\input texinfo @c -*-Texinfo-*-
@comment $Id: sos.texinfo,v 2.22 2007/01/05 21:21:08 cph Exp $
@comment %**start of header
@setfilename mit-scheme-sos
@set EDITION 2.11
@set VERSION 1.9
@set UPDATED 2006-05-16
@settitle SOS @value{VERSION}
@comment %**end of header
@setchapternewpage on
@finalout

@syncodeindex ky fn
@syncodeindex vr fn
@syncodeindex pg fn
@syncodeindex tp fn

@copying
This manual documents @acronym{SOS} @value{VERSION}.

Copyright @copyright{} 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993,
    1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004,
    2005, 2006, 2007 Massachusetts Institute of Technology

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
and with the Back-Cover Texts as in (a) below.  A copy of the
license is included in the section entitled ``GNU Free Documentation
License.''

(a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
this GNU Manual, like GNU software.  Copies published by the Free
Software Foundation raise funds for GNU development.''
@end quotation
@end copying

@dircategory Programming Languages
@direntry
* SOS: (mit-scheme-sos).		MIT/GNU Scheme Object System
@end direntry

@titlepage
@title The SOS Reference Manual
@subtitle Edition @value{EDITION} for SOS @value{VERSION}
@subtitle @value{UPDATED}
@author by Chris Hanson
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@headings double
@contents

@ifnottex
@node Top, Introduction, (dir), (dir)
@top SOS

@insertcopying
@end ifnottex

@menu
* Introduction::                
* Classes::                     
* Instances::                   
* Slots::                       
* Generic Procedures::          
* Methods::                     
* Printing::                    
* GNU Free Documentation License::  
* Binding Index::               
* Concept Index::               
@end menu

@node Introduction, Classes, Top, Top
@unnumbered Introduction

@sc{sos} is a Scheme @dfn{object system} derived from Tiny @sc{clos}
@footnote{Tiny @sc{clos} was written by Gregor Kiczales of Xerox
@sc{parc}; @sc{sos} is derived from version 1.2 of Tiny @sc{clos}.},
which in turn was loosely derived from @sc{clos}, the Common Lisp Object
System.  Its basic design and philosophy is closely related to Tiny
@sc{clos}, but there are differences in naming and interface.

This document is a reference manual, and as such does not attempt to
teach the reader about object-oriented programming.  It is assumed that
you already have a passing familiarity with @sc{clos} and with Scheme.

In the procedure descriptions that follow, certain argument names imply
restrictions on the corresponding argument.  Here is a table of those
names.  The parenthesised name in each entry is the name of the
predicate procedure that the argument must satisfy.

@table @var
@item class
The argument must be a @dfn{class} (@code{class?}).

@item instance
The argument must be an @dfn{instance} (@code{instance?}).

@item name
The argument must be a symbol (@code{symbol?}); sometimes this is also
allowed to be @code{#f} (@code{false?}).

@item generic-procedure
The argument must be a @dfn{generic procedure}
(@code{generic-procedure?}).

@item method
The argument must be a @dfn{method} (@code{method?}).

@item specializer
The argument must be a @dfn{method specializer} (@code{specializer?}).

@item procedure
The argument must be a procedure (@code{procedure?}).

@item slot
The argument must be a @dfn{slot descriptor} (@code{slot-descriptor?}).
@end table

@menu
* Classes::                     
* Instances::                   
* Slots::                       
* Generic Procedures::          
* Methods::                     
* Printing::                    
@end menu

@node Classes, Instances, Introduction, Top
@chapter Classes

@cindex class
@cindex instance
A @dfn{class} is an object that determines the structure and behavior of
a set of other objects, which are called its @dfn{instances}.  However,
in this document, the word @dfn{instance} usually means an instance of
the class @code{<instance>}.

@cindex subclass
@cindex superclass
A class can inherit structure and behavior from other classes.  A class
whose definition refers to other classes for the purpose of inheriting
from them is said to be a @dfn{subclass} of each of those classes.  The
classes that are designated for purposes of inheritance are said to be
@dfn{superclasses} of the inheriting class.

@cindex class name
@cindex name, of class
A class can have a @dfn{name}.  The procedure @code{class-name} takes a
class object and returns its name.  The name of an anonymous class is
@code{#f}.

@cindex direct superclass
@cindex superclass, direct
@cindex direct subclass
@cindex subclass, direct
A class @var{C_1} is a @dfn{direct superclass} of a class @var{C_2} if
@var{C_2} explicitly designates @var{C_1} as a superclass in its
definition.  In this case, @var{C_2} is a @dfn{direct subclass} of
@var{C_1}.  A class @var{C_n} is a @dfn{superclass} of a class @var{C_1}
if there exists a series of classes @var{C_2}, @dots{}, @var{C_n-1} such
that @var{C_i+1} is a direct superclass of @var{C_i} for all @var{i}
between @var{1} and @var{n}.  In this case, @var{C_1} is a
@dfn{subclass} of @var{C_n}.  A class is considered neither a superclass
nor a subclass of itself.  That is, if @var{C_1} is a superclass of
@var{C_2}, then @var{C_1} is different from @var{C_2}.  The set of
classes consisting of some given class @var{C} along with all of its
superclasses is called ``@var{C} and its superclasses.''

@cindex class precedence list
@cindex precedence list, class
Each class has a @dfn{class precedence list}, which is a total ordering
on the set of the given class and its superclasses.  The total ordering
is expressed as a list ordered from the most specific to the least
specific.  The class precedence list is used in several ways.  In
general, more specific classes can @dfn{shadow}, or override, features
that would otherwise be inherited from less specific classes.  The
method selection and combination process uses the class precedence list
to order methods from most specific to least specific.

@cindex local precedence order
@cindex precedence order, local
@cindex order, local precedence
When a class is defined, the order in which its direct superclasses are
mentioned in the defining form is important.  Each class has a
@dfn{local precedence order}, which is a list consisting of the class
followed by its direct superclasses in the order mentioned in the
defining form.

A class precedence list is always consistent with the local precedence
order of each class in the list.  The classes in each local precedence
order appear within the class precedence list in the same order.  If the
local precedence orders are inconsistent with each other, no class
precedence list can be constructed, and an error is signalled.

Classes are organized into a @dfn{directed acyclic graph}.  There are
two distinguished classes, named @code{<object>} and @code{<instance>}.
The class named @code{<object>} has no superclasses.  It is a superclass
of every class except itself.  The class named @code{<instance>} is a
direct subclass of @code{<object>} and is the base class for
@dfn{instance} objects.  Instances are special because @sc{sos} has
efficient mechanisms for dispatching on them and for accessing their
slots.

@menu
* Class Datatype::              
* Predefined Classes::          
* Record Classes::              
* Specializers::                
@end menu

@node Class Datatype, Predefined Classes, Classes, Classes
@section Class Datatype

The procedures in this section may be used to construct and inspect
classes.

@deffn Procedure make-class name direct-superclasses direct-slots
Creates and returns a new class object.

@var{Name} is used for debugging: it is a symbol that appears in the
printed representation of the class and has no role in the semantics of
the class.  Alternatively, @var{name} may be @code{#f} to indicate that
the class is anonymous.

