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<!DOCTYPE HTML>
<HTML LANG="en-us"
><HEAD
><TITLE
>4.1 Introduction | Common Lisp Nova Spec</TITLE
><META CHARSET="US-ASCII"
><LINK REL="canonical" HREF="4_1_Introduction.html"
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><DIV CLASS="breadcrumb"
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>Common Lisp Nova Spec</A
></SPAN
> <SPAN CLASS="breadcrumb-item"
>&#8594; <A HREF="4_Types_and_Classes.html"
>4. Types and Classes</A
></SPAN
> <SPAN CLASS="breadcrumb-item"
>&#8594; <A HREF="4_1_Introduction.html"
>4.1 Introduction</A
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><DIV CLASS="matter"
><SECTION
><H2 ID="sec_4_1" CLASS="HeadII"
>4.1 Introduction</H2
><UL CLASS="subtoc"
></UL
><P CLASS="j"
>A <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
> is a (possibly infinite) set of <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>objects</EM
></A
>. An <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> can belong to more than one <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
>. <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>Types</EM
></A
> are never explicitly represented as <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>objects</EM
></A
> by Common Lisp. Instead, they are referred to indirectly by the use of <A HREF="26_1_Glossary.html#type_specifier"
><EM CLASS="term"
>type specifiers</EM
></A
>, which are <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>objects</EM
></A
> that denote <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>types</EM
></A
>. </P
><P CLASS="j"
>New <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>types</EM
></A
> can be defined using <A HREF="f_deftype.html" CLASS="macref"
><B
>deftype</B
></A
>, <A HREF="f_defstruct.html" CLASS="macref"
><B
>defstruct</B
></A
>, <A HREF="f_defclass.html" CLASS="macref"
><B
>defclass</B
></A
>, and <A HREF="f_define-condition.html" CLASS="macref"
><B
>define-condition</B
></A
>. </P
><P CLASS="j"
>The <A HREF="26_1_Glossary.html#function"
><EM CLASS="term"
>function</EM
></A
> <A HREF="f_typep.html" CLASS="funref"
><B
>typep</B
></A
>, a set membership test, is used to determine whether a given <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> is of a given <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
>. The function <A HREF="f_subtypep.html" CLASS="funref"
><B
>subtypep</B
></A
>, a subset test, is used to determine whether a given <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
> is a <A HREF="26_1_Glossary.html#subtype"
><EM CLASS="term"
>subtype</EM
></A
> of another given <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
>. The function <A HREF="f_type-of.html" CLASS="funref"
><B
>type-of</B
></A
> returns a particular <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
> to which a given <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> belongs, even though that <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> must belong to one or more other <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>types</EM
></A
> as well. (For example, every <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> is of <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
> <A HREF="t_t.html" CLASS="typeref"
><B
>t</B
></A
>, but <A HREF="f_type-of.html" CLASS="funref"
><B
>type-of</B
></A
> always returns a <A HREF="26_1_Glossary.html#type_specifier"
><EM CLASS="term"
>type specifier</EM
></A
> for a <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
> more specific than <A HREF="t_t.html" CLASS="typeref"
><B
>t</B
></A
>.) </P
><P CLASS="j"
><A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>Objects</EM
></A
>, not <A HREF="26_1_Glossary.html#variable"
><EM CLASS="term"
>variables</EM
></A
>, have <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>types</EM
></A
>. Normally, any <A HREF="26_1_Glossary.html#variable"
><EM CLASS="term"
>variable</EM
></A
