#+LaTeX_HEADER: \DeclareTextFontCommand{\texttt}{\ttfamily\hyphenchar\font=45\relax}
#+begin_src elisp :exports none
+;;; use C-x C-e on this if org refuses to export
(add-to-list 'org-latex-classes
'("acm_proc_article-sp" "\\documentclass{acm_proc_article-sp}"
("\\section{%s}" . "\\section*{%s}")
("\\subsubsection{%s}" . "\\subsubsection*{%s}")
("\\paragraph{%s}" . "\\paragraph*{%s}")
("\\subparagraph{%s}" . "\\subparagraph*{%s}")))
+(set (make-local-variable 'org-latex-title-command)
+ "\\numberofauthors{3}
+\\author{
+\\alignauthor Christophe Rhodes\\\\
+ \\affaddr{Department of Computing}\\\\
+ \\affaddr{Goldsmiths, University of London}\\\\
+ \\affaddr{London SE14 6NW}\\\\
+ \\email{c.rhodes@gold.ac.uk}
+\\alignauthor Jan Moringen\\\\
+ \\affaddr{Universität Bielefeld}\\\\
+ \\affaddr{Technische Fakultät}\\\\
+ \\affaddr{33594 Bielefeld}\\\\
+ \\email{jmoringe@techfak.uni-bielefeld.de}
+\\alignauthor David Lichteblau\\\\
+ \\affaddr{ZenRobotics Ltd}\\\\
+ \\affaddr{Vilhonkatu 5 A}\\\\
+ \\affaddr{FI-00100 Helsinki}\\\\
+ \\email{david@lichteblau.com}
+}
+\\maketitle")
#+end_src
#+begin_abstract
-1. This paper introduces a new metaobject, the generalizer, which
- complements the existing specializer metaobject.
-2. With the help of examples, we show that this metaobject allows for
- the efficient implementation of complex non-class-based dispatch
- within the framework of existing metaobject protocols
-3. We present the generalizer protocol, implemented within the SBCL
- implementation of Common Lisp
-4. In combination with previous work, this produces a fully-functional
- extension of the existing mechanism for method selection and
- effective method computation, including support for standard and
- user-defined method combination independent from method selection.
+This paper introduces a new metaobject, the generalizer, which
+complements the existing specializer metaobject. With the help of
+examples, we show that this metaobject allows for the efficient
+implementation of complex non-class-based dispatch within the
+framework of existing metaobject protocols. We present our
+modifications to the generic function invocation protocol from the
+/Art of the Metaobject Protocol/; in combination with previous work,
+this produces a fully-functional extension of the existing mechanism
+for method selection and combination, including support for method
+combination completely independent from method selection. We discuss
+our implementation, within the SBCL implementation of Common Lisp, and
+in that context compare the performance of the new protocol with the
+standard one, demonstrating that the new protocol can be tolerably
+efficient.
#+end_abstract
+#+begin_LaTeX
+\category{D.1}{Software}{Programming Techniques}[Object-oriented Programming]
+\category{D.3.3}{Programming Languages}{Language Constructs and Features}
+\terms{Languages, Design}
+\keywords{generic functions, specialization-oriented programming, method selection, method combination}
+#+end_LaTeX
+
* Introduction
The revisions to the original Common Lisp language \cite{CLtL}
included the detailed specification of an object system, known as
while many implementations have derived their implementations of
CLOS from either the Closette illustrative implementation in
\cite{AMOP}, or the Portable Common Loops implementation of CLOS
- from Xerox Parc, there have been from-scratch reimplementations of
- CLOS (in at least CLISP; check for others -- Lisp500? CCL?)
+ from Xerox Parc, there have been largely from-scratch
+ reimplementations of CLOS (in CLISP[fn:1] and CCL[fn:2], at least)
incorporating substantial fractions of the Metaobject Protocol as
described.
+ #+CAPTION: MOP Design Space
+ #+LABEL: fig:mopdesign
+ #+ATTR_LATEX: width=\linewidth float
+ [[file:figures/mop-design-space.pdf]]
+
Although it has stood the test of time, the CLOS MOP is neither
without issues (e.g. semantic problems with =make-method-lambda=
\cite{Costanza.Herzeel:2008}; useful functions such as
implement all conceivable variations of object-oriented behaviour.
While metaprogramming offers some possibilities for customization of
the object system behaviour, those possibilities cannot extend
- arbitrarily in all directions. There is still an expectation that
- functionality is implemented with methods on generic functions,
- acting on objects with slots. Nevertheless, the MOP is flexible,
- and is used for a number of things, including: documentation
- generation (where introspective functionality in the MOP is used to
- extract information from a running system); object-relational
- mapping and other approaches to object persistence; alternative
- backing stores for slots (hash-tables or symbols); and programmatic
- construction of metaobjects, for example for IDL compilers and model
- transformations.
