X-Git-Url: http://christophe.rhodes.io/gitweb/?p=paper-els-specializers.git;a=blobdiff_plain;f=els-specializers.org;h=5b5731c4fda94d428d46019fc4c0de539a229f80;hp=d7e129df5a32090cae7ee9c98e8ac209ad957948;hb=d42453fadbac3bab620fa079b894b143a8c24946;hpb=fc053e7df033d7fb70f389579105326e2f13c9a0 diff --git a/els-specializers.org b/els-specializers.org index d7e129d..5b5731c 100644 --- a/els-specializers.org +++ b/els-specializers.org @@ -2,7 +2,18 @@ #+AUTHOR: Christophe Rhodes, Jan Moringen, David Lichteblau #+OPTIONS: toc:nil -#+LaTeX_HEADER: \usepackage[margin=1in]{geometry} +#+LaTeX_CLASS: acm_proc_article-sp +#+LaTeX_HEADER: \DeclareTextFontCommand{\texttt}{\ttfamily\hyphenchar\font=45\relax} + +#+begin_src elisp :exports none +(add-to-list 'org-latex-classes + '("acm_proc_article-sp" "\\documentclass{acm_proc_article-sp}" + ("\\section{%s}" . "\\section*{%s}") + ("\\subsection{%s}" . "\\subsection*{%s}") + ("\\subsubsection{%s}" . "\\subsubsection*{%s}") + ("\\paragraph{%s}" . "\\paragraph*{%s}") + ("\\subparagraph{%s}" . "\\subparagraph*{%s}"))) +#+end_src #+begin_abstract 1. This paper introduces a new metaobject, the generalizer, which @@ -19,39 +30,50 @@ #+end_abstract * Introduction - The revisions to the original Common Lisp language \cite{CLtL1} + The revisions to the original Common Lisp language \cite{CLtL} included the detailed specification of an object system, known as the Common Lisp Object System (CLOS), which was eventually standardized as part of the ANSI Common Lisp standard \cite{CLtS}. The object system as presented to the standardization committee was - formed of three parts, the first two of which covered XXX [what?] - and were incorporated into the final standard, and the third, - covering a Metaobject Protocol (MOP) for CLOS, was not. + formed of three chapters. The first two chapters covered programmer + interface concepts and the functions in the programmer interface + \cite[Chapter 28]{CLtL2} and were largely incorporated into the + final standard; the third chapter, covering a Metaobject Protocol + (MOP) for CLOS, was not. Nevertheless, the CLOS MOP has proven to be a robust design, and 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 -- ABCL? Lisp500?!) - incorporating the majority of the Metaobject Protocol as described. - - Although it has stood the test of time, the MOP is neither without issues - (e.g. M-M-L considered harmful; slot-definition initargs issue) nor - a complete framework for the metaprogrammer to implement all - conceivable variations of object-oriented behaviour; indeed, 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 + CLOS (in at least CLISP; check for others -- Lisp500? CCL?) + incorporating substantial fractions of the Metaobject Protocol as + described. + + 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 + =compute-effective-slot-definition-initargs= being missing from the + standard) nor is it a complete framework for the metaprogrammer to + 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. [XXX find Paepke picture here? Not - Paepke; AMOP?]. XXX include typical examples of MOP: object - persistence; maybe ref. Kizcales "MOPs: why we want them and what - else they can do"? (Fig. 2 in that is good) ORMs; sparse slots. - jmoringe: - + introspection, e.g. documentation generation - + programmatic construction of classes and generic functions - e.g. for IDL compilers, model transformations + 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 ] One area of functionality where there is scope for customization by the metaprogrammer is in the mechanics and semantics of method @@ -61,30 +83,27 @@ =compute-applicable-methods=, =compute-applicable-methods-using-classes=), for example, in practice implementation support for this was weak until relatively - recently (ref. closer, also check how ContextL and filtered dispatch - are implemented). - jmoringe: filtered dispatch uses a custom method combination, i - think + recently[fn:1]. Another potential mechanism for customizing dispatch is implicit in the class structure defined by AMOP: standard specializer objects (instances of =class= and =eql-specializer=) are generalized instances of the =specializer= protocol class, and in principle there are no restrictions on the metaprogrammer constructing - additional subclasses. Previous work [Newton/Rhodes] 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. + 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. This paper introduces a protocol for efficient and controlled - handling of arbitrary subclasses of =specializer=. In particular, - it introduces the =generalizer= protocol class, which generalizes - (ahem) the return value of =class-of=, and allows the metaprogrammer - to hook into cacheing schemes to avoid needless recomputation of - effective methods for sufficiently similar generic function - arguments (See