> can have any <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> as its <A HREF="26_1_Glossary.html#value"
><EM CLASS="term"
>value</EM
></A
>. It is possible to declare that a <A HREF="26_1_Glossary.html#variable"
><EM CLASS="term"
>variable</EM
></A
> takes on only values of a given <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
> by making an explicit <A HREF="26_1_Glossary.html#type_declaration"
><EM CLASS="term"
>type declaration</EM
></A
>. <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>Types</EM
></A
> are arranged in a directed acyclic graph, except for the presence of equivalences. </P
><P CLASS="j"
><A HREF="26_1_Glossary.html#declaration"
><EM CLASS="term"
>Declarations</EM
></A
> can be made about <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>types</EM
></A
> using <A HREF="m_declare.html" CLASS="misc"
><B
>declare</B
></A
>, <A HREF="f_proclaim.html" CLASS="funref"
><B
>proclaim</B
></A
>, <A HREF="f_declaim.html" CLASS="macref"
><B
>declaim</B
></A
>, or <A HREF="f_the.html" CLASS="specref"
><B
>the</B
></A
>. For more information about <A HREF="26_1_Glossary.html#declaration"
><EM CLASS="term"
>declarations</EM
></A
>, see <A HREF="3_3_Declarations.html#sec_3_3" CLASS="secref"
><SPAN CLASS="cmr"
>Section</SPAN
> <SPAN CLASS="cmr"
>3.3</SPAN
> <SPAN CLASS="cmr"
>(Declarations)</SPAN
></A
>. </P
><P CLASS="j"
>Among the fundamental <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>objects</EM
></A
> of the object system are <A HREF="26_1_Glossary.html#class"
><EM CLASS="term"
>classes</EM
></A
>. A <A HREF="26_1_Glossary.html#class"
><EM CLASS="term"
>class</EM
></A
> determines the structure and behavior of a set of other <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>objects</EM
></A
>, which are called its <A HREF="26_1_Glossary.html#instance"
><EM CLASS="term"
>instances</EM
></A
>. Every <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> is a <A HREF="26_1_Glossary.html#direct_instance"
><EM CLASS="term"
>direct instance</EM
></A
> of a <A HREF="26_1_Glossary.html#class"
><EM CLASS="term"
>class</EM
></A
>. The <A HREF="26_1_Glossary.html#class"
><EM CLASS="term"
>class</EM
></A
> of an <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> determines the set of operations that can be performed on the <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
>. For more information, see <A HREF="4_3_Classes.html#sec_4_3" CLASS="secref"
><SPAN CLASS="cmr"
>Section</SPAN
> <SPAN CLASS="cmr"
>4.3</SPAN
> <SPAN CLASS="cmr"
>(Classes)</SPAN
></A
>. </P
><P CLASS="j"
>It is possible to write <A HREF="26_1_Glossary.html#function"
><EM CLASS="term"
>functions</EM
></A
> that have behavior <A HREF="26_1_Glossary.html#specialized"
><EM CLASS="term"
>specialized</EM
></A
> to the class of the <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>objects</EM
></A
> which are their <A HREF="26_1_Glossary.html#argument"
><EM CLASS="term"
>arguments</EM
></A
>. For more information, see <A HREF="7_6_Generic_Functions_and_Methods.html#sec_7_6" CLASS="secref"
><SPAN CLASS="cmr"
>Section</SPAN
> <SPAN CLASS="cmr"
>7.6</SPAN
> <SPAN CLASS="cmr"
>(Generic</SPAN
> <SPAN CLASS="cmr"
>Functions</SPAN
> <SPAN CLASS="cmr"
>and</SPAN
> <SPAN CLASS="cmr"
>Methods)</SPAN
></A
>. </P
><P CLASS="j"
>The <A HREF="26_1_Glossary.html#class"
><EM CLASS="term"
>class</EM
></A
> of the <A HREF="26_1_Glossary.html#class"
><EM CLASS="term"
>class</EM
></A
> of an <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> is called its <DFN CLASS="newterm"
><SPAN CLASS="cmbxsl"
>metaclass</SPAN
></DFN
><SPAN CLASS="idx" DATA-KIND="idxterm" DATA-TERM="metaclass"
></SPAN
>. For more information about <A HREF="26_1_Glossary.html#metaclass"
><EM CLASS="term"
>metaclasses</EM
></A
>, see <A HREF="7_4_Meta-Objects.html#sec_7_4" CLASS="secref"
><SPAN CLASS="cmr"
>Section</SPAN
> <SPAN CLASS="cmr"
>7.4</SPAN
> <SPAN CLASS="cmr"
>(Meta-Objects)</SPAN
></A
>.</P
></SECTION
></DIV
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