1 #+TITLE: Generalizers: New Metaobjects for Generalized Dispatch
2 #+AUTHOR: Christophe Rhodes, Jan Moringen, David Lichteblau
5 #+LaTeX_HEADER: \usepackage[margin=1in]{geometry}
8 1. This paper introduces a new metaobject, the generalizer, which
9 complements the existing specializer metaobject.
10 2. With the help of examples, we show that this metaobject allows for
11 the efficient implementation of complex non-class-based dispatch
12 within the framework of existing metaobject protocols
13 3. We present the generalizer protocol, implemented within the SBCL
14 implementation of Common Lisp
15 4. In combination with previous work, this produces a fully-functional
16 extension of the existing mechanism for method selection and
17 effective method computation, including support for standard and
18 user-defined method combination independent from method selection.
22 The revisions to the original Common Lisp language \cite{CLtL1}
23 included the detailed specification of an object system, known as
24 the Common Lisp Object System (CLOS), which was eventually
25 standardized as part of the ANSI Common Lisp standard \cite{CLtS}.
26 The object system as presented to the standardization committee was
27 formed of three parts, the first two of which covered XXX [what?]
28 and were incorporated into the final standard, and the third,
29 covering a Metaobject Protocol (MOP) for CLOS, was not.
31 Nevertheless, the CLOS MOP has proven to be a robust design, and
32 while many implementations have derived their implementations of
33 CLOS from either the Closette illustrative implementation in
34 \cite{AMOP}, or the Portable Common Loops implementation of CLOS
35 from Xerox Parc, there have been from-scratch reimplementations of
36 CLOS (in at least CLISP; check for others -- ABCL? Lisp500?!)
37 incorporating the majority of the Metaobject Protocol as described.
39 Although it has stood the test of time, the MOP is neither without issues
40 (e.g. M-M-L considered harmful; slot-definition initargs issue) nor
41 a complete framework for the metaprogrammer to implement all
42 conceivable variations of object-oriented behaviour; indeed, while
43 metaprogramming offers some possibilities for customization of the
44 object system behaviour, those possibilities cannot extend
45 arbitrarily in all directions. There is still an expectation that
46 functionality is implemented with methods on generic functions,
47 acting on objects with slots. [XXX find Paepke picture here? Not
48 Paepke; AMOP?]. XXX include typical examples of MOP: object
49 persistence; maybe ref. Kizcales "MOPs: why we want them and what
50 else they can do"? (Fig. 2 in that is good) ORMs; sparse slots.
52 + introspection, e.g. documentation generation
53 + programmatic construction of classes and generic functions
54 e.g. for IDL compilers, model transformations
56 One area of functionality where there is scope for customization by
57 the metaprogrammer is in the mechanics and semantics of method
58 applicability and dispatch. While in principle AMOP allows
59 customization of dispatch in various different ways (the
60 metaprogrammer can define methods on protocol functions such as
61 =compute-applicable-methods=,
62 =compute-applicable-methods-using-classes=), for example, in
63 practice implementation support for this was weak until relatively
64 recently (ref. closer, also check how ContextL and filtered dispatch
66 jmoringe: filtered dispatch uses a custom method combination, i
69 Another potential mechanism for customizing dispatch is implicit in
70 the class structure defined by AMOP: standard specializer objects
71 (instances of =class= and =eql-specializer=) are generalized
72 instances of the =specializer= protocol class, and in principle
73 there are no restrictions on the metaprogrammer constructing
74 additional subclasses. Previous work [Newton/Rhodes] has explored
75 the potential for customizing generic function dispatch using
76 extended specializers, but as of that work the metaprogrammer must
77 override the entirety of the generic function invocation protocol
78 (from =compute-discriminating-function= on down), leading to toy
79 implementations and duplicated effort.
81 This paper introduces a protocol for efficient and controlled
82 handling of arbitrary subclasses of =specializer=. In particular,
83 it introduces the =generalizer= protocol class, which generalizes
84 (ahem) the return value of =class-of=, and allows the metaprogrammer
85 to hook into cacheing schemes to avoid needless recomputation of
86 effective methods for sufficiently similar generic function
87 arguments (See Figure\nbsp\ref{fig:dispatch}).
89 #+CAPTION: Dispatch Comparison
91 #+ATTR_LATEX: width=0.9\linewidth float
92 [[file:figures/dispatch-comparison.pdf]]
94 The remaining sections in this paper can be read in any order. We
95 give some motivating examples in section XX, including
96 reimplementations of examples from previous work, as well as
97 examples which are poorly supported by previous protocols. We
98 describe the protocol itself in section YY, describing each protocol
99 function in detail and, where applicable, relating it to existing
100 protocol functions within the CLOS MOP. We survey related work in
101 more detail in section ZZ, touching on work on customized dispatch
102 schemes in other environments. Finally, we draw our conclusions
103 from this work, and indicate directions for further development, in
104 section WW; reading that section before the others indicates
105 substantial trust in the authors' work.