@var{Direct-superclasses} must be a list of class objects.  The new
class inherits both methods and slots from the classes in this list.
Specifying the empty list for @var{direct-superclasses} is equivalent to
specifying @code{(list <instance>)}.

@var{Direct-slots} describes additional slots that instances of this
class will have.  It is a list, each element of which must have one of
the following forms:

@example
@var{name}
(@var{name} . @var{plist})
@end example

@noindent
where @var{name} is a symbol, and @var{plist} is a property list.  The
first of these two forms is equivalent to the second with an empty
@var{plist}.

Each of the elements of @var{direct-slots} defines one slot named
@var{name}.  @var{Plist} is used to describe additional properties of
that slot.  The following properties are recognized:

@table @code
@item initial-value
This property specifies the default initial value for the slot, i.e.@:
the value stored in the slot when an instance is created and no value is
explicitly specified by the instance constructor.  If neither the
@code{initial-value} nor the @code{initializer} property is specified,
the slot has no default initial value.

@item initializer
This property specifies a procedure of no arguments that is called by an
instance constructor whenever an instance containing this slot is
created.  The value returned by the @code{initializer} procedure is the
initial value of the slot.

@item accessor
This property specifies a generic procedure; @code{make-class} will add
an accessor method for this slot to the procedure.  @xref{Slots}.

@item modifier
This property specifies a generic procedure; @code{make-class} will add
a modifier method for this slot to the procedure.  @xref{Slots}.

@item initpred
This property specifies a generic procedure; @code{make-class} will add
an ``initialized?''@: predicate method for this slot to the procedure.
@xref{Slots}.
@end table

Slot properties are combined in slightly complicated ways.

@itemize @bullet
@item
It is not allowed to specify both @code{initial-value} and
@code{initializer} for a slot in a given call to @code{make-class}; at
most one of these properties may be given.

@item
If a slot is specified for a given class, and a slot of the same name is
inherited from a superclass, then the slot properties for the two slots
are combined.  Slot properties from the subclass shadow those of the
superclass.  However, if a superclass has a slot property, and the
subclass does not, the property is inherited.  The resulting class never
has more than one slot of a given name.

@item
When combining superclass and subclass slots, @code{initial-value} and
@code{initializer} shadow one another.  In other words, regardless of
the inherited slot properties, the resulting slot has at most one of
these two properties.
@end itemize

Examples of @code{make-class}:

@lisp
@group
(define <cell>
  (make-class '<cell> '() '()))
@end group

@group
(define-generic cell-name (cell))
(define-generic cell-width (cell))
(define-generic cell-height (cell))
(define-generic cell-components (cell))
(define-generic set-cell-components! (cell components))
@end group

@group
(define <contact>
  (make-class '<contact>
              (list <cell>)
              `((name accessor ,cell-name))))
@end group

@group
(define <compound-cell>
  (make-class '<compound-cell>
              (list <cell>)
              `((width accessor ,cell-width)
                (height accessor ,cell-height)
                (components accessor ,cell-components
                            modifier ,set-cell-components!
                            initial-value ()))))
@end group
@end lisp
@end deffn

@deffn Syntax define-class name direct-superclasses direct-slot @dots{}
Define @var{name} to be a class.  In its basic form, @code{define-class}
might have been defined by

@lisp
@group
(define-syntax define-class
  (syntax-rules ()
    ((define-class name (class ...) slot ...)
     (define name
       (make-class (quote name)
                   (list class ...)
                   (quote (slot ...)))))))
@end group
@end lisp

Note that slot properties are handled specially by @code{define-class}.
If a @var{direct-slot} specifies a slot properties property list, the
keys of the property list (i.e.@: the even-numbered elements) are not
evaluated, while the datums of the property list @emph{are} evaluated.
The expansion above does not show the proper treatment of slot
properties.

In addition to the slot properties recognized by @code{make-class},
@code{define-class} recognizes a special slot property, called
@code{define}.  The @code{define} property specifies that some or all of
the slot accessors should be defined here; that is, generic procedures
should be constructed and bound to variables, and then the accessor
methods added to them.

The argument to the @code{define} property is a list containing any
combination of the symbols @code{accessor}, @code{modifier}, and
@code{initpred}.  As an abbreviation, the argument may be one of these
symbols by itself, which is equivalent to the list containing that
symbol.  Also, the argument may be the symbol @code{standard}, which is
equivalent to @code{(accessor modifier)}.

The argument to @code{define} specifies the accessors that will be
defined by this form.  The accessors are defined using default names,
unless the names are overridden by the corresponding slot property.  The
default names for a class @code{<foo>} and a slot @code{bar} are
@code{foo-bar}, @code{set-foo-bar!}, and @code{foo-bar-initialized?},
respectively for the accessor, modifier, and initpred.  For example,

@lisp
@group
(define-class foo
  (bar define accessor))
@end group
@end lisp

@noindent
defines an accessor called @code{foo-bar}, but

@lisp
@group
(define-class foo
  (bar define accessor accessor foo/bar))
@end group
@end lisp

@noindent
instead defines an accessor called @code{foo/bar}.  Finally,

@lisp
@group
(define-class foo
  (bar accessor foo/bar))
@end group
@end lisp

@noindent
doesn't define any accessor, but assumes that @code{foo/bar} is a
previously-defined generic procedure and adds an accessor method to it.

@cindex class options
@code{define-class} permits the specification of @dfn{class options},
which are options that pertain to the class as a whole.  Class options
are specified by overloading @var{name}: instead of a symbol, specify a
pair whose @sc{car} is a symbol and whose @sc{cdr} is an alist.  The
following class options are recognized:

@table @code
@item (predicate [@var{name}])
@findex predicate
@cindex predicate, class option
Specifies that a predicate procedure should be defined for this class.
@var{Name} must be either a symbol or @code{#f}: a symbol specifies the
name that will be bound to the predicate procedure, and @code{#f}
specifies that no predicate procedure should be defined.  If @var{name}
is omitted, or if no @code{predicate} option is specified, a predicate
procedure is defined by appending @code{?} to the name of the class.  If
the class name is surrounded by angle brackets, they are stripped off
first.  For example, the default predicate name for the class
@code{<foo>} is @code{foo?}.

@item (constructor [@var{name}] @var{slot-names} [@var{n-init-args}])
@findex constructor
@cindex constructor, class option
Specifies that a constructor procedure should be defined for this class.
@var{Name} must be a symbol, which is the name that will be bound to the
constructor procedure; if omitted, a default name is formed by
prepending @code{make-} to the name of the class.  If the class name is
surrounded by angle brackets, they are stripped off first.  For example,
the default constructor name for the class @code{<foo>} is
@code{make-foo}.

@var{Slot-names} and @var{n-init-args} correspond to the arguments of
the respective names accepted by @code{instance-constructor}, and can
take any of the allowed forms for those arguments.