Figure\nbsp\ref{fig:dispatch}). + handling of new subclasses of =specializer=. In particular, it + introduces the =generalizer= protocol class, which generalizes the + return value of =class-of= in method applicability computation, and + allows the metaprogrammer to hook into cacheing schemes to avoid + needless recomputation of effective methods for sufficiently similar + generic function arguments (See Figure\nbsp\ref{fig:dispatch}). #+CAPTION: Dispatch Comparison #+LABEL: fig:dispatch @@ -92,57 +111,145 @@ [[file:figures/dispatch-comparison.pdf]] The remaining sections in this paper can be read in any order. We - give some motivating examples in section XX, including + give some motivating examples in section [[#Examples]], including reimplementations of examples from previous work, as well as examples which are poorly supported by previous protocols. We - describe the protocol itself in section YY, describing each protocol - function in detail and, where applicable, relating it to existing - protocol functions within the CLOS MOP. We survey related work in - more detail in section ZZ, touching on work on customized dispatch - schemes in other environments. Finally, we draw our conclusions - from this work, and indicate directions for further development, in - section WW; reading that section before the others indicates - substantial trust in the authors' work. + describe the protocol itself in section [[#Protocol]], describing each + protocol function in detail and, where applicable, relating it to + existing protocol functions within the CLOS MOP. We survey related + work in more detail in section [[#Related Work]], touching on work on + customized dispatch schemes in other environments. Finally, we draw + our conclusions from this work, and indicate directions for further + development, in section [[#Conclusions]]; reading that section before the + others indicates substantial trust in the authors' work. * Examples - - [ ] =cons-specializer= (can be done using filtered dispatch) - - [ ] factorial (like filtered dispatch) - - [ ] HTTP Accept header - - [ ] xpattern - - [ ] prototype/multimethod -** car-of-cons - NB this example can be done using filtered dispatch, with a filter - calling =car= on cons arguments. - - Note also that there's no real need for =cons-specializer= and - =cons-generalizer= to be distinct classes (as with =class= and - =class=). Also true for =signum=, below; but more interesting - dispatch reveals the need to split. + :PROPERTIES: + :CUSTOM_ID: Examples + :END: + In this section, we present a number of examples of dispatch + 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. + + A note on terminology: we will attempt to distinguish between the + user of an individual case of generalized dispatch (the + “programmer”), the implementor of a particular case of generalized + dispatch (the “metaprogrammer”), and the authors as the designers + and implementors of our generalized dispatch protocol (the + “metametaprogammer”, or more likely “we”). +** CONS specializers + :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. + + 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. + #+begin_src lisp (defclass cons-specializer (specializer) ((%car :reader %car :initarg :car))) (defclass cons-generalizer (generalizer) ((%car :reader %car :initarg :car))) -(defmethod generalizer-of-using-class ((gf cons-generic-function) arg) +(defmethod generalizer-of-using-class + ((gf cons-generic-function) arg) (typecase arg - ((cons symbol) (make-instance 'cons-generalizer :car (car arg))) + ((cons symbol) + (make-instance 'cons-generalizer + :car (car arg))) (t (call-next-method)))) -(defmethod generalizer-equal-hash-key ((gf cons-generic-function) - (g cons-generalizer)) +(defmethod generalizer-equal-hash-key + ((gf cons-generic-function) + (g cons-generalizer)) (%car g)) -(defmethod specializer-accepts-generalizer-p ((gf cons-generic-function) - (s cons-specializer) - (g cons-generalizer)) +(defmethod specializer-accepts-generalizer-p + ((gf cons-generic-function) + (s cons-specializer) + (g cons-generalizer)) (if (eql (%car s) (%car g)) (values t t) (values nil t))) -(defmethod specializer-accepts-p ((s cons-specializer) o) +(defmethod specializer-accepts-p + ((s cons-specializer) o) (and (consp o) (eql (car o) (%car s)))) +#+end_src -#| less interesting methods elided: jmoringe: (un)parsing, specializer) - (t '<)))))) - -;; here are the only methods that actually know about TBNL -(defmethod generalizer-of-using-class ((gf accept-generic-function) (arg tbnl:request)) +(defmethod specializer-accepts-generalizer-p + ((gf accept-generic-function) + (s specializer) + (generalizer accept-generalizer)) + (specializer-accepts-generalizer-p + gf s (next generalizer))) + +(defmethod specializer< + ((gf accept-generic-function) + (s1 accept-specializer) + (s2 accept-specializer) + (generalizer