-
- [ A picture on MOP flexibility here would be good; I have in my mind
- one where an object system is a point and the MOP opens up a blob
- around that point, and I'm sure I've seen it somewhere but I can't
- remember where. Alternatively, there's Kiczales et al "MOPs: why we
- want them and what else they can do", fig. 2 ]
+ arbitrarily in all directions (conceptually, if a given object
+ system is a point in design space, then a MOP for that object system
+ allows exploration of a region of design space around that point;
+ see figure \ref{fig:mopdesign}). In the case of the CLOS MOP, there is
+ still an expectation that functionality is implemented with methods
+ on generic functions, acting on objects with slots; it is not
+ possible, for example, to transparently implement support for
+ “message not understood” as in the message-passing paradigm, because
+ the analogue of messages (generic functions) need to be defined
+ before they are used.
+
+ Nevertheless, the MOP is flexible, and is used for a number of
+ things, including: documentation generation (where introspection in
+ the MOP is used to extract information from a running system);
+ object-relational mapping and other approaches to object
+ persistence; alternative backing stores for slots (hash-tables or
+ symbols); and programmatic construction of metaobjects, for example
+ for IDL compilers and model transformations.
One area of functionality where there is scope for customization by
the metaprogrammer is in the mechanics and semantics of method
=compute-applicable-methods=,
=compute-applicable-methods-using-classes=), for example, in
practice implementation support for this was weak until relatively
- recently[fn:1].
+ recently[fn:3].
Another potential mechanism for customizing dispatch is implicit in
the class structure defined by AMOP: standard specializer objects
there are no restrictions on the metaprogrammer constructing
additional subclasses. Previous work \cite{Newton.Rhodes:2008} has
explored the potential for customizing generic function dispatch
- using extended specializers, but as of that work the metaprogrammer
- must override the entirety of the generic function invocation
- protocol (from =compute-discriminating-function= on down), leading
- to toy implementations and duplicated effort.
+ using extended specializers, but there the metaprogrammer must
+ override the entirety of the generic function invocation protocol
+ (from =compute-discriminating-function= on down), leading to toy
+ implementations and duplicated effort.
This paper introduces a protocol for efficient and controlled
handling of new subclasses of =specializer=. In particular, it
#+CAPTION: Dispatch Comparison
#+LABEL: fig:dispatch
- #+ATTR_LATEX: width=0.9\linewidth float
- [[file:figures/dispatch-comparison.pdf]]
+ #+ATTR_LATEX: width=\linewidth float
+ [[file:figures/dispatch-relationships.pdf]]
The remaining sections in this paper can be read in any order. We
give some motivating examples in section [[#Examples]], including
implemented using our protocol, which we describe in section
[[#Protocol]]. For reasons of space, the metaprogram code examples in
this section do not include some of the necessary support code to
- run; complete implementations of each of these cases are included in
- an appendix / in the accompanying repository snapshot / at this
- location.
+ run; complete implementations of each of these cases, along with the
+ integration of this protocol into the SBCL implementation
+ \cite{Rhodes:2008} of Common Lisp, are included in an appendix / in
+ the accompanying repository snapshot / at this location.
A note on terminology: we will attempt to distinguish between the
user of an individual case of generalized dispatch (the
:PROPERTIES:
:CUSTOM_ID: Cons
:END:
- We start by presenting our original use case, performing
- dispatching on the first element of lists. Semantically, we allow
- the programmer to specialize any argument of methods with a new
- kind of specializer, =cons-specializer=, which is applicable if and
- only if the corresponding object is a =cons= whose =car= is =eql=
- to the symbol associated with the =cons-specializer=; these
- specializers are more specific than the =cons= class, but less
- specific than an =eql-specializer= on any given =cons=.
-
- One motivation for the use of this specializer is in an extensible
- code walker: a new special form can be handled simply by writing an
- additional method on the walking generic function, seamlessly
- interoperating with all existing methods.
-
+ One motivation for the use of generalized dispatch is in an
+ extensible code walker: a new special form can be handled simply by
+ writing an additional method on the walking generic function,
+ seamlessly interoperating with all existing methods. In this
+ use-case, dispatch is performed on the first element of lists.
+ Semantically, we allow the programmer to specialize any argument of
+ methods with a new kind of specializer, =cons-specializer=, which
+ is applicable if and only if the corresponding object is a =cons=
+ whose =car= is =eql= to the symbol associated with the
+ =cons-specializer=; these specializers are more specific than the
+ =cons= class, but less specific than an =eql-specializer= on any
+ given =cons=.
+
The programmer code using these specializers is unchanged from
\cite{Newton.Rhodes:2008}; the benefits of the protocol described
- here are centered on performance and generality: in an application
- such as walking source code, we would expect to encounter special
- forms (distinguished by particular atoms in the =car= position)
- multiple times, and hence to dispatch to the same effective method
- repeatedly. We discuss this in more detail in section [[#Memoization]];
- we present the metaprogrammer code below.
