1.0.8.48: Update comment.

[sbcl.git] / doc / manual / beyond-ansi.texinfo

@node  Beyond the ANSI Standard
@comment  node-name,  next,  previous,  up
@chapter Beyond the ANSI Standard

SBCL is derived from CMUCL, which implements many extensions to the
ANSI standard. SBCL doesn't support as many extensions as CMUCL, but
it still has quite a few.  @xref{Contributed Modules}.

@menu
* Garbage Collection::          
* Metaobject Protocol::         
* Support For Unix::            
* Customization Hooks for Users::  
* Tools To Help Developers::    
* Resolution of Name Conflicts::  
* Stale Extensions::            
* Efficiency Hacks::            
@end menu

@node  Garbage Collection
@comment  node-name,  next,  previous,  up
@section Garbage Collection

SBCL provides additional garbage collection functionality not
specified by ANSI. Weak pointers allow references to objects to be
maintained without keeping them from being garbage collected, and
``finalization'' hooks are available to cause code to be executed when
an object has been garbage collected. Additionally users can specify
their own cleanup actions to be executed with garbage collection.

@include fun-sb-ext-finalize.texinfo
@include fun-sb-ext-cancel-finalization.texinfo
@include fun-sb-ext-make-weak-pointer.texinfo
@include fun-sb-ext-weak-pointer-value.texinfo
@include var-sb-ext-star-after-gc-hooks-star.texinfo

@node Metaobject Protocol
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@section Metaobject Protocol

SBCL supports a metaobject protocol which is intended to be compatible
with AMOP; present exceptions to this (as distinct from current bugs)
are:

@itemize
  
@item
@findex compute-effective-method
@findex sb-mop:compute-effective-method
@code{compute-effective-method} only returns one value, not two.

There is no record of what the second return value was meant to
indicate, and apparently no clients for it.

@item
@tindex generic-function
@tindex standard-generic-function
@tindex funcallable-standard-object
@tindex sb-mop:funcallable-standard-object
@tindex standard-object
@tindex function
The direct superclasses of @code{sb-mop:funcallable-standard-object} are
@code{(function standard-object)}, not @code{(standard-object function)}.

This is to ensure that the @code{standard-object} class is the last of
the standardized classes before @code{t} appearing in the class
precedence list of @code{generic-function} and
@code{standard-generic-function}, as required by section 1.4.4.5 of the
ANSI specification.
  
@item
@findex ensure-generic-function
@findex generic-function-declarations
@findex sb-mop:generic-function-declarations
the arguments @code{:declare} and @code{:declarations} to
@code{ensure-generic-function} are both accepted, with the leftmost
argument defining the declarations to be stored and returned by
@code{generic-function-declarations}.

Where AMOP specifies @code{:declarations} as the keyword argument to
@code{ensure-generic-function}, the Common Lisp standard specifies
@code{:declare}.  Portable code should use @code{:declare}.

@item
@findex validate-superclass
@findex finalize-inheritance
@findex sb-mop:validate-superclass
@findex sb-mop:finalize-inheritance
@tindex standard-class
@tindex funcallable-standard-class
@tindex sb-mop:funcallable-standard-class
@tindex function
@findex sb-mop:class-prototype
@findex class-prototype
although SBCL obeys the requirement in AMOP that
@code{validate-superclass} should treat @code{standard-class} and
@code{funcallable-standard-class} as compatible metaclasses, we
impose an additional requirement at class finalization time: a class
of metaclass @code{funcallable-standard-class} must have
@code{function} in its superclasses, and a class of metaclass
@code{standard-class} must not.

@findex typep
@findex class-of
@findex subtypep
After a class has been finalized, it is associated with a class
prototype which is accessible by a standard mop function
@code{sb-mop:class-prototype}.  The user can then ask whether this
object is a @code{function} or not in several different ways: whether it
is a function according to @code{typep}; whether its @code{class-of} is
@code{subtypep} @code{function}, or whether @code{function} appears in
the superclasses of the class.  The additional consistency requirement
comes from the desire to make all of these answers the same.

The following class definitions are bad, and will lead to errors
either immediately or if an instance is created:
@lisp
(defclass bad-object (funcallable-standard-object)
  ()
  (:metaclass standard-class))
@end lisp
@lisp
(defclass bad-funcallable-object (standard-object)
  ()
  (:metaclass funcallable-standard-class))
@end lisp
The following definition is acceptable:
@lisp
(defclass mixin ()
  ((slot :initarg slot)))
(defclass funcallable-object (funcallable-standard-object mixin)
  ()
  (:metaclass funcallable-standard-class))
@end lisp
and leads to a class whose instances are funcallable and have one slot.