accept-generalizer)) + (let ((m1 (media-type s1)) + (m2 (media-type s2)) + (tree (tree generalizer))) + (cond + ((string= m1 m2) '=) + (t (let ((q1 (q m1 tree))) + (q2 (q m2 tree)))) + (cond + ((= q1 q2) '=) + ((< q1 q2) '>) + (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). + +#+begin_src +(defmethod generalizer-of-using-class + ((gf accept-generic-function) + (arg tbnl:request)) (make-instance 'accept-generalizer :header (tbnl:header-in :accept arg) :next (class-of arg))) -(defmethod specializer-accepts-p ((specializer accept-specializer) (obj tbnl:requ -est)) - (q (media-type specializer) (parse-accept-string (tbnl:header-in :accept obj))) -) +(defmethod specializer-accepts-p + ((specializer accept-specializer) + (obj tbnl:request)) + (let* ((accept (tbnl:header-in :accept obj)) + (tree (parse-accept-string accept)) + (q (q (media-type specializer) tree))) + (and q (> q 0)))) +#+end_src + + This dispatch cannot be implemented using filtered dispatch, except + by generating anonymous classes with all the right mime-types as + direct superclasses in dispatch order; the filter would generate +#+begin_src lisp +(ensure-class nil :direct-superclasses + '(text/html image/webp ...)) +#+end_src + and dispatch the operates 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 + they accept, rather than merely the set of mime-types that a + particular generic function is capable of serving; handling + wildcards in accept strings is particularly awkward in the + 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 + single extended specializer could be applicable to any given + argument). + + Also note that the accept specializer protocol is straightforwardly + extensible to other suitable objects; for example, one simple + debugging aid is to define that an =accept-specializer= should be + applicable to =string= objects. This can be done in a modular + fashion (see the code below, which can be completely disconnected + from the code for Hunchentoot request objects), and generalizes to + dealing with multiple web server libraries, so that + content-negotiation methods are applicable to each web server's + request objects. -;; we can define methods on STRING too, for debugging/simulation purposes -(defmethod generalizer-of-using-class ((gf accept-generic-function) (s string)) +#+begin_src lisp +(defmethod generalizer-of-using-class + ((gf accept-generic-function) + (s string)) (make-instance 'accept-generalizer :header s :next (class-of s))) -(defmethod specializer-accepts-p ((s accept-specializer) (string string)) - (q (media-type s) (parse-accept-string string))) +(defmethod specializer-accepts-p + ((s accept-specializer) (string string)) + (let* ((tree (parse-accept-string string)) + (q (q (media-type s) tree))) + (and q (> q 0)))) #+end_src - - jmoringe: The role of =accept-generalizer.tree= and the =q= - function are hard to understand and may require some - explanation. However, the example with its distinct, asymmetric - specializers/generalizers, =accept-generalizer.next= and - =specializer<= is likely worth it. - ** 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. A second paper? Future work? * Protocol -** Generalizer - - [ ] =generalizer-of-using-class= (NB class of gf not class of object) - - [ ] =compute-applicable-methods-using-generalizers= - - [ ] =generalizer-equal-hash-key= - - [ ] =specializer-accepts-generalizer-p= - - [ ] =specializer-accepts-p= - - [ ] =specializer<= - jmoringe: If I remember correctly, closette has - =method-more-specific-p= should we aim for parity with that and - use =specializer-more-specific-p=? The downside would be that - =-p= indicates a Boolean return value which is not the case here. + :PROPERTIES: + :CUSTOM_ID: Protocol + :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. + +** Generalizer metaobjects + :PROPERTIES: + :CUSTOM_ID: Generalizer metaobjects + :END: + +*** Generic function invocation + As in the standard generic function invocation protocol, the + generic function's actual functionality is provided by a + discriminating function. The functionality described in this + protocol is implemented by having a distinct subclass of + =standard-generic-function=, and a method on + =compute-discriminating-function= which produces a custom + discriminating function. The basic outline of the discriminating + function is the same as the standard one: it must first compute the + set of applicable methods given particular arguments; from that, it + must compute the effective method by combining the methods + appropriately according to the generic function's method + combination; finally, it must call the effective method with the + arguments. + + Computing the set of applicable methods is done using a pair of + functions: =compute-applicable-methods=, the standard metaobject + function, and a new function + =compute-applicable-methods-using-generalizers=. We