+ here are: that the separation of concerns is complete – method
+ selection is independent of method combination – and that the
+ protocol allows for efficient implementation where possible, even
+ when method selection is customized. In an application such as
+ walking source code, we would expect to encounter special forms
+ (distinguished by particular atoms in the =car= position) multiple
+ times, and hence to dispatch to the same effective method
+ repeatedly. We discuss the efficiency aspects of the protocol in
+ more detail in section [[#Memoization]]; we present the metaprogrammer
+ code to implement the =cons-specializer= below.
#+begin_src lisp
(defclass cons-specializer (specializer)
(and (consp o) (eql (car o) (%car s))))
#+end_src
-The code above shows the core of the use of our protocol. We have
-elided some support code for parsing and unparsing specializers, and
-for handling introspective functions such as finding generic functions
-for a given specializer. We have also elided methods on the protocol
-function =specializer<=; for =cons-specializers= here, specializer
-ordering is trivial, as only one =cons-specializer= can ever be
-applicable to any given argument. See section [[#Accept]] for a case
-where specializer ordering is substantially different.
-
-As in \cite{Newton.Rhodes:2008}, we can use these specializers to
-implement a modular code walker, where we define one method per
-special operator. We show two of those methods below, in the context
-of a walker which checks for unused bindings and uses of unbound
-variables.
+The code above shows a minimal use of our protocol. We have elided
+some support code for parsing and unparsing specializers, and for
+handling introspective functions such as finding generic functions for
+a given specializer. We have also elided methods on the protocol
+functions =specializer<= and =same-specializer-p=; for
+=cons-specializer= objects, specializer ordering is trivial, as only
+one =cons-specializer= (up to equality) can ever be applicable to any
+given argument. See section [[#Accept]] for a case where specializer
+ordering is non-trivial.
+
+As in \cite{Newton.Rhodes:2008}, the programmer can use these
+specializers to implement a modular code walker, where they define one
+method per special operator. We show two of those methods below, in
+the context of a walker which checks for unused bindings and uses of
+unbound variables.
#+begin_src
(defgeneric walk (form env stack)
(:generic-function-class cons-generic-function))
-(defmethod walk ((expr (cons lambda)) env call-stack)
+(defmethod walk
+ ((expr (cons lambda)) env call-stack)
(let ((lambda-list (cadr expr))
(body (cddr expr)))
(with-checked-bindings
- ((bindings-from-ll lambda-list) env call-stack)
+ ((bindings-from-ll lambda-list)
+ env call-stack)
(dolist (form body)
(walk form env (cons form call-stack))))))
-(defmethod walk ((expr (cons let)) env call-stack)
+(defmethod walk
+ ((expr (cons let)) env call-stack)
(flet ((let-binding (x)
- (walk (cadr x) env (cons (cadr x) call-stack))
- (cons (car x) (make-instance 'binding))))
+ (walk (cadr x) env
+ (cons (cadr x) call-stack))
+ (cons (car x)
+ (make-instance 'binding))))
(with-checked-bindings
- ((mapcar #'let-binding (cadr expr)) env call-stack)
+ ((mapcar #'let-binding (cadr expr))
+ env call-stack)
(dolist (form (cddr expr))
(walk form env (cons form call-stack))))))
#+end_src
Note that in this example there is no strict need for
- =cons-specializer= and =cons-generalizer= to be distinct classes –
- just as in the normal protocol involving
- =compute-applicable-methods= and
- =compute-applicable-methods-using-classes=, the specializer object
- for mediating dispatch contains the same information as the object
- representing the equivalence class of objects to which that
- specializer is applicable: here it is the =car= of the =cons=
- (which we wrap in a distinct object); in the standard dispatch it
- is the =class= of the object. This feature also characterizes
- those use cases where the metaprogrammer could straightforwardly
- use filtered dispatch \cite{Costanza.etal:2008} to implement their
- dispatch semantics. We will see in section [[#Accept]] an example
- of a case where filtered dispatch is incapable of straightforwardly
- expressing the dispatch, but first we present our implementation of
- the motivating case from \cite{Costanza.etal:2008}.
+ =cons-specializer= and =cons-generalizer= to be distinct classes.
+ In standard generic function dispatch, the =class= functions both
+ as the specializer for methods and as the generalizer for generic
+ function arguments; we can think of the dispatch implemented by
+ =cons-specializer= objects as providing for subclasses of the
+ =cons= class distinguished by the =car= of the =cons=. This
+ analogy also characterizes those use cases where the metaprogrammer
+ could straightforwardly use filtered dispatch
+ \cite{Costanza.etal:2008} to implement their dispatch semantics.
+ We will see in section [[#Accept]] an example of a case where filtered
+ dispatch is incapable of straightforwardly expressing the dispatch,
+ but first we present our implementation of the motivating case from
+ \cite{Costanza.etal:2008}.