@item (separator @var{string})
@findex separator
@cindex separator, class option
Specifies how names for slot accessors are constructed.  If this option
isn't given, the name of a slot accessor is formed by concatenating the
name of the class with the name of the slot, with a hyphen between them.
When this option is given, @var{string} is used instead of the hyphen.
For example, normally a slot accessor for the slot @code{bar} in the
class @code{foo} is called @code{foo-bar}.  A class option
@code{(separator ".")} will cause the slot accessor to be called
@code{foo.bar}, the modifier to be called @code{set-foo.bar!}, and the
initialization predicate to be called @code{foo.bar?}.
@end table

Examples of @code{define-class} (compare these to the similar examples
for @code{make-class}):

@lisp
(define-class <cell> ())

@group
(define-generic cell-name (cell))
(define-generic cell-width (cell))
(define-generic cell-height (cell))
(define-generic cell-components (cell))
(define-generic set-cell-components! (cell components))
@end group

@group
(define-class (<contact> (constructor (name) no-init)) (<cell>)
  (name accessor cell-name))
@end group

@group
(define-class (<compound-cell> (constructor ())) (<cell>)
  (width accessor cell-width)
  (height accessor cell-height)
  (components accessor cell-components
              modifier set-cell-components!
              initial-value '()))
@end group
@end lisp
@end deffn

@deffn Procedure make-trivial-subclass superclass1 superclass2 @dots{}
This convenience procedure makes a subclass that defines no new slots,
and that inherits from the given superclasses.  It is equivalent to the
following

@lisp
@group
(make-class (class-name @var{superclass1})
            (list @var{superclass1} @var{superclass2} @dots{})
            '())
@end group
@end lisp
@end deffn

@deffn Procedure class? object
Returns @code{#t} if @var{object} is a class, otherwise returns
@code{#f}.
@end deffn

@deffn Procedure subclass? class specializer
Returns @code{#t} if @var{class} is a subclass of @var{specializer},
otherwise returns @code{#f}.  If @var{specializer} is a class, the
result follows from the above definition of subclass, except that a
class is a subclass of itself.  If @var{specializer} is a record type,
it is equivalent to having used the @code{record-type-class} of the
record type.  Finally, if @var{specializer} is a union specializer,
@code{subclass?} is true if @var{class} is a subclass of one or more of
the component classes of @var{specializer}.
@end deffn

@deffn Procedure object-class object
Returns the class of @var{object}.  @var{Object} may be any Scheme
object; if @var{object} is known to be an instance,
@code{instance-class} is faster than @code{object-class}.
@end deffn

@deffn Procedure class-name class
Returns the name of @var{class}.  This is the @var{name} argument passed
to @code{make-class} when @var{class} was created.
@end deffn

@deffn Procedure class-direct-superclasses class
Returns a list of the direct superclasses of @var{class}.  If a
non-empty @var{direct-superclasses} argument was passed to
@code{make-class} when @var{class} was created, this list is
@code{equal?} to that argument.  The returned value must not be
modified.
@end deffn

@deffn Procedure class-direct-slot-names class
Returns a list of symbols that are the names of the direct slots of
@var{class}.  This list contains only those slots that were defined in
the call to @code{make-class} that created @var{class}; it does not
contain slots that were inherited.  The returned value must not be
modified.
@end deffn

@deffn Procedure class-precedence-list class
Returns a list of the superclasses of @var{class}.  The order of this
list is significant: it is the method resolution order.  This list will
always have @var{class} as its first element, and @code{<object>} as its
last element.  The returned value must not be modified.
@end deffn

@node Predefined Classes, Record Classes, Class Datatype, Classes
@section Predefined Classes

@cindex predefined classes
@sc{sos} provides a rich set of predefined classes that can be used to
specialize methods to any of Scheme's built-in datatypes.

@defvr Class <object>
This is the class of all Scheme objects.  It has no direct superclasses,
and all other classes are subclasses of this class.
@end defvr

@defvr Class <instance>
This is the class of instances.  It is a direct subclass of
@code{<object>}.  The members of this class are the objects that satisfy
the predicate @code{instance?}.
@end defvr

@defvr Class <boolean>
@defvrx Class <char>
@defvrx Class <entity>
@defvrx Class <pair>
@defvrx Class <procedure>
@defvrx Class <record>
@defvrx Class <string>
@defvrx Class <symbol>
@defvrx Class <vector>
These are the classes of their respective Scheme objects.  They are all
direct subclasses of @code{<object>}.  The members of each class are the
objects that satisfy the corresponding predicate; for example, the
members of @code{<procedure>} are the objects that satisfy
@code{procedure?}.
@end defvr

@defvr Class <generic-procedure>
This is the class of generic procedure instances.  It is a direct
subclass of @code{<procedure>}.
@end defvr

@defvr Class <method>
This is the class of method objects.  It is a direct subclass of
@code{<instance>}.
@end defvr

@defvr Class <chained-method>
@defvrx Class <computed-method>
@defvrx Class <computed-emp>
These classes specify additional method objects with special properties.
Each class is a subclass of @code{<method>}.
@end defvr

The following are the classes of Scheme numbers.  Note that
@code{object-class} will never return one of these classes; instead it
returns an implementation-specific class that is associated with a
particular numeric representation.  The implementation-specific class is
a subclass of one or more of these implementation-independent classes,
so you should use these classes for specialization.

@defvr Class <number>
@defvrx Class <complex>
@defvrx Class <real>
@defvrx Class <rational>
@defvrx Class <integer>
These are the classes of the Scheme numeric tower.  @code{<number>} is a
direct subclass of @code{<math-object>}, @code{<complex>} is a direct
subclass of @code{<number>}, @code{<real>} is a direct subclass of
@code{<complex>}, etc.
@end defvr

@defvr Class <exact>
@defvrx Class <exact-complex>
@defvrx Class <exact-real>
@defvrx Class <exact-rational>
@defvrx Class <exact-integer>
These are the classes of exact numbers.  @code{<exact>} is a direct
subclass of @code{<number>}, @code{<exact-complex>} is a direct
subclass of @code{<exact>} and @code{<complex>}, and in general, each is
a direct subclass of preceding class and of the class without the
@code{exact-} prefix.
@end defvr

@defvr Class <inexact>
@defvrx Class <inexact-complex>
@defvrx Class <inexact-real>
@defvrx Class <inexact-rational>
@defvrx Class <inexact-integer>
These are the classes of inexact numbers.  @code{<inexact>} is a direct
subclass of @code{<number>}, @code{<inexact-complex>} is a direct
subclass of @code{<inexact>} and @code{<complex>}, and in general, each
is a direct subclass of preceding class and of the class without the
@code{inexact-} prefix.
@end defvr

@node Record Classes, Specializers, Predefined Classes, Classes
@section Record Classes

@cindex record class
@sc{sos} allows generic procedures to discriminate on record types.
This means that a record structure defined by means of
@code{make-record-type} or @code{define-structure} can be passed as an
argument to a generic procedure, and the generic procedure can use the
record's type to determine which method to be invoked.@footnote{If the
@code{type} option of @code{define-structure} is used, the resulting
data structure is @emph{not} a record and thus cannot be used in this
manner.}

In order to support this, @sc{sos} accepts record type descriptors in
all contexts that accept classes.  Additionally, every record type
descriptor has an associated @sc{sos} class; either the class or the
record type can be used with equivalent results.

@deffn Procedure record-type-class record-type
@var{Record-type} must be a record type descriptor (in other words, it
must satisfy the predicate @code{record-type?}).  Returns the class
associated with @var{record-type}.
@end deffn

@deffn Procedure record-class record
@var{Record} must be a record (in other words, it must satisfy the
predicate @code{record?}).  Returns the class associated with
@var{record}.  This is equivalent to

@lisp
(record-type-class (record-type-descriptor @var{record}))
@end lisp
@end deffn

@node Specializers,  , Record Classes, Classes
@section Specializers

@cindex specializer
A @dfn{specializer} is a generalization of a class.  A specializer is
any one of the following:

@itemize @bullet
@item
A class.

