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>defparameter, defvar | Common Lisp Nova Spec</TITLE
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><TABLE WIDTH="100%" CELLSPACING="0" CELLPADDING="0"
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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="defparameter, defvar"
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></SPAN
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><B
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>defparameter, defvar</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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>Macro</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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>defparameter</B
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> <SPAN CLASS="cmssi"
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>name</SPAN
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> <SPAN CLASS="cmssi"
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>initial-value</SPAN
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> [<SPAN CLASS="cmssi"
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>documentation</SPAN
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>] <SPAN CLASS="arrow"
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>→</SPAN
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> <SPAN CLASS="cmssi"
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>name</SPAN
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></P
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><P CLASS="j"
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><B
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>defvar</B
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> <SPAN CLASS="cmssi"
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>name</SPAN
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> <SPAN CLASS="cmtt"
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>[</SPAN
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><SPAN CLASS="cmssi"
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>initial-value</SPAN
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> [<SPAN CLASS="cmssi"
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>documentation</SPAN
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>]<SPAN CLASS="cmtt"
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>]</SPAN
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> <SPAN CLASS="arrow"
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>→</SPAN
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> <SPAN CLASS="cmssi"
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>name</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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>name</VAR
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> — a <A HREF="26_1_Glossary.html#symbol"
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><EM CLASS="term"
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>symbol</EM
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></A
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>; not evaluated. </P
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><P CLASS="j"
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><VAR CLASS="param"
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>initial-value</VAR
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> — a <A HREF="26_1_Glossary.html#form"
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><EM CLASS="term"
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>form</EM
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></A
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>; for <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defparameter</B
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></A
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>, it is always <A HREF="26_1_Glossary.html#evaluate"
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><EM CLASS="term"
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>evaluated</EM
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></A
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>, but for <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defvar</B
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></A
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> it is <A HREF="26_1_Glossary.html#evaluate"
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><EM CLASS="term"
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>evaluated</EM
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></A
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> only if <VAR CLASS="param"
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>name</VAR
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> is not already <A HREF="26_1_Glossary.html#bound"
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><EM CLASS="term"
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>bound</EM
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></A
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>. </P
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><P CLASS="j"
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><VAR CLASS="param"
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>documentation</VAR
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> — a <VAR CLASS="param"
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>string</VAR
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>; not evaluated.</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_defparameter.html" CLASS="macref"
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><B
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>defparameter</B
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></A
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> and <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defvar</B
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></A
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> <A HREF="26_1_Glossary.html#establish"
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><EM CLASS="term"
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>establish</EM
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></A
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> <VAR CLASS="param"
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>name</VAR
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> as a <A HREF="26_1_Glossary.html#dynamic_variable"
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><EM CLASS="term"
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>dynamic variable</EM
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></A
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>. </P
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><P CLASS="j"
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><A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defparameter</B
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></A
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> unconditionally <A HREF="26_1_Glossary.html#assign"
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><EM CLASS="term"
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>assigns</EM
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></A
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> the <VAR CLASS="param"
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>initial-value</VAR
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> to the <A HREF="26_1_Glossary.html#dynamic_variable"
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><EM CLASS="term"
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>dynamic variable</EM
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></A
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> named <VAR CLASS="param"
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>name</VAR
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>. <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defvar</B