** SIGNUM specializers
:PROPERTIES:
:CUSTOM_ID: Signum
Our second example of the implementation and use of generalized
specializers is a reimplementation of one of the examples in
\cite{Costanza.etal:2008}: specifically, the factorial function.
- Here, we will perform dispatch based on the =signum= of the
+ Here, dispatch will be performed based on the =signum= of the
argument, and again, at most one method with a =signum= specializer
- will be appliable to any given argument, which makes the structure
+ will be applicable to any given argument, which makes the structure
of the specializer implementation very similar to the =cons=
specializers in the previous section.
- We have chosen to compare signum values using \texttt{=}, which
- means that a method with specializer =(signum 1)= will be
- applicable to positive floating-point arguments (see the first
- method on =specializer-accepts-generalizer-p= and the method on
- =specializer=accepts-p= below). This leads to one subtle
+ The metaprogrammer has chosen in the example below to compare
+ signum values using \texttt{=}, which means that a method with
+ specializer =(signum 1)= will be applicable to positive
+ floating-point arguments (see the first method on
+ =specializer-accepts-generalizer-p= and the method on
+ =specializer-accepts-p= below). This leads to one subtle
difference in behaviour compared to that of the =cons=
specializers: in the case of =signum= specializers, the /next/
method after any =signum= specializer can be different, depending
on the class of the argument. This aspect of the dispatch is
handled by the second method on =specializer-accepts-generalizer-p=
below.
+
#+begin_src lisp
(defclass signum-specializer (specializer)
((%signum :reader %signum :initarg :signum)))
(defclass signum-generalizer (generalizer)
((%signum :reader %signum :initarg :signum)))
(defmethod generalizer-of-using-class
- ((gf signum-generic-function) arg)
- (typecase arg
- (real (make-instance 'signum-generalizer
- :signum (signum arg)))
- (t (call-next-method))))
+ ((gf signum-generic-function) (arg real))
+ (make-instance 'signum-generalizer
+ :signum (signum arg)))
(defmethod generalizer-equal-hash-key
((gf signum-generic-function)
- (g signum-specializer))
+ (g signum-generalizer))
(%signum g))
(defmethod specializer-accepts-generalizer-p
((gf signum-generic-function)
(s signum-specializer)
(g signum-generalizer))
- (if (= (%signum s) (%signum g)) ; or EQL?
+ (if (= (%signum s) (%signum g))
(values t t)
(values nil t)))
(defmethod specializer-accepts-generalizer-p
((gf signum-generic-function)
- (specializer sb-mop:specializer)
- (thing signum-specializer))
+ (s specializer)
+ (g signum-generalizer))
(specializer-accepts-generalizer-p
- gf specializer (class-of (%signum thing))))
+ gf s (class-of (%signum g))))
(defmethod specializer-accepts-p
((s signum-specializer) o)
#+end_src
Given these definitions, and once again some more straightforward
- ones elided for reasons of space, we can implement the factorial
- function as follows:
+ ones elided for reasons of space, the programmer can implement the
+ factorial function as follows:
#+begin_src lisp
(defgeneric fact (n)
(defmethod fact ((n (signum 1))) (* n (fact (1- n))))
#+end_src
- We do not need to include a method on =(signum -1)=, as the
- standard =no-applicable-method= protocol will automatically apply to
- negative real or non-real arguments.
+ The programmer does not need to include a method on =(signum -1)=,
+ as the standard =no-applicable-method= protocol will automatically
+ apply to negative real or non-real arguments.
** Accept HTTP header specializers
:PROPERTIES:
:CUSTOM_ID: Accept
it has available to satisfy this request, and sends the best
matching resource in its response.
- For example, a graphical web browser might by default send an
- =Accept= header such as
- =text/html,application/xml;q=0.9,*/*;q=0.8=. This should be
- interpreted by a web server as meaning that if for a given resource
- the server can provide content of type =text/html= (i.e. HTML),
- then it should do so. Otherwise, if it can provide
- =application/xml= content (i.e. XML of any schema), then that
- should be provided; failing that, any other content type is
- acceptable.
+ For example, a graphical web browser might send an =Accept= header
+ of =text/html,application/xml;q=0.9,*/*;q=0.8= for a request of a
+ resource typed in to the URL bar. This should be interpreted as
+ meaning that: if the server can provide content of type =text/html=
+ (i.e. HTML) for that resource, then it should do so. Otherwise, if
+ it can provide =application/xml= content (i.e. XML of any schema),
+ then that should be provided; failing that, any other content type
+ is acceptable.
In the case where there are static files on the filesystem, and the
web server must merely select between them, there is not much more
generic function must then perform method selection against the
request's =Accept= header to compute the appropriate response.