@item
A record type, which is equivalent to its associated class.

@item
A union specializer, which is a set of classes.
@end itemize

A specializer may be used in many contexts where a class is required,
specifically, as a method specializer (hence the name), as the second
argument to @code{subclass?}, and elsewhere.

@deffn Procedure specializer? object
Returns @code{#t} if @var{object} is a specializer, otherwise returns
@code{#f}.
@end deffn

@deffn Procedure specializer-classes specializer
Returns a list of the classes in @var{specializer}.  If
@var{specializer} is a class, the result is a list of that class.  If
@var{specializer} is a record type, the result is a list of the record
type's class.  If @var{specializer} is a union specializer, the result
is a list of the component classes of the specializer.
@end deffn

@deffn Procedure specializer=? specializer1 specializer2
Returns @code{#t} if @var{specializer1} and @var{specializer2} are
equivalent, otherwise returns @code{#f}.  Two specializers are
equivalent if the lists returned by @code{specializer-classes} contain
the same elements.
@end deffn

@deffn Procedure union-specializer specializer @dots{}
Returns a union specializer consisting of the union of the classes of
the arguments.  This is equivalent to converting all of the specializer
arguments to sets of classes, then taking the union of those sets.
@end deffn

@deffn Procedure union-specializer? object
Returns @code{#t} if @var{object} is a union specializer, otherwise
returns @code{#f}.
@end deffn

@deffn Procedure specializers? object
Returns @code{#t} if @var{object} is a list of specializers, otherwise
returns @code{#f}.
@end deffn

@deffn Procedure specializers=? specializers1 specializers2
@var{Specializers1} and @var{specializers2} must be lists of
specializers.  Returns @code{#t} if @var{specializers1} and
@var{specializers2} are equivalent, otherwise returns @code{#f}.
Two specializers lists are equivalent if each of their corresponding
elements is equivalent.
@end deffn

@node Instances, Slots, Classes, Top
@chapter Instances

@cindex instance
An @dfn{instance} is a compound data structure much like a record,
except that it is defined by a class rather than a record type
descriptor.  Instances are more powerful than records, because their
representation is designed to support inheritance, while the
representation of records is not.

@deffn Procedure instance-constructor class slot-names [n-init-args]
Creates and returns a procedure that, when called, will create and
return a newly allocated instance of @var{class}.

@var{Class} must be a subclass of @code{<instance>}.  @var{Slot-names}
must be a list of symbols, each of which must be the name of a slot in
@var{class}.  @var{N-init-args} will be described below.

In its basic operation, @code{instance-constructor} works much like
@code{record-constructor}: the @var{slot-names} argument specifies how
many arguments the returned constructor accepts, and each of those
arguments is stored in the corresponding slot of the returned instance.
Any slots that are not specified in @var{slot-names} are given their
initial values, as specified by the @code{initial-value} or
@code{initializer} slot properties; otherwise they are left
uninitialized.

@cindex initialize-instance
@findex initialize-instance
After the new instance is created and its slots filled in, but before it
is returned, it is passed to the generic procedure
@code{initialize-instance}.  Normally, @code{initialize-instance} does
nothing, but because it is always called, the programmer can add methods
to it to specify an initialization that is to be performed on every
instance of the class.

By default, @code{initialize-instance} is called with one argument, the
newly created instance.  However, the optional argument
@var{n-init-args} can be used to specify additional arguments that will
be passed to @code{initialize-instance}.

The way this works is that the returned constructor procedure accepts
additional arguments after the specified number of slot values, and
passes these extra arguments to @code{initialize-instance}.  When
@var{n-init-args} is not supplied or is @code{#t}, any number of extra
arguments are accepted and passed along.  When @var{n-init-args} is an
exact non-negative integer, exactly that number of extra arguments must
be supplied when the constructor is called.  Finally, if
@var{n-init-args} is the symbol @code{no-initialize-instance}, then the
constructor accepts no extra arguments and does not call
@code{initialize-instance} at all; this is desirable when
@code{initialize-instance} is not needed, because it makes the
constructor significantly faster.

For notational convenience, @var{n-init-args} may take two other forms.
First, it may be a list of symbols, which is equivalent to the integer
that is the length of the list.  Second, it may be the symbol
@code{no-init}, which is an abbreviation for
@code{no-initialize-instance}.

Note that the default method on @code{initialize-instance} accepts no
extra arguments and does nothing.

Examples of @code{instance-constructor}:

@lisp
@group
(define-class <simple-reference> (<reference>)
  (from accessor reference-from)
  (to accessor reference-to)
  (cx accessor reference-cx)
  (cy accessor reference-cy))
@end group

@group
(define make-simple-reference
  (instance-constructor <simple-reference>
                        '(from to cx cy)
                        'no-init))
@end group

@group
(define-class <simple-wirenet> (<wirenet>)
  (cell accessor wirenet-cell)
  (wires accessor wirenet-wires
         modifier set-wirenet-wires!
         initial-value '()))
@end group

@group
(define make-simple-wirenet
  (instance-constructor <simple-wirenet> '(cell)))
@end group
@end lisp
@end deffn

@deffn Procedure instance? object
Returns @code{#t} if @var{object} is an instance, otherwise returns
@code{#f}.
@end deffn

@deffn Procedure instance-class instance
Returns the class of @var{instance}.  This is faster than
@code{object-class}, but it works only for instances, and not for other
objects.
@end deffn

@deffn Procedure instance-of? object specializer
Returns @code{#t} if @var{object} is a general instance of
@var{specializer}, otherwise returns @code{#f}.  This is equivalent to

@lisp
(subclass? (object-class @var{object}) @var{specializer})
@end lisp
@end deffn

@deffn Procedure instance-predicate specializer
Returns a predicate procedure for @var{specializer}.  The returned
procedure accepts one argument and returns @code{#t} if the argument is
an instance of @var{specializer} and @code{#f} otherwise.
@end deffn

@node Slots, Generic Procedures, Instances, Top
@chapter Slots

@cindex slot
An instance has zero or more named slots; the name of a slot is a
symbol.  The slots of an instance are determined by its class.

@cindex uninitialized slot
@cindex slot, uninitialized
Each slot can hold one value.  When a slot does not have a value, the
slot is said to be @dfn{uninitialized}.  The default initial value for a
slot is defined by the @code{initial-value} and @code{initializer} slot
properties.

@cindex accessibility of slots
A slot is said to be @dfn{accessible} in an instance of a class if the
slot is defined by the class of the instance or is inherited from a
superclass of that class.  At most one slot of a given name can be
accessible in an instance.  Slots are accessed by means of slot-access
methods (usually generated by @code{make-class}).

@menu
* Slot Descriptors::            
* Slot Access Methods::         
* Slot Access Constructors::    
* Slot Access Procedures::      
@end menu

@node Slot Descriptors, Slot Access Methods, Slots, Slots
@section Slot Descriptors

@cindex slot descriptor
Slots are represented by @dfn{slot descriptors}, which are data
structures providing information about the slots, such as their name.
Slot descriptors are stored inside of classes, and may be retrieved from
there and subsequently inspected.