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></A
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>, by contrast, <A HREF="26_1_Glossary.html#assign"
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><EM CLASS="term"
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>assigns</EM
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></A
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> <VAR CLASS="param"
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>initial-value</VAR
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> (if supplied) to the <A HREF="26_1_Glossary.html#dynamic_variable"
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><EM CLASS="term"
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>dynamic variable</EM
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></A
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> named <VAR CLASS="param"
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>name</VAR
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> only if <VAR CLASS="param"
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>name</VAR
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> is not already <A HREF="26_1_Glossary.html#bound"
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><EM CLASS="term"
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>bound</EM
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></A
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>. </P
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><P CLASS="j"
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>If no <VAR CLASS="param"
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>initial-value</VAR
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> is supplied, <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defvar</B
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></A
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> leaves the <A HREF="26_1_Glossary.html#value_cell"
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><EM CLASS="term"
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>value cell</EM
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></A
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> of the <A HREF="26_1_Glossary.html#dynamic_variable"
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><EM CLASS="term"
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>dynamic variable</EM
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></A
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> named <VAR CLASS="param"
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>name</VAR
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> undisturbed; if <VAR CLASS="param"
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>name</VAR
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> was previously <A HREF="26_1_Glossary.html#bound"
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><EM CLASS="term"
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>bound</EM
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></A
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>, its old <A HREF="26_1_Glossary.html#value"
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><EM CLASS="term"
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>value</EM
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></A
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> persists, and if it was previously <A HREF="26_1_Glossary.html#unbound"
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><EM CLASS="term"
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>unbound</EM
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></A
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>, it remains <A HREF="26_1_Glossary.html#unbound"
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><EM CLASS="term"
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>unbound</EM
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></A
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>. </P
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><P CLASS="j"
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>If <VAR CLASS="param"
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>documentation</VAR
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> is supplied, it is attached to <VAR CLASS="param"
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>name</VAR
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> as a <A HREF="26_1_Glossary.html#documentation_string"
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><EM CLASS="term"
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>documentation string</EM
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></A
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> of kind <SPAN CLASS="misc"
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><B
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>variable</B
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></SPAN
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>. </P
|
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><P CLASS="j"
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><A HREF="f_defparameter.html" CLASS="macref"
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><B
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|
>defparameter</B
|
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></A
|
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> and <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defvar</B
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></A
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> normally appear as a <A HREF="26_1_Glossary.html#top_level_form"
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><EM CLASS="term"
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>top level form</EM
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></A
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>, but it is meaningful for them to appear as <A HREF="26_1_Glossary.html#non-top-level_form"
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><EM CLASS="term"
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>non-top-level forms</EM
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></A
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>. However, the compile-time side effects described below only take place when they appear as <A HREF="26_1_Glossary.html#top_level_form"
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><EM CLASS="term"
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>top level forms</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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>Examples</B
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></DT
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><DD
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><PRE CLASS="screen"
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>(defparameter *p* 1) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> *P*
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*p* <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 1
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(constantp '*p*) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> <A HREF="26_1_Glossary.html#false"
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><EM CLASS="term"
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>false</EM
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></A
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>
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(setq *p* 2) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 2
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(defparameter *p* 3) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> *P*
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*p* <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 3
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(defvar *v* 1) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> *V*