107 - [ ] =cons-specializer= (can be done using filtered dispatch)
108 - [ ] factorial (like filtered dispatch)
109 - [ ] HTTP Accept header
111 - [ ] prototype/multimethod
113 NB this example can be done using filtered dispatch, with a filter
114 calling =car= on cons arguments.
116 Note also that there's no real need for =cons-specializer= and
117 =cons-generalizer= to be distinct classes (as with =class= and
118 =class=). Also true for =signum=, below; but more interesting
119 dispatch reveals the need to split.
121 (defclass cons-specializer (specializer)
122 ((%car :reader %car :initarg :car)))
123 (defclass cons-generalizer (generalizer)
124 ((%car :reader %car :initarg :car)))
125 (defmethod generalizer-of-using-class ((gf cons-generic-function) arg)
127 ((cons symbol) (make-instance 'cons-generalizer :car (car arg)))
128 (t (call-next-method))))
129 (defmethod generalizer-equal-hash-key ((gf cons-generic-function)
130 (g cons-generalizer))
132 (defmethod specializer-accepts-generalizer-p ((gf cons-generic-function)
134 (g cons-generalizer))
135 (if (eql (%car s) (%car g))
138 (defmethod specializer-accepts-p ((s cons-specializer) o)
139 (and (consp o) (eql (car o) (%car s))))
141 #| less interesting methods elided: jmoringe: (un)parsing, specializer<?, more? |#
143 #| XXX insert motivating example from Newton/Rhodes here |#
146 NB this example can definitely be done using filtered dispatch.
148 Point out obvious similarity between this and car-of-cons. Note
149 the subtlety, though, in generalizer-of-using-class / signum wrt
150 rational vs floating point arguments
152 (defclass signum-specializer (specializer)
153 ((%signum :reader %signum :initarg :signum)))
154 (defclass signum-generalizer (generalizer)
155 ((%signum :reader %signum :initarg :signum)))
156 (defmethod generalizer-of-using-class ((gf signum-generic-function) arg)
158 (real (make-instance 'signum-generalizer :signum (signum arg)))
159 (t (call-next-method))))
160 (defmethod generalizer-equal-hash-key ((gf signum-generic-function)
161 (g signum-specializer))
162 (%signum g)) ; this will create multiple entries for the same emf, but that's OK
163 (defmethod specializer-accepts-generalizer-p ((gf signum-generic-function)
164 (s signum-specializer)
165 (g signum-generalizer))
166 (if (= (%signum s) (%signum g)) ; or EQL?
170 ;; this method is perhaps interesting enough to talk about?
171 (defmethod specializer-accepts-generalizer-p ((gf signum-generic-function) (specializer sb-mop:specializer) (thing signum-specializer))
172 (specializer-accepts-generalizer-p gf specializer (class-of (%signum thing))))
175 (defmethod specializer-accepts-p ((s signum-specializer) o)
176 (and (realp o) (= (%signum s) (signum o))))
178 #| again elide more boring methods |#
181 (:generic-function-class signum-generic-function))
182 (defmethod fact ((n (signum 0))) 1)
183 (defmethod fact ((n (signum 1))) (* n (fact (1- n))))
184 (defmethod fact ((n (signum -1)))
185 (error "factorial of negative number: ~D" n))
187 ** HTTP Accept header
188 implement RFC2616 content negotiation
190 NB this definitely can't be done with filtered dispatch, except by
191 generating anonymous classes with all the right mime-types as
192 direct superclasses in dispatch order,so the filter does
194 (ensure-class nil :direct-superclasses '(text/html image/webp ...))
196 and dispatch is defined by using those classes. And that's even
197 more awkward than it sounds, because that means that in principle
198 all the mime types in the universe need an existence as classes, to
199 cater for arbitrary mime types in accept headers. And handling
200 wildcards is pretty much impossible, too. See that in
201 =specializer<= which involves a nontrivial ordering of methods
202 (whereas in two above previous cases only a single extended
203 specializer could be applicable to any given argument)
205 Also of interest: note that we can have these
206 specializer/generalizers handle arbitrary objects: the =string= and
207 =tbnl:request= methods are independent of each other; this
208 generalizes to dealing with multiple web server libraries.
210 jmoringe: the name =accept-specializer=, while sensible, may
211 confusing in this context because "accept" occurs as part of the
212 protocol with a different semantic.