define a + custom method on =compute-applicable-methods= which tests the + 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). + + The new =compute-applicable-methods-using-generalizers= is the + analogue of the MOP's =compute-applicable-methods-using-classes=. + Instead of calling it with the =class-of= each argument, we compute + the generalizers of each argument using the new function + =generalizer-of-using-class= (where the =-using-class= refers to + the class of the generic function rather than the class of the + object), and call it with the list of generalizers. As with the + standard function, a secondary return value indicates whether the + result of the function is definitive for that list of generalizers. + + Thus, in generic function invocation, we first compute the + generalizers of the arguments; we compute the ordered set of + applicable methods, either from the generalizers or (if that is + not definitive) from the arguments themselves; then the normal + effective method computation and call can occur. Unfortunately, + the nature of an effective method object is not specified, so we + have to reach into implementation internals a little in order to + call it, but otherwise the remainder of the generic function + invocation protocol is unchanged from the standard one. In + particular, method combination is completely unchanged; + programmers can choose arbitrary method combinations, including + user-defined long form combinations, for their generic functions + involving generalized dispatch. + +*** Effective method memoization + :PROPERTIES: + :CUSTOM_ID: Memoization + :END: + The potential efficiency benefit to having =generalizer= + metaobjects lies in the use of + =compute-applicable-methods-using-generalizers=. If a particular + generalized specializer accepts a variety of objects (such as the + =signum= specializer accepting all reals with a given sign, or the + =accept= specializer accepting all HTTP requests with a particular + =Accept= header), then there is the possibility of cacheing and + reusing the results of the applicable and effective method + computation. If the computation of the applicable method from + =compute-applicable-methods-using-generalizers= is definitive, + then the ordered set of applicable methods and the effective + method can be cached. + + One issue is what to use as the key for that cache. We cannot use + the generalizers themselves, as two generalizers that should be + considered equal for cache lookup will not compare as =equal= – + and indeed even the standard generalizer, the =class=, cannot be + used as we must be able to invalidate cache entries upon class + redefinition. The issue of =class= generalizers we can solve as + in \cite{Kiczales.Rodriguez:1990} by using the =wrapper= of a + class, which is distinct for each distinct (re)definition of a + class; for arbitrary generalizers, however, there is /a priori/ no + good way of computing a suitable hash key automatically, so we + allow the metaprogrammer to specify one by defining a method on + =generalizer-equal-hash-key=, and combining the hash keys for all + 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 + generalizer's class name and initargs?] + + - [X] =generalizer-of-using-class= (NB class of gf not class of object) + - [X] =compute-applicable-methods-using-generalizers= + - [X] =generalizer-equal-hash-key= + - [X] =specializer-accepts-generalizer-p= + - [X] =specializer-accepts-p= + - [X] =specializer<= +** Performance + :PROPERTIES: + :CUSTOM_ID: Generalizer performance + :END: + We have argued that the protocol presented here allows for + expressive control of method dispatch while preserving the + possibility of efficiency. In this section, we quantify the + 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. + + In the case of the =cons-specializer=, we benchmark the walker + acting on a small but non-trivial form. The implementation + strategies in the table below refer to: an implementation in a + single function with a large =typecase= to dispatch between all the + cases; the natural implementation in terms of a standard generic + function with multiple methods (the method on =cons= having a + slightly reduced =typecase= to dispatch on the first element, and + other methods handling =symbol= and other atoms); and three + separate cases using =cons-specializer= objects. As well as + measuring the effect of memoization against the full invocation + protocol, we can also introduce a special case: when only one + argument participates in method selection (all the other required + arguments only being specialized on =t=), we can avoid the + 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 | | + | standard-gf/methods | 3.6 | +14% | + | cons-gf/one-arg-cache | 7.4 | +130% | + | cons-gf | 15 | +370% | + | cons-gf/no-cache | 90 | +2700% | + + The benchmarking results from this exercise are promising: in + particular, the introduction of the effective method cache speeds + up the use of generic specializers in this case by a factor of 6, + and the one-argument special case by another factor of 2. For this + workload, even the one-argument special case only gets to within a + factor of 2-3 of the function and standard generic function + implementations, but the overall picture is that the memoizability + in the protocol does indeed drastically reduce the overhead + compared with the full invocation. + + For the =signum-specializer= case, we choose to benchmark the + computation of 20!, because that is the largest factorial whose + 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 + obscure the picture. + +#+begin_src lisp :exports none +(progn (gc :full t) (time (dotimes (i 10000) (%fact 20)))) +#+end_src + + | implementation | time (µs/call) | overhead | + |-------------------------+----------------+----------| + | function | 0.6 | | + | standard-gf/fixnum | 1.2 | +100% | + | signum-gf/one-arg-cache | 7.5 | +1100% | + | signum-gf | 23 | +3800% | + | signum-gf/no-cache | 240 | +41000% | + + The relative picture is similar to the =cons-specializer= case; + including a cache saves a factor of 10 in this case, and another + factor of 3 for the one-argument cache special case. The cost of + the genericity of the protocol here is starker; even the + one-argument cache is a factor of 6 slower than the standard + generic-function implementation, and a further factor of 2 away + from the implementation of factorial as a function. We discuss + ways in which we expect to be able to improve performance in + section [[#Future Work]]. + + We could allow the metaprogrammer to improve on the one-argument + performance by constructing a specialized cache: for =signum= + arguments of =rational= arguments, the logical cache structure is + to index a three-element vector with =(1+ signum)=. The current + protocol does not provide a way of eliding the two generic function + 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= @@ -356,16 +721,25 @@ est)) 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 - - [ ] Newton/Rhodes + :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. + 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 + 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 @@ -375,9 +749,79 @@ est)) 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. + 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 * 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 + :PROPERTIES: + :CUSTOM_ID: Future Work + :END: + Although the protocol described in this paper allows for a more + efficient implementation, as described in section [[#Memoization]], + than computing the applicable and effective methods at each generic + function call, the efficiency is still some way away from a + baseline of the standard generic-function, let alone a standard + function. Most of the invocation protocol is memoized, but there + are still two full standard generic-function calls – + =generalizer-of-using-class= and =generalizer-equal-hash-key= – per + argument per call to a generic function with extended specializers, + not to mention a hash table lookup. + + For many applications, the additional flexibility afforded by + generalized specializers might be worth the cost in efficiency, but + it would still be worth investigating how much the overhead from + generalized specializers can be reduced; one possible avenue for + investigation is giving greater control over the cacheing strategy + to the metaprogrammer. + + As an example, consider the =signum-specializer=. The natural + cache structure for a single argument generic function specializing + on =signum= is probably a four-element vector, where the first + three elements hold the effective methods for =signum= values of + -1, 0, and 1, and the fourth holds the cached effective methods for + everything else. This would make the invocation of such functions + very fast for the (presumed) common case where the argument is in + fact a real number. We hope to develop and show the effectiveness + of an appropriate protocol to allow the metaprogrammer to construct + and exploit such cacheing strategies, and (more speculatively) to + implement the lookup of an effective method function in other ways. + + We also aim to demonstrate support within this protocol for some + particular cases of generalized specializers which seem to have + 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 + 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. ** Acknowledgments - We thank Lee Salzman, Pascal Costanza, Mikel Evins for their - helpful discussions + 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. + +\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:2] https://github.com/m2ym/optima