- The =accept-specializer= below implements the dispatch. It depends
+ The =accept-specializer= below implements this dispatch. It depends
on a lazily-computed =tree= slot to represent the information in
the accept header (generated by =parse-accept-string=), and a
function =q= to compute the (defaulted) preference level for a
(defmethod specializer-accepts-generalizer-p
((gf accept-generic-function)
(s accept-specializer)
- (generalizer accept-generalizer))
- (values (q (media-type s) (tree generalizer)) t))
+ (g accept-generalizer))
+ (values (q (media-type s) (tree g)) t))
(defmethod specializer-accepts-generalizer-p
((gf accept-generic-function)
(s specializer)
- (generalizer accept-generalizer))
+ (g accept-generalizer))
(specializer-accepts-generalizer-p
- gf s (next generalizer)))
+ gf s (next g)))
(defmethod specializer<
((gf accept-generic-function)
(s1 accept-specializer)
(s2 accept-specializer)
- (generalizer accept-generalizer))
+ (g accept-generalizer))
(let ((m1 (media-type s1))
(m2 (media-type s2))
- (tree (tree generalizer)))
+ (tree (tree g)))
(cond
((string= m1 m2) '=)
(t (let ((q1 (q m1 tree)))
(t '<))))))
#+end_src
- The metaprogrammer can then support dispatching in this way for
- suitable objects, such as the =request= object representing a
- client request in the Hunchentoot web server. The code below
- implements this, by defining the computation of a suitable
- =generalizer= object for a given request, and specifying how to
- compute whether the specializer accepts the given request object
- (=q= returns a number between 0 and 1 if any pattern in the =tree=
- matches the media type, and =nil= if the media type cannot be
- matched at all).
+ The metaprogrammer can then add support for objects representing
+ client requests, such as instances of the =request= class in the
+ Hunchentoot web server, by translating these into
+ =accept-generalizer= instances. The code below implements this, by
+ defining the computation of a =generalizer= object for a given
+ request, and specifying how to compute whether the specializer
+ accepts the given request object (=q= returns a number between 0
+ and 1 if any pattern in the =tree= matches the media type, and
+ =nil= if the media type cannot be matched at all).
#+begin_src
(defmethod generalizer-of-using-class
(arg tbnl:request))
(make-instance 'accept-generalizer
:header (tbnl:header-in :accept arg)
- :next (class-of arg)))
+ :next (call-next-method)))
(defmethod specializer-accepts-p
- ((specializer accept-specializer)
- (obj tbnl:request))
- (let* ((accept (tbnl:header-in :accept obj))
+ ((s accept-specializer)
+ (o tbnl:request))
+ (let* ((accept (tbnl:header-in :accept o))
(tree (parse-accept-string accept))
- (q (q (media-type specializer) tree)))
+ (q (q (media-type s) tree)))
(and q (> q 0))))
#+end_src
(ensure-class nil :direct-superclasses
'(text/html image/webp ...))
#+end_src
- and dispatch the operates using those anonymous classes. While
+ and dispatch would operate using those anonymous classes. While
this is possible to do, it is awkward to express content-type
negotiation in this way, as it means that the dispatcher must know
about the universe of mime-types that clients might declare that
filtering paradigm.
Note that in this example, the method on =specializer<= involves a
- nontrivial ordering of methods based on the =q= values specified in
- the accept header (whereas in sections [[#Cons]] and [[#Signum]] only a
+ non-trivial ordering of methods based on the =q= values specified
+ in the accept header (whereas in sections [[#Cons]] and [[#Signum]] only a
single extended specializer could be applicable to any given
argument).
(s string))
(make-instance 'accept-generalizer
:header s
- :next (class-of s)))
+ :next (call-next-method)))
(defmethod specializer-accepts-p
- ((s accept-specializer) (string string))
- (let* ((tree (parse-accept-string string))
+ ((s accept-specializer) (o string))
+ (let* ((tree (parse-accept-string o))
(q (q (media-type s) tree)))
(and q (> q 0))))
#+end_src
-** Pattern / xpattern / regex / optima
+
+ The =next= slot in the =accept-generalizer= is used to deal with
+ the case of methods specialized on the classes of objects as well
+ as on the acceptable media types; there is a method on
+ =specializer-accepts-generalizer-p= for specializers that are not
+ of type =accept-specializer= which calls the generic function again
+ with the next generalizer, so that methods specialized on the
+ classes =tbnl:request= and =string= are treated as applicable to
+ corresponding objects, though less specific than methods with
+ =accept-specializer= specializations.
+
+** COMMENT Pattern / xpattern / regex / optima
Here's the /really/ interesting bit, but on the other hand we're
probably going to run out of space, and the full description of
these is going to take us into =make-method-lambda= territory.
:END:
In section [[#Examples]], we have seen a number of code fragments as
- partial implementations of particular non-standard method dispatch,
- using =generalizer= metaobjects to mediate between the methods of
- the generic function and the actual arguments passed to it. In
- section [[#Generalizer metaobjects]], we go into more detail regarding
- these =generalizer= metaobjects, describing the generic function
- invocation protocol in full, and showing how this protocol allows a
- similar form of effective method cacheing as the standard one does.
