dpans/concept-type-intro.tex

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%% Introduction to Objects and Types

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A \term{type} is a (possibly infinite) set of \term{objects}.
An \term{object} can belong to more than one \term{type}.  
\term{Types} are never explicitly represented as \term{objects} by \clisp.
Instead, they are referred to indirectly by the use of \term{type specifiers},
which are \term{objects} that denote \term{types}.

New \term{types} can be defined using \macref{deftype}, \macref{defstruct}, 
\macref{defclass}, and \macref{define-condition}.

\Thefunction{typep}, a set membership test, is used to determine
whether a given \term{object} is of a given \term{type}.  The function
\funref{subtypep}, a subset test, is used to determine whether a
given \term{type} is a \term{subtype} of another given \term{type}.  The
function \funref{type-of} returns a particular \term{type} to
which a given \term{object} belongs, even though that \term{object}
must belong to one or more other \term{types} as well.
(For example, every \term{object} is \oftype{t}, 
 but \funref{type-of} always returns a \term{type specifier}
 for a \term{type} more specific than \typeref{t}.)

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\term{Objects}, not \term{variables}, have \term{types}.
Normally, any \term{variable} can have any \term{object} as its \term{value}.
It is possible to declare that a \term{variable} takes on only 
values of a given \term{type} by making an explicit \term{type declaration}.
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\term{Types} are arranged in a directed acyclic graph, except
for the presence of equivalences. 

\term{Declarations} can be made about \term{types} using \misc{declare}, 
\funref{proclaim}, \macref{declaim}, or \specref{the}.
For more information about \term{declarations},
\seesection\Declarations.

Among the fundamental \term{objects} of the \CLOS\ are \term{classes}.
A \term{class} determines the structure and behavior of a set of
other \term{objects}, which are called its \term{instances}. 
Every \term{object} is a \term{direct instance} of a \term{class}.
The \term{class} of an \term{object} determines the set of
operations that can be performed on the \term{object}.
For more information, \seesection\Classes.

It is possible to write \term{functions} that have behavior \term{specialized}
to the class of the \term{objects} which are their \term{arguments}.
For more information, \seesection\GFsAndMethods.

The \term{class} of the \term{class} of an \term{object} 
is called its \newterm{metaclass}.
For more information about \term{metaclasses},
\seesection\MetaObjects.