@tindex funcallable-standard-object
@tindex sb-mop:funcallable-standard-object
Note that this requirement also applies to the class
@code{sb-mop:funcallable-standard-object}, which has metaclass
@code{sb-mop:funcallable-standard-class} rather than
@code{standard-class} as AMOP specifies.

@item
the requirement that ``No portable class @math{C_p} may inherit, by
virtue of being a direct or indirect subclass of a specified class, any
slot for which the name is a symbol accessible in the
@code{common-lisp-user} package or exported by any package defined in
the ANSI Common Lisp standard.'' is interpreted to mean that the
standardized classes themselves should not have slots named by external
symbols of public packages.

The rationale behind the restriction is likely to be similar to the ANSI
Common Lisp restriction on defining functions, variables and types named
by symbols in the Common Lisp package: preventing two independent pieces
of software from colliding with each other.

@item
@findex slot-value-using-class
@findex sb-mop:slot-value-using-class
@findex (setf slot-value-using-class)
@findex (setf sb-mop:slot-value-using-class)
@findex slot-boundp-using-class
@findex sb-mop:slot-boundp-using-class
specializations of the @code{new-value} argument to @code{(setf
sb-mop:slot-value-using-class)} are not allowed: all user-defined
methods must have a specializer of the class @code{t}.

This prohibition is motivated by a separation of layers: the
@code{slot-value-using-class} family of functions is intended for use in
implementing different and new slot allocation strategies, rather than
in performing application-level dispatching.  Additionally, with this
requirement, there is a one-to-one mapping between metaclass, class and
slot-definition-class tuples and effective methods of @code{(setf
slot-value-using-class)}, which permits optimization of @code{(setf
slot-value-using-class)}'s discriminating function in the same manner as
for @code{slot-value-using-class} and @code{slot-boundp-using-class}.

Note that application code may specialize on the @code{new-value}
argument of slot accessors.

@item
@findex defclass
@findex ensure-class
@findex ensure-class-using-class
@findex sb-mop:ensure-class
@findex sb-mop:ensure-class-using-class
@findex find-class
@findex class-name
the class named by the @code{name} argument to @code{ensure-class}, if
any, is only redefined if it is the proper name of that class;
otherwise, a new class is created.

This is consistent with the description of @code{ensure-class} in AMOP
as the functional version of @code{defclass}, which has this behaviour;
however, it is not consistent with the weaker requirement in AMOP, which
states that any class found by @code{find-class}, no matter what its
@code{class-name}, is redefined.

@end itemize

In addition, SBCL supports extensions to the Metaobject protocol from
AMOP; at present, they are:

@itemize

@item
@findex defmethod
@findex find-class
@findex intern-eql-specializer
@findex sb-mop:intern-eql-specializer
@findex make-method-specializers-form
@findex sb-pcl:make-method-specializers-form
@findex make-method-lambda
@findex sb-mop:make-method-lambda
compile-time support for generating specializer metaobjects from
specializer names in @code{defmethod} forms is provided by the
@code{make-method-specializers-form} function, which returns a form
which, when evaluated in the lexical environment of the
@code{defmethod}, returns a list of specializer metaobjects.  This
operator suffers from similar restrictions to those affecting
@code{make-method-lambda}, namely that the generic function must be
defined when the @code{defmethod} form is expanded, so that the
correct method of @code{make-method-specializers-form} is invoked.
The system-provided method on @code{make-method-specializers-form}
generates a call to @code{find-class} for each symbol specializer
name, and a call to @code{intern-eql-specializer} for each @code{(eql
@var{x})} specializer name.

@item
@findex find-method
@findex parse-specializer-using-class
@findex unparse-specializer-using-class
@findex sb-pcl:parse-specializer-using-class
@findex sb-pcl:unparse-specializer-using-class
run-time support for converting between specializer names and
specializer metaobjects, mostly for the purposes of
@code{find-method}, is provided by
@code{parse-specializer-using-class} and
@code{unparse-specializer-using-class}, which dispatch on their first
argument, the generic function associated with a method with the given
specializer.  The system-provided methods on those methods convert
between classes and proper names and between lists of the form
@code{(eql @var{x})} and interned eql specializer objects.

@end itemize

@node  Support For Unix
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@section Support For Unix

The UNIX command line can be read from the variable
@code{sb-ext:*posix-argv*}. The UNIX environment can be queried with
the @code{sb-ext:posix-getenv} function.

@include fun-sb-ext-posix-getenv.texinfo

External programs can be run with @code{sb-ext:run-program}.