- In section [[#Generalizer performance]], we show the results of some
- simple performance measurements to highlight the improvement that
- this protocol can bring over a naïve implementation of generalized
- dispatch, as well as highlighting the potential for further
- improvement.
+ partial implementations of particular non-standard method dispatch
+ strategies, using =generalizer= metaobjects to mediate between the
+ methods of the generic function and the actual arguments passed to
+ it. In section [[#Generalizer metaobjects]], we go into more detail
+ regarding these =generalizer= metaobjects, describing the generic
+ function invocation protocol in full, and showing how this protocol
+ allows a similar form of effective method cacheing as the standard
+ one does. In section [[#Generalizer performance]], we show the results
+ of some simple performance measurements on our implementation of
+ this protocol in the SBCL implementation \cite{Rhodes:2008} of
+ Common Lisp to highlight the improvement that this protocol can
+ bring over a naïve implementation of generalized dispatch, as well
+ as to make the potential for further improvement clear.
** Generalizer metaobjects
:PROPERTIES:
applicability of a particular specializer against a given argument
using =specializer-accepts-p=, a new protocol function with
default implementations on =class= and =eql-specializer= to
- implement the expected behaviour. In order to order the methods,
- as required by the protocol, we define a pairwise comparison
- operator =specializer<= which defines an ordering between
- specializers for a given generalizer argument (remembering that
- even in standard CLOS the ordering between =class= specializers
- can change depending on the actual class of the argument).
+ implement the expected behaviour. To order the methods, as
+ required by the protocol, we define a pairwise comparison operator
+ =specializer<= which defines an ordering between specializers for
+ a given generalizer argument (remembering that even in standard
+ CLOS the ordering between =class= specializers can change
+ depending on the actual class of the argument).
The new =compute-applicable-methods-using-generalizers= is the
analogue of the MOP's =compute-applicable-methods-using-classes=.
required arguments in a list to use as a key in an =equal=
hash-table.
- [XXX could we actually compute a suitable hash key using the
+#+begin_comment
+ [could we actually compute a suitable hash key using the
generalizer's class name and initargs?]
+#+end_comment
+*** COMMENT
- [X] =generalizer-of-using-class= (NB class of gf not class of object)
- [X] =compute-applicable-methods-using-generalizers=
- [X] =generalizer-equal-hash-key=
efficiency that the memoization protocol described in section
[[#Memoization]] achieves, by comparing it both to the same protocol
with no memoization, as well as with equivalent dispatch
- implementations in the context of methods with regular
- specializers, and with implementation in straightforward functions.
+ implementations in the context of methods with regular specializers
+ (in an implementation similar to that in
+ \cite{Kiczales.Rodriguez:1990}), and with implementation in
+ straightforward functions.
In the case of the =cons-specializer=, we benchmark the walker
acting on a small but non-trivial form. The implementation
construction of a list of hash keys and simply use the key
from the single active generalizer directly.
- Refer to \cite{Kiczales.Rodriguez:1990}
-
| implementation | time (µs/call) | overhead |
|-----------------------+----------------+----------|
| function | 3.17 | |
answer fits in SBCL's 63-bit fixnums – in an attempt to measure the
worst case for generic dispatch, where the work done within the
methods is as small as possible without being meaningless, and in
- particular does not cause allocation or garbage collection to
+ particular does not cause heap allocation or garbage collection to
obscure the picture.
#+begin_src lisp :exports none
calls for the generic cache; we discuss possible approaches in
section [[#Conclusions]].
** Full protocol
- Description and specification left for reasons of space (we'll see?)
- - [ ] =same-specializer-p=
- - [ ] =parse/unparse-specializer-using-class=
- - [ ] =make-method-specializers-form=
- - [ ] jmoringe: In an email, I suggested
- =make-specializer-form-using-class=:
-
- #+begin_quote
- Could we change =make-method-specializers-form='s default
- behaviour to call a new generic function
- #+begin_src
- make-specializer-form-using-class gf method name env
- #+end_src
- with builtin methods on =sb-mop:specializer=, =symbol=, =cons= (for
- eql-specializers)? This would make it unnecessary to repeat
- boilerplate along the lines of
- #+begin_src lisp
- (flet ((make-parse-form (name)
- (if <name-is-interesting>
- <handle-interesting-specializer>
- <repeat-handling-of-standard-specializers>)))
- `(list ,@(mapcar #'make-parse-form specializer-names)))
- #+end_src
- for each generic function class.
- #+end_quote
- - [ ] =make-method-lambda= revision (use environment arg?)