@deffn Procedure class-slots class
Returns a list of the slot descriptors for @var{class}.  This contains
all slots for @var{class}, both direct slots and inherited slots.  The
returned value must not be modified.
@end deffn

@deffn Procedure class-slot class name error?
Returns the slot descriptor for the slot named @var{name} in
@var{class}.  If there is no such slot: if @var{error?} is @code{#f},
returns @code{#f}, otherwise signals an error of type
@code{condition-type:no-such-slot}.
@end deffn

@deffn Procedure slot-descriptor? object
Returns @code{#t} if @var{object} is a slot descriptor, otherwise
returns @code{#f}.
@end deffn

@deffn Procedure slot-name slot
Returns the name of @var{slot}.
@end deffn

@deffn Procedure slot-class slot
Returns the class of @var{slot}.  This is the class with which
@var{slot} is associated.  This is not necessarily the class that
defines @var{slot}; it could also be a subclass of that class.  If the
slot was returned from @code{class-slots} or @code{class-slot}, then
this class is the argument passed to that procedure.
@end deffn

@deffn Procedure slot-properties slot
Returns an alist of the properties of @var{slot}.  This list must not be
modified.
@end deffn

@deffn Procedure slot-property slot name default
If @var{slot} has a property named @var{name}, it is returned; otherwise
@var{default} is returned.
@end deffn

@deffn Procedure slot-initial-value? slot
Returns @code{#t} if @var{slot} has an initial value, and @code{#f}
otherwise.  The initial value is specified by the @code{initial-value}
slot property when a class is made.
@end deffn

@deffn Procedure slot-initial-value slot
Returns the initial value for @var{slot}, if it has one; otherwise it
returns an unspecified value.  The initial value is specified by the
@code{initial-value} slot property when a class is made.
@end deffn

@deffn Procedure slot-initializer slot
Returns the initializer for @var{slot}; the initializer is specified by
the @code{initializer} slot property when a class is made.  This is a
procedure of no arguments that is called to produce an initial value for
@var{slot}.  The result may also be @code{#f} meaning that the slot has
no initializer.
@end deffn

@node Slot Access Methods, Slot Access Constructors, Slot Descriptors, Slots
@section Slot Access Methods

@cindex accessor, for slot
@cindex slot accessor
@cindex modifier, for slot
@cindex slot modifier
The procedure @code{make-class} provides slot properties that generate
methods to read and write slots.  If an @dfn{accessor} is requested, a
method is automatically generated for reading the value of the slot.  If
a @dfn{modifier} is requested, a method is automatically generated for
storing a value into the slot.  When an accessor or modifier is
specified for a slot, the generic procedure to which the generated
method belongs is directly specified.  The procedure specified for the
accessor takes one argument, the instance.  The procedure specified for
the modifier takes two arguments, the instance and the new value, in
that order.

All of the procedures described here signal an error of type
@code{condition-type:no-such-slot} if the given class or object does not
have a slot of the given name.

Slot-access methods can be generated by the procedures
@code{slot-accessor-method}, @code{slot-modifier-method}, and
@code{slot-initpred-method}.  These methods may be added to a generic
procedure by passing them as arguments to @code{add-method}.  The
methods generated by these procedures are equivalent to those generated
by the slot properties in @code{make-class}.

@deffn Procedure slot-accessor-method class name
Returns an accessor method for the slot @var{name} in @var{class}.  The
returned method has one required argument, an instance of @var{class},
and the specializer for that argument is @var{class}.  When invoked, the
method returns the contents of the slot specified by @var{name} in the
instance; if the slot is uninitialized, an error of type
@code{condition-type:uninitialized-slot} is signalled.

@lisp
@group
(define-generic get-bar (object))

(add-method get-bar
  (slot-accessor-method <foo> 'bar))
@end group
@end lisp
@end deffn

@deffn Procedure slot-modifier-method class name
Returns a modifier method for the slot @var{name} in @var{class}.  The
returned method has two required arguments, an instance of @var{class}
and an object.  The specializer for the first argument is @var{class}
and the second argument is not specialized.  When invoked, the method
stores the second argument in the slot specified by @var{name} in the
instance.

@lisp
@group
(define-generic set-bar! (object bar))

(add-method set-bar!
  (slot-modifier-method <foo> 'bar))
@end group
@end lisp
@end deffn

@deffn Procedure slot-initpred-method class name
Returns an ``initialized?''@: predicate method for the slot @var{name}
in @var{class}.  The returned method has one required argument, an
instance of @var{class}, and the specializer for that argument is
@var{class}.  When invoked, the method returns @code{#t} if the slot
specified by @var{name} is initialized in the instance; otherwise it
returns @code{#f}.

@lisp
@group
(define-generic has-bar? (object))

(add-method has-bar?
  (slot-initpred-method <foo> 'bar))
@end group
@end lisp
@end deffn

@node Slot Access Constructors, Slot Access Procedures, Slot Access Methods, Slots
@section Slot Access Constructors

For convenience, and for consistency with the record-accessor procedures
@code{record-accessor} and @code{record-modifier}, each of the above
method-generating procedures has a corresponding accessor-generator.
Each of these procedures creates a generic procedure, adds an
appropriate method to it by calling the corresponding method-generating
procedure, and returns the generic procedure.  Thus, for example, the
following are equivalent:

@lisp
@group
(slot-accessor <foo> 'bar)

(let ((g (make-generic-procedure 1)))
  (add-method g (slot-accessor-method <foo> 'bar))
  g)
@end group
@end lisp

@deffn Procedure slot-accessor class name
Returns a generic procedure of one argument that is an accessor for the
slot @var{name} in @var{class}.  The argument to the returned procedure
must be an instance of @var{class}.  When the procedure is called, it
returns the contents of the slot @var{name} in that instance; if the
slot is uninitialized, an error of type
@code{condition-type:uninitialized-slot} is signalled.
@end deffn

@deffn Procedure slot-modifier class name
Returns a generic procedure of two arguments that is a modifier for the
slot @var{name} in @var{class}.  The first argument to the returned
procedure must be an instance of @var{class}, and the second argument
may be any object.  When the procedure is called, it modifies the slot
@var{name} in the instance to contain the second argument.
@end deffn

@deffn Procedure slot-initpred class name
Returns a generic procedure of one argument that is an
``initialized?''@: predicate for the slot @var{name} in @var{class}.
The argument to the returned procedure must be an instance of
@var{class}.  When the procedure is called, it returns @code{#t} if the
slot @var{name} in that instance is initialized, otherwise it returns
@code{#f}.
@end deffn

@node Slot Access Procedures,  , Slot Access Constructors, Slots
@section Slot Access Procedures

Finally, there is another set of three procedures, which access the
contents of a slot directly, given an instance and a slot name.  These
procedures are very slow by comparison with the above.