@include fun-sb-ext-run-program.texinfo

@include fun-sb-ext-process-p.texinfo

@include fun-sb-ext-process-input.texinfo

@include fun-sb-ext-process-output.texinfo

@include fun-sb-ext-process-error.texinfo

@include fun-sb-ext-process-alive-p.texinfo

@include fun-sb-ext-process-status.texinfo

@include fun-sb-ext-process-wait.texinfo

@include fun-sb-ext-process-exit-code.texinfo

@include fun-sb-ext-process-core-dumped.texinfo

@include fun-sb-ext-process-close.texinfo

@include fun-sb-ext-process-kill.texinfo

@node  Customization Hooks for Users
@comment  node-name,  next,  previous,  up
@section Customization Hooks for Users

The toplevel repl prompt may be customized, and the function
that reads user input may be replaced completely.
@c <!-- FIXME but I don't currently remember how -->

The behaviour of @code{require} when called with only one argument is
implementation-defined.  In SBCL, @code{require} behaves in the
following way:

@include fun-common-lisp-require.texinfo
@include var-sb-ext-star-module-provider-functions-star.texinfo

Although SBCL does not provide a resident editor, the @code{ed}
function can be customized to hook into user-provided editing
mechanisms as follows:

@include fun-common-lisp-ed.texinfo
@include var-sb-ext-star-ed-functions-star.texinfo

@node Tools To Help Developers
@comment  node-name,  next,  previous,  up
@section Tools To Help Developers

SBCL provides a profiler and other extensions to the ANSI @code{trace}
facility.  For more information, see @ref{Macro common-lisp:trace}.

The debugger supports a number of options. Its documentation is
accessed by typing @kbd{help} at the debugger prompt. @xref{Debugger}.

Documentation for @code{inspect} is accessed by typing @kbd{help} at
the @code{inspect} prompt.

@node Resolution of Name Conflicts
@section Resolution of Name Conflicts

The ANSI standard (section 11.1.1.2.5) requires that name conflicts in
packages be resolvable in favour of any of the conflicting symbols.  In
the interactive debugger, this is achieved by prompting for the symbol
in whose favour the conflict should be resolved; for programmatic use,
the @code{sb-ext:resolve-conflict} restart should be invoked with one
argument, which should be a member of the list returned by the condition
accessor @code{sb-ext:name-conflict-symbols}.

@node Stale Extensions
@comment  node-name,  next,  previous,  up
@section Stale Extensions

SBCL has inherited from CMUCL various hooks to allow the user to
tweak and monitor the garbage collection process. These are somewhat
stale code, and their interface might need to be cleaned up. If you
have urgent need of them, look at the code in @file{src/code/gc.lisp}
and bring it up on the developers' mailing list.

SBCL has various hooks inherited from CMUCL, like
@code{sb-ext:float-denormalized-p}, to allow a program to take
advantage of IEEE floating point arithmetic properties which aren't
conveniently or efficiently expressible using the ANSI standard. These
look good, and their interface looks good, but IEEE support is
slightly broken due to a stupid decision to remove some support for
infinities (because it wasn't in the ANSI spec and it didn't occur to
me that it was in the IEEE spec). If you need this stuff, take a look
at the code and bring it up on the developers' mailing
list.


@node  Efficiency Hacks
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@section Efficiency Hacks

The @code{sb-ext:purify} function causes SBCL first to collect all
garbage, then to mark all uncollected objects as permanent, never again
attempting to collect them as garbage. This can cause a large increase
in efficiency when using a primitive garbage collector, or a more
moderate increase in efficiency when using a more sophisticated garbage
collector which is well suited to the program's memory usage pattern. It
also allows permanent code to be frozen at fixed addresses, a
precondition for using copy-on-write to share code between multiple Lisp
processes.  This is less important with modern generational garbage
collectors, but not all SBCL platforms use such a garbage collector.

@include fun-sb-ext-purify.texinfo

The @code{sb-ext:truly-the} special form declares the type of the
result of the operations, producing its argument; the declaration is
not checked. In short: don't use it.

@include special-operator-sb-ext-truly-the.texinfo

The @code{sb-ext:freeze-type} declaration declares that a
type will never change, which can make type testing
(@code{typep}, etc.) more efficient for structure types.

The @code{sb-ext:constant-function} declaration specifies
that a function will always return the same value for the same
arguments, which may allow the compiler to optimize calls
to it. This is appropriate for functions like @code{sqrt}, but
is @emph{not} appropriate for functions like @code{aref},
which can change their return values when the underlying data are
changed.
@c <!-- FIXME: This declaration does not seem to be supported in the 
@c      current compiler. -->