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*v* <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 1
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(constantp '*v*) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> <A HREF="26_1_Glossary.html#false"
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><EM CLASS="term"
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>false</EM
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></A
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>
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(setq *v* 2) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 2
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(defvar *v* 3) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> *V*
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*v* <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 2
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(defun foo ()
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(let ((*p* 'p) (*v* 'v))
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(bar))) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> FOO
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(defun bar () (list *p* *v*)) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> BAR
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(foo) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> (P V)</PRE
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><P CLASS="j"
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>The principal operational distinction between <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defparameter</B
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></A
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> and <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defvar</B
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></A
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> is that <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defparameter</B
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></A
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> makes an unconditional assignment to <VAR CLASS="param"
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>name</VAR
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>, while <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defvar</B
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></A
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> makes a conditional one. In practice, this means that <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defparameter</B
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></A
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|
> is useful in situations where loading or reloading the definition would want to pick up a new value of the variable, while <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defvar</B
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></A
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> is used in situations where the old value would want to be retained if the file were loaded or reloaded. For example, one might create a file which contained:</P
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><PRE CLASS="screen"
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>(defvar *the-interesting-numbers* '())
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(defmacro define-interesting-number (name n)
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`(progn (defvar ,name ,n)
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(pushnew ,name *the-interesting-numbers*)
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',name))
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(define-interesting-number *my-height* 168) ;cm
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(define-interesting-number *my-weight* 13) ;stones</PRE
|
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><P CLASS="j"
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>Here the initial value, <CODE CLASS="f"
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>()</CODE
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>, for the variable <CODE CLASS="f"
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>*the-interesting-numbers*</CODE
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> is just a seed that we are never likely to want to reset to something else once something has been grown from it. As such, we have used <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defvar</B
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></A
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> to avoid having the <CODE CLASS="f"
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>*interesting-numbers*</CODE
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> information reset if the file is loaded a second time. It is true that the two calls to <SPAN CLASS="macref"
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><B
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>define-interesting-number</B
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></SPAN
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> here would be reprocessed, but if there were additional calls in another file, they would not be and that information would be lost. On the other hand, consider the following code:</P
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><PRE CLASS="screen"
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>(defparameter *default-beep-count* 3)
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(defun beep (&optional (n *default-beep-count*))
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(dotimes (i n) (si:%beep 1000. 100000.) (sleep 0.1)))</PRE
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><P CLASS="j"
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>Here we could easily imagine editing the code to change the initial value of <CODE CLASS="f"
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>*default-beep-count*</CODE
|
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>, and then reloading the file to pick up the new value. In order to make value updating easy, we have used <A HREF="f_defparameter.html" CLASS="macref"
|
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><B
|
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>defparameter</B
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></A
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|
>. </P
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><P CLASS="j"
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>On the other hand, there is potential value to using <A HREF="f_defparameter.html" CLASS="macref"
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><B
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>defvar</B
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></A
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> in this situation. For example, suppose that someone had predefined an alternate value for <CODE CLASS="f"
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>*default-beep-count*</CODE
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>, or had loaded the file and then manually changed the value. In both cases, if we had used <A HREF="f_defparameter.html" CLASS="macref"
|
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><B
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>defvar</B
|
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></A
|
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> instead of <A HREF="f_defparameter.html" CLASS="macref"
|
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><B
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>defparameter</B
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></A
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>, those user preferences would not be overridden by (re)loading the file. </P
|
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><P CLASS="j"
|
|
