Re: [Rd] [R] S4 method dispatch matrixOrArray (fwd)

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Sat 15 Apr 2006 - 14:28:25 GMT

You could just place the array/matrix code into the vector method rather than using the inheritance mechanism. Note that if m is a matrix then is(m, "array") is TRUE. This avoids creating an ugly dummy class, fighting with the inheritance mechanism, having fragile code, etc. Also its shorter.

setGeneric("foo",

         function(A, ...) {
             cat("generic", match.call()[[1]], "\n")
             standardGeneric("foo")
         })

setMethod("foo",
        signature(A = "vector"),
        function(A, ...) {
             if (!is(A, "array")) A <- matrix(A, 1)
             cat("A =", A, "\n")
        })

## Test

foo(1:4)
foo(matrix(1:4, 1, 4))
foo(array(1:4, c(1, 4, 1)))

On 4/15/06, Paul Roebuck <roebuck@mdanderson.org> wrote:
> Thanks for your in-depth explanation. I had noticed the difference
> in order in showMethods() output but was unsure whether that was
> indicative of the problem or if I was somehow taking advantage of
> an undocumented implementation-specific detail.
>
> If I could, I'd like to go back to the original question from
> the R-help post here. The original method was doing its real
> dispatching on the arguments of the example that were represented
> by the dots. The meat of the method was to manipulate array
> objects and as such I didn't want to repeat numerous array methods
> as matrix methods (code duplication with the only difference being
> the first argument). From another recent posting on a similar topic,
> I wanted to do something along the lines of
>
> setClassUnion("matrixOrArray", c("matrix", "array"))
>
> but that didn't work as originally presented (probably due to
> your comment on matrix/array weirdness). So my other question
> would be:
>
> 3 - Is there a way to use the class union such that I don't have
> to duplicate the code unnecessarily yet avoid the setMethod
> ordering issue and clarify the original intention?
>
>
>
> On Thu, 13 Apr 2006, John Chambers wrote:
>
> > You have two questions, presumably:
> >
> > 1 - how could the order of the setMethod calls make a difference in the
> > results?
> > 2 - what's causing the infinite loop & how could it be avoided, reliably?
> >
> > Second question first. The danger sign is the "vector" method:
> >
> > setMethod("foo",
> > signature(A = "vector"),
> > function(A, ...) {
> > foo(matrix(A, nrow = 1), ...)
> > })
> >
> > This handles a vector by recalling the generic with a matrix. But
> > "vector" is a superclass of "matrix" (see getClass("matrix")) and you
> > DON'T have an explicit method defined for "matrix". So for sure if
> > "vector" was the only method, you would be in immediate trouble.
> > Coercing down the inheritance tree is potentially bad news.
> >
> > Generally, if you're going to take over dispatch by coercing an argument
> > & then recalling the generic, the result is simplest if you have an
> > exact match for the new call, not relying on inheritance. After all, if
> > you're doing the work to coerce the argument, might as well pick one
> > that works right away. Not a universal rule, but other things being
> > equal ...
> >
> > There's an additional issue with methods for "matrix" and "array"
> > because R never allows 2-way arrays to have class "array", which means
> > using array() here with the same dimensions would not have helped.
> > Also, matrix and array objects are weird in that they are not basic
> > vectors but also have no class attribute, and is.object() is FALSE for them.
> >
> > More interesting though--how can the order of the setMethod() calls
> > matter? To see that we need to look at the methods list object. (The
> > discussion is simplified to the case that only one argument is involved,
> > which doesn't affect the result.)
> >
> > The MethodsList object has a slot containing a named list of methods,
> > with the names being those of the classes that appeared in the setMethod
> > calls, in the order that the calls occurred(note!). All classes are
> > essentially equal from the view of the generic function, so there's no
> > ordering favoring the "more relevant".
> >
> > When method dispatch occurs, the code first looks for an exact match to
> > the class of the actual argument--that's a quick search in the names of
> > the list.
> >
> > If the direct search fails, the code now looks for an inherited method.
> > The key point is that this second search is "greedy"--the first
> > inherited method found is used.