However, note the following.  You can use these procedures in the body
of a @code{define-method} special form in an efficient way.  If the
@code{define-method} specifies the correct number of arguments, the body
of the form contains a call to one of these procedures and nothing else,
and the specified slot name is quoted, the form is rewritten during
macro-expansion time as a call to the corresponding method-generating
procedure.  For example, the following are equivalent:

@lisp
@group
(define-method p ((v <foo>))
  (slot-value v 'bar))

(add-method p
            (slot-accessor-method <foo> 'bar))
@end group
@end lisp

@deffn Procedure slot-value instance name
Returns the contents of the slot @var{name} in @var{instance}; if the
slot is uninitialized, an error of type
@code{condition-type:uninitialized-slot} is signalled.
@end deffn

@deffn Procedure set-slot-value! instance name object
Modifies the slot @var{name} in @var{instance} to contain @var{object}.
@end deffn

@deffn Procedure slot-initialized? instance name
Returns @code{#t} if the slot @var{name} in @var{instance} is
initialized, otherwise returns @code{#f}.
@end deffn

@node Generic Procedures, Methods, Slots, Top
@chapter Generic Procedures

@cindex generic procedure
Like an ordinary Scheme procedure, a generic procedure takes arguments,
performs a series of operations, and perhaps returns useful values.  An
ordinary procedure has a single body of code that is always executed
when the procedure is called.  A generic procedure has a set of multiple
bodies of code, called methods, from which a subset is selected for
execution.  The selected bodies of code and the manner of their
combination are determined by the classes of one or more of the
arguments to the generic procedure.

Ordinary procedures and generic procedures are called with identical
procedure-call syntax.

Generic procedures are true procedures that can be passed as arguments,
returned as values, and otherwise used in all the ways an ordinary
procedure may be used.  In particular, generic procedures satisfy the
predicate @code{procedure?}.

@menu
* Generic Procedure Datatype::  
* Method Storage::              
* Effective Method Procedure::  
@end menu

@node Generic Procedure Datatype, Method Storage, Generic Procedures, Generic Procedures
@section Generic Procedure Datatype

The following definitions are used to construct and inspect generic
procedures.

@deffn Procedure make-generic-procedure arity [name]
Creates and returns a new generic procedure.  The generic procedure
requires @var{arity} arguments.

@var{Arity} may take one of the following forms.  An exact positive
integer specifies that the procedure will accept exactly that number of
arguments.  A pair of two exact positive integers specifies inclusive
lower and upper bounds, respectively, on the number of arguments
accepted; the @sc{cdr} may be @code{#f} indicating no upper bound.

@var{Name} is used for debugging: it is a symbol that has no role in the
semantics of the generic procedure.  @var{Name} may be @code{#f} to
indicate that the generic procedure is anonymous.  If @var{name} is not
specified, it defaults to @code{#f}.

Examples:

@lisp
(define foo-bar (make-generic-procedure 2))

(define foo-baz (make-generic-procedure '(1 . 2) 'foo-baz))

(define foo-mum (make-generic-procedure '(1 . #f)))
@end lisp
@end deffn

@deffn Syntax define-generic name lambda-list
Defines @var{name} to be a generic procedure.  @var{Lambda-list} is an
ordinary parameter list, which is exactly like the parameter list in a
@code{lambda} special form.  This expands into

@lisp
@group
(define @var{name}
  (make-generic-procedure @var{arity}
                          (quote @var{name})))
@end group
@end lisp

@noindent
where @var{arity} is determined from @var{lambda-list}.

Examples (compare to examples of @code{make-generic-procedure}):

@lisp
(define-generic foo-bar (x y))

(define-generic foo-baz (x #!optional y))

(define-generic foo-mum (x . y))
@end lisp
@end deffn

@deffn Procedure generic-procedure? object
Returns @code{#t} if @var{object} is a generic procedure, otherwise
returns @code{#f}.  Note that every generic procedure satisfies the
predicate @code{procedure?}.
@end deffn

@deffn Procedure generic-procedure-arity generic-procedure
Returns the arity of @var{generic-procedure}, as specified in the call
to @code{make-generic-procedure}.  The returned arity must not be
modified.
@end deffn

@deffn Procedure generic-procedure-name generic-procedure
Returns the name of @var{generic-procedure}, as specified in the call to
@code{make-generic-procedure}.
@end deffn

@node Method Storage, Effective Method Procedure, Generic Procedure Datatype, Generic Procedures
@section Method Storage

Methods are stored in generic procedures.  When a generic procedure is
called, it selects a subset of its stored methods (using
@code{method-applicable?}), and arranges to invoke one or more of the
methods as necessary.  The following definitions provide the means for
adding methods to and removing them from a generic procedure.

@deffn Procedure add-method generic-procedure method
Adds @var{method} to @var{generic-procedure}.  If
@var{generic-procedure} already has a method with the same specializers
as @var{method}, then the old method is discarded and @var{method} is
used in its place.
@end deffn

@deffn Procedure delete-method generic-procedure method
Removes @var{method} from @var{generic-procedure}.  Does nothing if
@var{generic-procedure} does not contain @var{method}.
@end deffn

@deffn Procedure add-methods generic-procedure methods
Adds @var{methods}, which must be a list of methods, to
@var{generic-procedure}.  Equivalent to calling @code{add-method} on
each method in @var{methods}.
@end deffn

@deffn Procedure generic-procedure-methods generic-procedure
Returns a list of the methods contained in @var{generic-procedure}.  The
returned list must not be modified.
@end deffn

@node Effective Method Procedure,  , Method Storage, Generic Procedures
@section Effective Method Procedure

@cindex effective method procedure
@cindex emp
When a generic procedure is called, it arranges to invoke a subset of
its methods.  This is done by @dfn{combining} the selected methods into
an @dfn{effective method procedure}, or @sc{emp}, then tail-recursively
invoking the @sc{emp}.  @code{compute-effective-method-procedure} is the
procedure that is called to select the applicable methods and combine
them into an @sc{emp}.

@deffn Procedure compute-effective-method-procedure generic-procedure classes
Collects the applicable methods of @var{generic-procedure} by calling
@code{method-applicable?} on each method and on @var{classes}.  Combines
the resulting methods together into an effective method procedure, and
returns that @sc{emp}.
@end deffn

@deffn Procedure compute-method generic-procedure classes
This procedure is like @code{compute-effective-method-procedure}, except
that it returns the result as a method whose specializers are
@var{classes}.

@code{compute-method} is equivalent to

@lisp
@group
(make-method @var{classes}
             (compute-effective-method-procedure @var{generic-procedure}
                                                 @var{classes}))
@end group
@end lisp
@end deffn

@node Methods, Printing, Generic Procedures, Top
@chapter Methods

@cindex method
A method contains a method procedure and a sequence of @dfn{parameter
specializers} that specify when the given method is applicable.

A method is not a procedure and cannot be invoked as a procedure.
Methods are invoked by the effective method procedure when a generic
procedure is called.

@menu
* Method Datatype::             
* Method Syntax::               
* Chained Methods::             
* Computed Methods::            
@end menu

@node Method Datatype, Method Syntax, Methods, Methods
@section Method Datatype

The following procedures are used to construct and inspect methods.

@deffn Procedure make-method specializers procedure
Creates and returns a new method.  Note that @var{specializers} may have
fewer elements than the number of required parameters in
@var{procedure}; the trailing parameters are considered to be
specialized by @code{<object>}.

