1)
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Error: -1 is not a valid argument to FACTORIAL.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> To continue, type :CONTINUE followed by an option number:
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> 1: Return to Lisp Toplevel.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Debug></PRE
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><PRE CLASS="screen"
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> (setq a 'fred)
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<SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> FRED
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(if (numberp a) (1+ a) (error "~S is not a number." A))
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Error: FRED is not a number.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> To continue, type :CONTINUE followed by an option number:
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> 1: Return to Lisp Toplevel.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Debug> <U CLASS="underline"
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><SPAN CLASS="cmtt"
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>:Continue</SPAN
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> <SPAN CLASS="cmtt"
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>1</SPAN
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></U
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>
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Return to Lisp Toplevel.
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(define-condition not-a-number (error)
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((argument :reader not-a-number-argument :initarg :argument))
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(:report (lambda (condition stream)
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(format stream "~S is not a number."
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(not-a-number-argument condition)))))
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<SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> NOT-A-NUMBER
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(if (numberp a) (1+ a) (error 'not-a-number :argument a))
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Error: FRED is not a number.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> To continue, type :CONTINUE followed by an option number:
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> 1: Return to Lisp Toplevel.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Debug> <U CLASS="underline"
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><SPAN CLASS="cmtt"
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>:Continue</SPAN
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> <SPAN CLASS="cmtt"
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>1</SPAN
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></U
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>
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Return to Lisp Toplevel.</PRE
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></DD
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><DT
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><B
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>Side Effects</B
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></DT
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><DD
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><P CLASS="j"
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><A HREF="26_1_Glossary.html#handler"
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><EM CLASS="term"
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>Handlers</EM
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></A
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> for the specified condition, if any, are invoked and might have side effects. Program execution might stop, and the debugger might be entered.</P
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></DD
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><DT
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><B
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>Affected By</B
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></DT
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><DD
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><P CLASS="j"
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>Existing handler bindings. </P
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><P CLASS="j"
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><A HREF="v_break-on-signals.html" CLASS="varref"
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><B
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>*break-on-signals*</B
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></A
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> </P
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><P CLASS="j"
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>Signals an error of <A HREF="26_1_Glossary.html#type"
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><EM CLASS="term"
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>type</EM
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></A
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> <A HREF="t_type-error.html" CLASS="typeref"
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><B
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>type-error</B
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></A
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> if <VAR CLASS="param"
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>datum</VAR
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> and <VAR CLASS="param"
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>arguments</VAR
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> are not <A HREF="26_1_Glossary.html#designator"
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><EM CLASS="term"
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>designators</EM
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></A
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> for a <A HREF="26_1_Glossary.html#condition"
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><EM CLASS="term"
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>condition</EM
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></A
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>.</P
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></DD
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><DT
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><B
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>See Also</B
|
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></DT
|
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><DD
|
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><P CLASS="j"
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><A HREF="f_cerror.html" CLASS="funref"
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><B
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>cerror</B
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></A
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>, <A HREF="f_signal.html" CLASS="funref"
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><B
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>signal</B
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></A
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>, <A HREF="f_format.html" CLASS="funref"
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><B
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>format</B
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></A
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>, <A HREF="f_ignore-errors.html" CLASS="macref"
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><B
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>ignore-errors</B
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></A
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>, <A HREF="v_break-on-signals.html" CLASS="varref"
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><B
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>*break-on-signals*</B
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></A
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>, <A HREF="f_handler-bind.html" CLASS="macref"
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><B
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>handler-bind</B
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></A
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>, <A HREF="9_1_Condition_System_Concepts.html#sec_9_1" CLASS="secref"
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><SPAN CLASS="cmr"
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>Section</SPAN
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> <SPAN CLASS="cmr"
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>9.1</SPAN
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> <SPAN CLASS="cmr"
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>(Condition</SPAN
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> <SPAN CLASS="cmr"
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>System</SPAN
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> <SPAN CLASS="cmr"
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>Concepts)</SPAN
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></A
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></P
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></DD
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><DT
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><B
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>Notes</B
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></DT
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><DD
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><P CLASS="j"
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>Some implementations may provide debugger commands for interactively returning from individual stack frames. However, it should be possible for the programmer to feel confident about writing code like:</P
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><PRE CLASS="screen"
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>(defun wargames:no-win-scenario ()
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(if (error "pushing the button would be stupid."))
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(push-the-button))</PRE
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><P CLASS="j"
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>In this scenario, there should be no chance that <A HREF="f_error.html" CLASS="funref"
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><B
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>error</B
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></A
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> will return and the button will get pushed. </P
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><P CLASS="j"
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>While the meaning of this program is clear and it might be proven ‘safe’ by a formal theorem prover, such a proof is no guarantee that the program is safe to execute. Compilers have been known to have bugs, computers to have signal glitches, and human beings to manually intervene in ways that are not always possible to predict. Those kinds of errors, while beyond the scope of the condition system to formally model, are not beyond the scope of things that should seriously be considered when writing code that could have the kinds of sweeping effects hinted at by this example.</P
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></DD
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></DL
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></DIV
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>Conversion to HTML copyright 2023 by Gilbert Baumann</DIV
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