215 (defclass accept-specializer (extended-specializer)
216 ((media-type :initarg :media-type :reader media-type)))
217 (defclass accept-generalizer ()
218 ((header :initarg :header :reader header)
220 (next :initarg :next :reader next)))
221 (defmethod generalizer-equal-hash-key
222 ((gf accept-generic-function) (g accept-generalizer))
223 `(accept-generalizer ,(header g)))
224 (defmethod specializer-accepts-generalizer-p ((gf accept-generic-function) (s acc
225 ept-specializer) (generalizer accept-generalizer))
226 (values (q (media-type s) (tree generalizer)) t))
227 (defmethod specializer-accepts-generalizer-p ((gf accept-generic-function) (s sb-
228 mop:specializer) (generalizer accept-generalizer))
229 (specializer-accepts-generalizer-p gf s (next generalizer)))
231 (defmethod specializer< ((gf accept-generic-function) (s1 accept-specializer) (s2
232 accept-specializer) generalizer)
234 ((string= (media-type s1) (media-type s2)) '=)
235 (t (let ((q1 (q (media-type s1) (tree generalizer)))
236 (q2 (q (media-type s2) (tree generalizer))))
242 ;; here are the only methods that actually know about TBNL
243 (defmethod generalizer-of-using-class ((gf accept-generic-function) (arg tbnl:request))
244 (make-instance 'accept-generalizer
245 :header (tbnl:header-in :accept arg)
246 :next (class-of arg)))
247 (defmethod specializer-accepts-p ((specializer accept-specializer) (obj tbnl:requ
249 (q (media-type specializer) (parse-accept-string (tbnl:header-in :accept obj)))
252 ;; we can define methods on STRING too, for debugging/simulation purposes
253 (defmethod generalizer-of-using-class ((gf accept-generic-function) (s string))
254 (make-instance 'accept-generalizer
257 (defmethod specializer-accepts-p ((s accept-specializer) (string string))
258 (q (media-type s) (parse-accept-string string)))
261 jmoringe: The role of =accept-generalizer.tree= and the =q=
262 function are hard to understand and may require some
263 explanation. However, the example with its distinct, asymmetric
264 specializers/generalizers, =accept-generalizer.next= and
265 =specializer<= is likely worth it.
267 ** Pattern / xpattern / regex / optima
268 Here's the /really/ interesting bit, but on the other hand we're
269 probably going to run out of space, and the full description of
270 these is going to take us into =make-method-lambda= territory.
271 A second paper? Future work?
274 - [ ] =generalizer-of-using-class= (NB class of gf not class of object)
275 - [ ] =compute-applicable-methods-using-generalizers=
276 - [ ] =generalizer-equal-hash-key=
277 - [ ] =specializer-accepts-generalizer-p=
278 - [ ] =specializer-accepts-p=
280 jmoringe: If I remember correctly, closette has
281 =method-more-specific-p= should we aim for parity with that and
282 use =specializer-more-specific-p=? The downside would be that
283 =-p= indicates a Boolean return value which is not the case here.
285 Description and specification left for reasons of space (we'll see?)
286 - [ ] =same-specializer-p=
287 - [ ] =parse/unparse-specializer-using-class=
288 - [ ] =make-method-specializers-form=
289 - [ ] jmoringe: In an email, I suggested
290 =make-specializer-form-using-class=:
293 Could we change =make-method-specializers-form='s default
294 behaviour to call a new generic function
296 make-specializer-form-using-class gf method name env
298 with builtin methods on =sb-mop:specializer=, =symbol=, =cons= (for
299 eql-specializers)? This would make it unnecessary to repeat
300 boilerplate along the lines of
302 (flet ((make-parse-form (name)
303 (if <name-is-interesting>
304 <handle-interesting-specializer>
305 <repeat-handling-of-standard-specializers>)))
306 `(list ,@(mapcar #'make-parse-form specializer-names)))
308 for each generic function class.
310 - [ ] =make-method-lambda= revision (use environment arg?)
312 jmoringe: would only be relevant for pattern dispatch, right? I
313 think, we didn't finish the discussion regarding special
314 variables vs. environment vs. new protocol function
317 - [ ] filtered dispatch -- the point is that our work continues to
318 be useful in cases where there are unbounded numbers of
319 equivalence classes but each given invokation involves a small
321 - [ ] ContextL / context-oriented programming -- dispatch occurs on
322 hidden layer argument being an instance of an anonymous class with
323 suitably arranged superclasses -- OK because set of layers is
324 bounded and under programmer control
325 - [ ] http://soft.vub.ac.be/Publications/2010/vub-tr-soft-10-04.pdf
326 - [ ] http://soft.vub.ac.be/lambic/files/lambic-ilc09.pdf
327 - [ ] http://soft.vub.ac.be/Publications/2011/vub-soft-phd-11-03.pdf
328 - [ ] Prototypes with Multiple Dispatch
329 http://sauerbraten.org/lee/ecoop.pdf -- extension of Self-style
330 object system to handle multiple equally-privileged "receivers".
331 A good test case for our protocol; handled adequately with
332 generalizer being the tuple of (roles,delegations), with some
333 thought needed for method redefinitions but otherwise working
338 We thank Lee Salzman, Pascal Costanza, Mikel Evins for their