After the returned method is stored in a generic procedure,
@var{Procedure} is called by the effective method procedure of the
generic procedure when the generic procedure is called with arguments
satisfying @var{specializers}.  In simple cases, when no method
combination occurs, @var{procedure} @emph{is} the effective method
procedure.
@end deffn

@deffn Procedure method? object
Returns @code{#t} iff @var{object} is a method, otherwise returns
@code{#f}.
@end deffn

@deffn {Generic Procedure} method-specializers method
Returns the specializers of @var{method}.  This list must not be
modified.
@end deffn

@deffn {Generic Procedure} method-procedure method
Returns the procedure of @var{method}.
@end deffn

@deffn Procedure method-applicable? method classes
This predicate is used to determine the applicability of @var{method}.
When a method is contained in a generic procedure, and the procedure is
applied to some arguments, the method is @dfn{applicable} if each
argument is an instance of the corresponding method specializer, or
equivalently, if each argument's class is a subclass of the
corresponding method specializer.

@code{method-applicable?} determines whether @var{method} would be
applicable if the given arguments had the classes specified by
@var{classes}.  It returns @code{#t} if each element of @var{classes} is
a subclass of the corresponding specializer of @var{method}, and
@code{#f} otherwise.
@end deffn

@node Method Syntax, Chained Methods, Method Datatype, Methods
@section Method Syntax

The following syntactic form greatly simplifies the definition of
methods, and of adding them to generic procedures.

@deffn Syntax define-method generic-procedure lambda-list body @dots{}
Defines a method of @var{generic-procedure}.  @var{Lambda-list} is like
the parameter list of a @code{lambda} special form, except that the
required parameters may have associated specializers.  A parameter with
an associated specializer is written as a list of two elements: the
first element is the parameter's name, and the second element is an
expression that evaluates to a class.

@var{Lambda-list} must contain at least one required parameter, and at
least one required parameter must be specialized.

A @code{define-method} special form expands into the following:

@lisp
@group
(add-method @var{generic-procedure}
  (make-method (list @var{specializer} @dots{})
    (lambda (call-next-method . @var{stripped-lambda-list})
      @var{body} @dots{})))
@end group
@end lisp

@noindent
where @var{stripped-lambda-list} is @var{lambda-list} with the
specialized parameters replaced by their names, and the
@var{specializer}s are the corresponding expressions from the
specialized parameters.  If necessary, the @var{specializer}s are
interspersed with references to @code{<object>} in order to make them
occur in the correct position in the sequence.

For example,

@lisp
(define-method add ((x <integer>) (y <rational>)) ...)
@end lisp

@noindent
expands into

@lisp
@group
(add-method add
  (make-method (list <integer> <rational>)
    (lambda (call-next-method x y) ...)))
@end group
@end lisp

Note that the list of specializers passed to @code{make-method} will
correspond to the required parameters of the method; the specializer
corresponding to a non-specialized required parameter is
@code{<object>}.

Further note that, within the body of a @code{define-method} special
form, the free variable @code{call-next-method} is bound to a
``call-next-method'' procedure (see @code{make-chained-method} for
details).  If the @code{define-method} body refers to this variable, the
defined method is a chained method, otherwise it is an ordinary method.
@end deffn

@node Chained Methods, Computed Methods, Method Syntax, Methods
@section Chained Methods

@cindex chained method
Sometimes it is useful to have a method that adds functionality to
existing methods.  @dfn{Chained methods} provide a mechanism to
accomplish this.  A chained method, when invoked, can call the method
that would have been called had this method not been defined: it is
passed a procedure that will call the inherited method.  The chained
method can run arbitrary code both before and after calling the
inherited method.

@deffn Procedure make-chained-method specializers procedure
Create and return a chained method.  @var{Procedure} must be a procedure
of one argument that returns a procedure.  When the chained method is
combined, its procedure will be called with one argument, a
``call-next-method'' procedure; it must then return another procedure
that will be called when the method is invoked.  The
``call-next-method'' procedure may called by the method procedure at any
time, which will invoke the next less-specific method.  The
``call-next-method'' procedure must be called with the same number of
arguments as the method procedure; normally these are the same
arguments, but that is not required.
@end deffn

@deffn Procedure chained-method? object
Returns @code{#t} if @var{object} is a chained method, otherwise returns
@code{#f}.  Note that every chained method satisfies @code{method?}.
@end deffn

@node Computed Methods,  , Chained Methods, Methods
@section Computed Methods

@cindex computed method
A @dfn{computed method} is a powerful mechanism that provides the
ability to generate methods ``on the fly''.  A computed method is like
an ordinary method, except that its procedure is called during method
combination, and is passed the classes of the arguments in place of the
arguments themselves.  Based on these classes, the computed method
returns an ordinary method, which is combined in the usual way.

Note that computed methods and computed @sc{emp}s both satisfy the
predicate @code{method?}.  They are not really methods in that they
cannot be combined with other methods to form an effective method
procedure; however, they are treated as methods by procedures such as
@code{add-method} and @code{method-specializers}.

@deffn Procedure make-computed-method specializers procedure
Create and return a computed method.  @var{Procedure} will be called
during method combination with the classes of the generic-procedure
arguments as its arguments.  It must return one of the following:

@itemize @bullet
@item
An ordinary method (as returned by @code{make-method} or
@code{make-chained-method}).  The returned method's specializers must be
restrictions of @var{specializers}, i.e.@: each specializer in the
returned method must be a subclass of the corresponding specializer in
@var{specializers}.  In the usual case, the returned method's
specializers are the same as @var{specializers}.

@item
A procedure, which is converted into an ordinary method by calling
@code{make-method} on @var{specializers} and the returned procedure.

@item
@code{#f}, which means that the computed method declines to generate a
method.
@end itemize
@end deffn

@deffn Procedure computed-method? object
Returns @code{#t} if @var{object} is a computed method, otherwise
returns @code{#f}.
@end deffn

@cindex computed emps
A @dfn{computed @sc{emp}} takes the computed-method mechanism one step
further.  A computed @sc{emp} is like a computed method, except that it
returns an effective method procedure rather than a method.
@code{compute-effective-method-procedure} tries each of the applicable
computed @sc{emp}s, and if exactly one of them returns an @sc{emp}, that
is the resulting effective method procedure.

@deffn Procedure make-computed-emp key specializers procedure
Create and return a computed @sc{emp}.  @var{Procedure} will be called
during method combination with the classes of the generic-procedure
arguments as its arguments.  It must return either an @sc{emp} or
@code{#f}.

@var{Key} is an arbitrary object that is used to identify the computed
@sc{emp}.  The @var{key} is used by @code{add-method} and
@code{delete-method} to decide whether two computed @sc{emp}s are
the same; they are the same if their @var{key}s are @code{equal?}.  This
is necessary because a generic procedure may have more than one computed
@sc{emp} with the same specializers.
@end deffn

@deffn Procedure computed-emp? object
Returns @code{#t} if @var{object} is a computed @sc{emp}, otherwise
returns @code{#f}.
@end deffn

@deffn {Generic Procedure} computed-emp-key computed-emp
Returns the key for @var{computed-emp}.
@end deffn

@node Printing, GNU Free Documentation License, Methods, Top
@chapter Printing

@cindex printing instances
The following procedures can be used to define a custom printed
representation for an instance.  It is highly recommended that instances
be printed by @code{write-instance-helper}, as this ensures a uniform
appearance for all objects.