-
- jmoringe: would only be relevant for pattern dispatch, right? I
- think, we didn't finish the discussion regarding special
- variables vs. environment vs. new protocol function
+ The protocol described in this paper is only part of a complete
+ protocol for =specializer= and =generalizer= metaobjects. Our
+ development of this protocol is as yet incomplete; the work
+ described here augments that in \cite{Newton.Rhodes:2008}, but is
+ yet relatively untested – and additionally our recent experience of
+ working with that earlier protocol suggests that there might be
+ useful additions to the handling of =specializer= metaobjects,
+ independent of the =generalizer= idea presented here.
+
+*** COMMENT ideas
+ Description and specification left for reasons of space (we'll see?)
+ - [ ] =same-specializer-p=
+ - [ ] =parse/unparse-specializer-using-class=
+ - [ ] =make-method-specializers-form=
+ - [ ] jmoringe: In an email, I suggested
+ =make-specializer-form-using-class=:
+
+ #+begin_quote
+ Could we change =make-method-specializers-form='s default
+ behaviour to call a new generic function
+ #+begin_src
+ make-specializer-form-using-class gf method name env
+ #+end_src
+ with builtin methods on =sb-mop:specializer=, =symbol=, =cons= (for
+ eql-specializers)? This would make it unnecessary to repeat
+ boilerplate along the lines of
+ #+begin_src lisp
+ (flet ((make-parse-form (name)
+ (if <name-is-interesting>
+ <handle-interesting-specializer>
+ <repeat-handling-of-standard-specializers>)))
+ `(list ,@(mapcar #'make-parse-form specializer-names)))
+ #+end_src
+ for each generic function class.
+ #+end_quote
+ - [ ] =make-method-lambda= revision (use environment arg?)
+
+ jmoringe: would only be relevant for pattern dispatch, right? I
+ think, we didn't finish the discussion regarding special
+ variables vs. environment vs. new protocol function
* Related Work
:PROPERTIES:
:CUSTOM_ID: Related Work
:END:
- - [ ] Newton/Rhodes, \cite{Newton.Rhodes:2008}
- - [ ] filtered dispatch -- the point is that our work continues to
- be useful in cases where there are unbounded numbers of
- equivalence classes but each given invokation involves a small
- number of methods. Filtered dispatch works by having a custom
- discriminating function which wraps the usual one, and augments
- the set of applicable methods computed with applicable methods
- from other (hidden) generic functions (one per filter group). It
- then also has a custom method combination to handle combining
- these applicable methods. \cite{Costanza.etal:2008}
- - [ ] ContextL / context-oriented programming -- dispatch occurs on
- hidden layer argument being an instance of an anonymous class with
- suitably arranged superclasses -- OK because set of layers is
- bounded and under programmer control. \cite{Hirschfeld.etal:2008,Vallejos.etal:2010}
- - [ ] http://soft.vub.ac.be/Publications/2010/vub-tr-soft-10-04.pdf
- - [ ] http://soft.vub.ac.be/lambic/files/lambic-ilc09.pdf
- - [ ] http://soft.vub.ac.be/Publications/2011/vub-soft-phd-11-03.pdf
- - [ ] Prototypes with Multiple Dispatch
- http://sauerbraten.org/lee/ecoop.pdf -- extension of Self-style
- object system to handle multiple equally-privileged "receivers".
- A good test case for our protocol; handled adequately with
- generalizer being the tuple of (roles,delegations), with some
- thought needed for method redefinitions but otherwise working
- fine. \cite{Salzman.Aldrich:2005}
- - [ ] Sheeple
- - [ ] Multiple dispatch in Clojure
- http://clojure.org/multimethods -- seems to allow hierarchy-based,
- eql and the equivalent of filtered dispatch
+
+ The work presented here builds on specializer-oriented programming
+ described in \cite{Newton.Rhodes:2008}. Approximately
+ contemporaneously, filtered dispatch \cite{Costanza.etal:2008} was
+ introduced to address some of the same use cases: filtered dispatch
+ works by having a custom discriminating function which wraps the
+ usual one, where the wrapping function augments the set of
+ applicable methods with applicable methods from other (hidden)
+ generic functions, one per filter group; this step is not memoized,
+ and using =eql= methods to capture behaviours of equivalence classes
+ means that it is hard to see how it could be. The methods are then
+ combined using a custom method combination to mimic the standard
+ one; in principle implementors of other method combinations could
+ cater for filtered dispatch, but they would have to explicitly
+ modify their method combinations. The Clojure programming language
+ supports multimethods[fn:5] with a variant of filtered dispatch as
+ well as hierarchical and identity-based method selectors.
+
+ In context-oriented programming
+ \cite{Hirschfeld.etal:2008,Vallejos.etal:2010}, context dispatch
+ occurs by maintaining the context state as an anonymous class with
+ the superclasses representing all the currently active layers; this
+ is then passed as a hidden argument to context-aware functions. The
+ set of layers is known and under programmer control, as layers must
+ be defined beforehand.