>The choice of whether to use <A HREF="f_defparameter.html" CLASS="macref"
|
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><B
|
|
>defparameter</B
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></A
|
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> or <A HREF="f_defparameter.html" CLASS="macref"
|
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><B
|
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>defvar</B
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></A
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> has visible consequences to programs, but is nevertheless often made for subjective reasons.</P
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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
|
|
><DD
|
|
><P CLASS="j"
|
|
>If a <A HREF="f_defparameter.html" CLASS="macref"
|
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><B
|
|
>defvar</B
|
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></A
|
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> or <A HREF="f_defparameter.html" CLASS="macref"
|
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><B
|
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>defparameter</B
|
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></A
|
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> <A HREF="26_1_Glossary.html#form"
|
|
><EM CLASS="term"
|
|
>form</EM
|
|
></A
|
|
> appears as a <A HREF="26_1_Glossary.html#top_level_form"
|
|
><EM CLASS="term"
|
|
>top level form</EM
|
|
></A
|
|
>, the <A HREF="26_1_Glossary.html#compiler"
|
|
><EM CLASS="term"
|
|
>compiler</EM
|
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></A
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> must recognize that the <VAR CLASS="param"
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>name</VAR
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> has been proclaimed <A HREF="m_special.html" CLASS="declref"
|
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><B
|
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>special</B
|
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></A
|
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>. However, it must neither <A HREF="26_1_Glossary.html#evaluate"
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><EM CLASS="term"
|
|
>evaluate</EM
|
|
></A
|
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> the <VAR CLASS="param"
|
|
>initial-value</VAR
|
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> <A HREF="26_1_Glossary.html#form"
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><EM CLASS="term"
|
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>form</EM
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></A
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> nor <A HREF="26_1_Glossary.html#assign"
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><EM CLASS="term"
|
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>assign</EM
|
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></A
|
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> the <A HREF="26_1_Glossary.html#dynamic_variable"
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><EM CLASS="term"
|
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>dynamic variable</EM
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></A
|
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> named <VAR CLASS="param"
|
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>name</VAR
|
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> at compile time. </P
|
|
><P CLASS="j"
|
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>There may be additional (<A HREF="26_1_Glossary.html#implementation-defined"
|
|
><EM CLASS="term"
|
|
>implementation-defined</EM
|
|
></A
|
|
>) compile-time or run-time side effects, as long as such effects do not interfere with the correct operation of <A HREF="26_1_Glossary.html#conforming_program"
|
|
><EM CLASS="term"
|
|
>conforming programs</EM
|
|
></A
|
|
>.</P
|
|
></DD
|
|
><DT
|
|
><B
|
|
>Affected By</B
|
|
></DT
|
|
><DD
|
|
><P CLASS="j"
|
|
><A HREF="f_defparameter.html" CLASS="macref"
|
|
><B
|
|
>defvar</B
|
|
></A
|
|
> is affected by whether <VAR CLASS="param"
|
|
>name</VAR
|
|
> is already <A HREF="26_1_Glossary.html#bound"
|
|
><EM CLASS="term"
|
|
>bound</EM
|
|
></A
|
|
>.</P
|
|
></DD
|
|
><DT
|
|
><B
|
|
>See Also</B
|
|
></DT
|
|
><DD
|
|
><P CLASS="j"
|
|
><A HREF="f_declaim.html" CLASS="macref"
|
|
><B
|
|
>declaim</B
|
|
></A
|
|
>, <A HREF="f_defconstant.html" CLASS="macref"
|
|
><B
|
|
>defconstant</B
|
|
></A
|
|
>, <A HREF="f_documentation.html" CLASS="funref"
|
|
><B
|
|
>documentation</B
|
|
></A
|
|
>, <A HREF="3_2_Compilation.html#sec_3_2" CLASS="secref"
|
|
><SPAN CLASS="cmr"
|
|
>Section</SPAN
|
|
> <SPAN CLASS="cmr"
|
|
>3.2</SPAN
|
|
> <SPAN CLASS="cmr"
|
|
>(Compilation)</SPAN
|
|
></A
|
|
></P
|
|
></DD
|
|
><DT
|
|
><B
|
|
>Notes</B
|
|
></DT
|
|
><DD
|
|
><P CLASS="j"
|
|
>It is customary to name <A HREF="26_1_Glossary.html#dynamic_variable"
|
|
><EM CLASS="term"
|
|
>dynamic variables</EM
|
|
></A
|
|
> with an <A HREF="26_1_Glossary.html#asterisk"
|
|
><EM CLASS="term"
|
|
>asterisk</EM
|
|
></A
|
|
> at the beginning and end of the name. e.g., <CODE CLASS="f"
|
|
>*foo*</CODE
|
|
> is a good name for a <A HREF="26_1_Glossary.html#dynamic_variable"
|
|
><EM CLASS="term"
|
|
>dynamic variable</EM
|
|
></A
|
|
>, but not for a <A HREF="26_1_Glossary.html#lexical_variable"
|
|
><EM CLASS="term"
|
|
>lexical variable</EM
|
|
></A
|
|
>; <CODE CLASS="f"
|
|
>foo</CODE
|
|
> is a good name for a <A HREF="26_1_Glossary.html#lexical_variable"
|
|
><EM CLASS="term"
|
|
>lexical variable</EM
|
|
></A
|
|
>, but not for a <A HREF="26_1_Glossary.html#dynamic_variable"
|
|
><EM CLASS="term"
|
|
>dynamic variable</EM
|
|
></A
|
|
>. This naming convention is observed for all <A HREF="26_1_Glossary.html#defined_name"
|
|
><EM CLASS="term"
|
|
>defined names</EM
|
|
></A
|
|
> in Common Lisp; however, neither <A HREF="26_1_Glossary.html#conforming_program"
|
|
><EM CLASS="term"
|
|
>conforming programs</EM
|
|
></A
|
|
> nor <A HREF="26_1_Glossary.html#conforming_implementation"
|
|
><EM CLASS="term"
|
|
>conforming implementations</EM
|
|
></A
|
|
> are obliged to adhere to this convention. </P
|
|
><P CLASS="j"
|
|
>The intent of the permission for additional side effects is to allow <A HREF="26_1_Glossary.html#implementation"
|
|
><EM CLASS="term"
|
|
>implementations</EM
|
|
></A
|
|
> to do normal “bookkeeping” that accompanies definitions. For example, the <A HREF="26_1_Glossary.html#macro_expansion"
|
|
><EM CLASS="term"
|
|
>macro expansion</EM
|
|
></A
|
|
> of a <A HREF="f_defparameter.html" CLASS="macref"
|
|
><B
|
|
>defvar</B
|
|
></A
|
|
> or <A HREF="f_defparameter.html" CLASS="macref"
|
|
><B
|
|
>defparameter</B
|
|
></A
|
|
> <A HREF="26_1_Glossary.html#form"
|
|
><EM CLASS="term"
|
|
>form</EM
|
|
></A
|
|
> might include code that arranges to record the name of the source file in which the definition occurs. </P
|
|
><P CLASS="j"
|
|
><A HREF="f_defparameter.html" CLASS="macref"
|
|
><B
|
|
>defparameter</B
|
|
></A
|
|
> and <A HREF="f_defparameter.html" CLASS="macref"
|
|
><B
|
|
>defvar</B
|
|
></A
|
|
> might be defined as follows:</P
|
|
><PRE CLASS="screen"
|
|
>(defmacro defparameter (name initial-value
|
|
&optional (documentation nil documentation-p))
|
|
`(progn (declaim (special ,name))
|
|
(setf (symbol-value ',name) ,initial-value)
|
|
,(when documentation-p
|
|
`(setf (documentation ',name 'variable) ',documentation))
|
|
',name))
|
|
(defmacro defvar (name &optional
|
|
(initial-value nil initial-value-p)
|
|
(documentation nil documentation-p))
|
|
`(progn (declaim (special ,name))
|
|
,(when initial-value-p
|
|
`(unless (boundp ',name)
|
|
(setf (symbol-value ',name) ,initial-value)))
|
|
,(when documentation-p
|
|
`(setf (documentation ',name 'variable) ',documentation))
|
|
',name))</PRE
|
|
></DD
|
|
></DL
|
|
></DIV
|
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></DIV
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