@deffn {Generic Procedure} write-instance instance port
This is called by the runtime system to generate the printed
representation of @var{instance}.  The methods of this procedure should
write the representation to @var{port}.
@end deffn

@deffn Procedure write-instance-helper name instance port thunk
This writes a standardized ``frame'' for a printed representation
method.  It generates the following output on @var{port}:

@lisp
#[@var{name} @var{hash-number}@dots{}]
@end lisp

@noindent
where @var{hash-number} is the result of calling @code{hash} on
@var{instance}, and @dots{} is the output generated by @var{thunk}.
@end deffn

@node GNU Free Documentation License, Binding Index, Printing, Top
@appendix GNU Free Documentation License

@cindex FDL, GNU Free Documentation License
@center Version 1.2, November 2002

@display
Copyright @copyright{} 2000,2001,2002 Free Software Foundation, Inc.
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display

@enumerate 0
@item
PREAMBLE

The purpose of this License is to make a manual, textbook, or other
functional and useful document @dfn{free} in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible
for modifications made by others.

This License is a kind of ``copyleft'', which means that derivative
works of the document must themselves be free in the same sense.  It
complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does.  But this License is not limited to software manuals;
it can be used for any textual work, regardless of subject matter or
whether it is published as a printed book.  We recommend this License
principally for works whose purpose is instruction or reference.

@item
APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that
contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License.  Such a notice grants a
world-wide, royalty-free license, unlimited in duration, to use that
work under the conditions stated herein.  The ``Document'', below,
refers to any such manual or work.  Any member of the public is a
licensee, and is addressed as ``you''.  You accept the license if you
copy, modify or distribute the work in a way requiring permission
under copyright law.

A ``Modified Version'' of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.

A ``Secondary Section'' is a named appendix or a front-matter section
of the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document's overall
subject (or to related matters) and contains nothing that could fall
directly within that overall subject.  (Thus, if the Document is in
part a textbook of mathematics, a Secondary Section may not explain
any mathematics.)  The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.

The ``Invariant Sections'' are certain Secondary Sections whose titles
are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License.  If a
section does not fit the above definition of Secondary then it is not
allowed to be designated as Invariant.  The Document may contain zero
Invariant Sections.  If the Document does not identify any Invariant
Sections then there are none.

The ``Cover Texts'' are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License.  A Front-Cover Text may
be at most 5 words, and a Back-Cover Text may be at most 25 words.

A ``Transparent'' copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, that is suitable for revising the document
straightforwardly with generic text editors or (for images composed of
pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or
for automatic translation to a variety of formats suitable for input
to text formatters.  A copy made in an otherwise Transparent file
format whose markup, or absence of markup, has been arranged to thwart
or discourage subsequent modification by readers is not Transparent.
An image format is not Transparent if used for any substantial amount
of text.  A copy that is not ``Transparent'' is called ``Opaque''.

Examples of suitable formats for Transparent copies include plain
@sc{ascii} without markup, Texinfo input format, La@TeX{} input
format, @acronym{SGML} or @acronym{XML} using a publicly available
@acronym{DTD}, and standard-conforming simple @acronym{HTML},
PostScript or @acronym{PDF} designed for human modification.  Examples
of transparent image formats include @acronym{PNG}, @acronym{XCF} and
@acronym{JPG}.  Opaque formats include proprietary formats that can be
read and edited only by proprietary word processors, @acronym{SGML} or
@acronym{XML} for which the @acronym{DTD} and/or processing tools are
not generally available, and the machine-generated @acronym{HTML},
PostScript or @acronym{PDF} produced by some word processors for
output purposes only.

The ``Title Page'' means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page.  For works in
formats which do not have any title page as such, ``Title Page'' means
the text near the most prominent appearance of the work's title,
preceding the beginning of the body of the text.

A section ``Entitled XYZ'' means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following
text that translates XYZ in another language.  (Here XYZ stands for a
specific section name mentioned below, such as ``Acknowledgements'',
``Dedications'', ``Endorsements'', or ``History''.)  To ``Preserve the Title''
of such a section when you modify the Document means that it remains a
section ``Entitled XYZ'' according to this definition.

The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document.  These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.

@item
VERBATIM COPYING

You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License.  You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute.  However, you may accept
compensation in exchange for copies.  If you distribute a large enough
number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and
you may publicly display copies.

@item
COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document's license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover.  Both covers must also clearly and legibly identify
you as the publisher of these copies.  The front cover must present
the full title with all words of the title equally prominent and
visible.  You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve
the title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.

If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.

@item
MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it.  In addition, you must do these things in the Modified Version:

@enumerate A
@item
Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document).  You may use the same title as a previous version
if the original publisher of that version gives permission.

@item
List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.

@item
State on the Title page the name of the publisher of the
Modified Version, as the publisher.

@item
Preserve all the copyright notices of the Document.

@item
Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

@item
Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

@item
Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document's license notice.

@item
Include an unaltered copy of this License.

@item
Preserve the section Entitled ``History'', Preserve its Title, and add
to it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page.  If
there is no section Entitled ``History'' in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.

@item
Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on.  These may be placed in the ``History'' section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

@item
For any section Entitled ``Acknowledgements'' or ``Dedications'', Preserve
the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.

@item
Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles.  Section numbers
or the equivalent are not considered part of the section titles.

@item
Delete any section Entitled ``Endorsements''.  Such a section
may not be included in the Modified Version.

@item
Do not retitle any existing section to be Entitled ``Endorsements'' or
to conflict in title with any Invariant Section.

@item
Preserve any Warranty Disclaimers.
@end enumerate

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant.  To do this, add their titles to the
list of Invariant Sections in the Modified Version's license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled ``Endorsements'', provided it contains
nothing but endorsements of your Modified Version by various
parties---for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version.  Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity.  If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.

@item
COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy.  If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled ``History''
in the various original documents, forming one section Entitled
``History''; likewise combine any sections Entitled ``Acknowledgements'',
and any sections Entitled ``Dedications''.  You must delete all
sections Entitled ``Endorsements.''

@item
COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.

@item
AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an ``aggregate'' if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation's users beyond what the individual works permit.
When the Document is included an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document's Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.

@item
TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections.  You may include a
translation of this License, and all the license notices in the
Document, and any Warrany Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers.  In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled ``Acknowledgements'',
``Dedications'', or ``History'', the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

@item
TERMINATION

You may not copy, modify, sublicense, or distribute the Document except
as expressly provided for under this License.  Any other attempt to
copy, modify, sublicense or distribute the Document is void, and will
automatically terminate your rights under this License.  However,
parties who have received copies, or rights, from you under this
License will not have their licenses terminated so long as such
parties remain in full compliance.

@item
FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time.  Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns.  See
@uref{http://www.gnu.org/copyleft/}.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License ``or any later version'' applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation.  If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.
@end enumerate

@page
@appendixsec ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

@smallexample
@group
  Copyright (C)  @var{year}  @var{your name}.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.2
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.
@end group
@end smallexample

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the ``with...Texts.'' line with this:

@smallexample
@group
    with the Invariant Sections being @var{list their titles}, with
    the Front-Cover Texts being @var{list}, and with the Back-Cover Texts
    being @var{list}.
@end group
@end smallexample

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.

@node Binding Index, Concept Index, GNU Free Documentation License, Top
@appendix Binding Index
@printindex fn

@node Concept Index,  , Binding Index, Top
@appendix Concept Index
@printindex cp

@bye