+
+ In some sense, all dispatch schemes are specializations of predicate
+ dispatch \cite{Ernst.etal:1998}. The main problem with predicate
+ dispatch is its expressiveness: with arbitrary predicates able to
+ control dispatch, it is essentially impossible to perform any
+ substantial precomputation, or even to automatically determine an
+ ordering of methods given a set of arguments. Even Clojure's
+ restricted dispatch scheme provides an explicit operator for stating
+ a preference order among methods, where here we provide an operator
+ to order specializers; in filtered dispatch the programmer
+ implicitly gives the system an order of precedence, through the
+ lexical ordering of filter specification in a filtered function
+ definition.
+
+ The Slate programming environment combines prototype-oriented
+ programming with multiple dispatch \cite{Salzman.Aldrich:2005}; in
+ that context, the analogue of an argument's class (in Common Lisp)
+ as a representation of the equivalence class of objects with the
+ same behaviour is the tuple of roles and delegations: objects with
+ the same roles and delegations tuple behave the same, much as
+ objects with the same generalizer have the same behaviour in the
+ protocol described in this paper.
+
+ The idea of generalization is of course not new, and arises in other
+ contexts. Perhaps of particular interest is generalization in the
+ context of partial evaluation; for example, \cite{Ruf:1993}
+ considers generalization in online partial evaluation, where sets of
+ possible values are represented by a type system construct
+ representing an upper bound. Exploring the relationship between
+ generalizer metaobjects and approximation in type systems might
+ yield strategies for automatically computing suitable generalizers
+ and cache functions for a variety of forms of generalized dispatch.
* Conclusions
:PROPERTIES:
:CUSTOM_ID: Conclusions
:END:
- - protocol for straightforward definition of custom dispatch
- + interoperates seamlessly with rest of CLOS: method combination,
- etc.
- + tolerably efficient: two extra standard gf invokations and one
- hash table lookup per call on the fast path (but more to be
- done)
- + expressive: handles forms of dispatch not handled elsewhere; all
- the usual niceties of redefinition, modularity, introspection
-** Future Work
+ In this paper, we have presented a new generalizer metaobject
+ protocol allowing the metaprogrammer to implement in a
+ straightforward manner metaobjects to implement custom method
+ selection, rather than the standard method selection as standardized
+ in Common Lisp. This protocol seamlessly interoperates with the
+ rest of CLOS and Common Lisp in general; the programmer (the user of
+ the custom specializer metaobjects) may without constraints use
+ arbitrary method combination, intercede in effective method
+ combination, or write custom method function implementations. The
+ protocol is expressive, in that it handles forms of dispatch not
+ possible in more restricted dispatch systems, while not suffering
+ from the indeterminism present in predicate dispatch through the use
+ of explicit ordering predicates.
+
+ The protocol is also reasonably efficient; the metaprogrammer can
+ indicate that a particular effective method computation can be
+ memoized, and under those circumstances much of the overhead is
+ amortized (though there remains a substantial overhead compared with
+ standard generic-function or regular function calls). We discuss
+ how the efficiency could be improved below.
+** Future work
:PROPERTIES:
:CUSTOM_ID: Future Work
:END:
widespread demand (in as much as any language extension can be said
to be in “demand”). In particular, we have preliminary work
towards supporting efficient dispatch over pattern specializers
- such as implemented in the \textsf{Optima} library[fn:2], and over
+ such as implemented in the \textsf{Optima} library[fn:4], and over
a prototype object system similar to that in Slate
\cite{Salzman.Aldrich:2005}. Our current source code for the work
described in this paper can be seen in the git source code
repository at [[http://christophe.rhodes.io/git/specializable.git]],
which will be updated with future developments.
+
+ Finally, after further experimentation (and, ideally, non-trivial
+ use in production) if this protocol stands up to use as we hope, we
+ aim to produce a standards-quality document so that other
+ implementors of Common Lisp can, if they choose, independently
+ reimplement the protocol, and so that users can use the protocol
+ with confidence that the semantics will not change in a
+ backwards-incompatible fashion.
** Acknowledgments
We thank Lee Salzman, Pascal Costanza and Mikel Evins for helpful
and informative discussions, and all the respondents to one
- author's call for imaginative uses for generalized specializers.
+ author's request for imaginative uses for generalized specializers.
\bibliographystyle{plain}
\bibliography{crhodes,specializers}
* Footnotes
-[fn:1] the \textsf{Closer to MOP} project attempts to harmonize the
- different implementations of the metaobject protocol in Common Lisp.
+[fn:1] GNU CLISP, at http://www.clisp.org/
+
+[fn:2] Clozure Common Lisp, at http://ccl.clozure.com/
+
+[fn:3] the \textsf{Closer to MOP} project, at
+ http://common-lisp.net/project/closer/, attempts to harmonize the
+ different implementations of the metaobject protocol in Common
+ Lisp.
+
+[fn:4] https://github.com/m2ym/optima
-[fn:2] https://github.com/m2ym/optima
+[fn:5] http://clojure.org/multimethods
projects / paper-els-specializers.git / blobdiff