> >
> > NOW it should be clear why the order of the setMethod() calls matters.
> > You have two potential inherited methods here for "matrix"; namely,
> > "array" and "vector". WE know that the "array" method is `closer', and
> > the R dispatcher could decide this also, if it were willing to look
> > through all possible inheritance paths and drop one possibility if a
> > better one was found.
> >
> > It currently doesn't do any further search, and doing so would be a
> > modest efficiency hit. I'm inclined to think the cost would be worth it
> > to eliminate unpleasant suprises like this one, but opinions may
> > differ. (Once found, the inherited method is stored directly in the
> > list used for the first lookup, so the hit is only the first time a
> > particular signature turns up.)
> >
> > To make the difference clearer, I added to your example another generic
> > "bar" with the same methods as "foo", but with the order of the
> > setMethod calls reversed.
> > By looking at the "methods" slot in the two cases, we can see why the
> > bad ("vector") method is selected for bar() but not for foo():
> >
> > > names(getMethods("foo")@methods)
> > [1] "array" "vector"
> > > names(getMethods("bar")@methods)
> > [1] "vector" "array"
> >
> > After running foo(1:10) and trying to run bar(1:10):
> >
> > > showMethods("foo")
> >
> > Function "foo":
> > A = "array"
> > A = "vector"
> > A = "integer"
> > (inherited from A = "vector")
> > A = "matrix"
> > (inherited from A = "array")
> > > showMethods("bar")
> >
> > Function "bar":
> > A = "vector"
> > A = "array"
> > A = "integer"
> > (inherited from A = "vector")
> > A = "matrix"
> > (inherited from A = "vector")
> >
> > But including setMethod("bar", "matrix", ...) in the source code makes
> > bar() work fine.
> >
> >
> >
> > Paul Roebuck wrote:
> >
> > >Sorry to bother but could you shed some light on this?
> > >I don't understand why order of setMethod calls makes
> > >any difference. Since it obviously does, it has shaken
> > >the foundations of what I thought I understood about
> > >S4 methods. Even Gabor was surprised...
> > >
> > >
> > >---------- Forwarded message ----------
> > >Date: Wed, 12 Apr 2006 18:24:46 -0400
> > >From: Gabor Grothendieck <ggrothendieck@gmail.com>
> > >To: Paul Roebuck <roebuck@mdanderson.org>
> > >Cc: R Help Mailing List <r-help@stat.math.ethz.ch>
> > >Subject: Re: [R] S4 method dispatch matrixOrArray
> > >
> > >On 4/12/06, Paul Roebuck <roebuck@mdanderson.org> wrote:
> > >
> > >
> > >
> > >>On Wed, 12 Apr 2006, Gabor Grothendieck wrote:
> > >>
> > >>
> > >>
> > >>>On 4/12/06, Paul Roebuck <roebuck@mdanderson.org> wrote:
> > >>>
> > >>>
> > >>>
> > >>>>I have some code where the primary dispatching is on
> > >>>>other parameters so I'd like not to have to create a
> > >>>>set of functions for "matrix" and another duplicate
> > >>>>set for "array". But the class union technique isn't
> > >>>>working as implemented below and I don't have my Green
> > >>>>book with me. How do I fix my infinite recursion problem?
> > >>>>
> > >>>>
> > >>>>##--------------------------------------------------------
> > >>>>library(methods)
> > >>>>
> > >>>>setGeneric("foo",
> > >>>> function(A, ...) {
> > >>>> cat("generic", match.call()[[1]], "\n")
> > >>>> standardGeneric("foo")
> > >>>> })
> > >>>>
> > >>>>setMethod("foo",
> > >>>> signature(A = "vector"),
> > >>>> function(A, ...) {
> > >>>> callGeneric(matrix(A, nrow = 1), ...)
> > >>>> })
> > >>>>
> > >>>>setClassUnion("matrixOrArray", c("matrix", "array"))
> > >>>>
> > >>>>setMethod("foo",
> > >>>> signature(A = "matrixOrArray"),
> > >>>> function(A, ...) {
> > >>>> cat("A =", A, "\n")
> > >>>> })
> > >>>>
> > >>>>## Test
> > >>>>foo(1:4)
> > >>>>foo(matrix(1:4, 1, 4))
> > >>>>foo(array(1:4, c(1, 4, 1)))
> > >>>>
> > >>>>
> > >>>I think its good enough to just define an array method, i.e. you
> > >>>don't need the matrix method or the matrixOrArray class, and the
> > >>>vector method can call foo(matrix(A,1), ...) so:
> > >>>
> > >>>setGeneric("foo",
> > >>> function(A, ...) {
> > >>> cat("generic", match.call()[[1]], "\n")
> > >>> standardGeneric("foo")
> > >>> })
> > >>>
> > >>>setMethod("foo",
> > >>> signature(A = "array"),
> > >>> function(A, ...) {
> > >>> cat("A =", A, "\n")
> > >>> })
> > >>>
> > >>>setMethod("foo",
> > >>> signature(A = "vector"),
> > >>> function(A, ...) {
> > >>> foo(matrix(A, nrow = 1), ...)
> > >>> })
> > >>>
> > >>>
> > >>Something didn't seem right here. That was pretty close
> > >>to what I had started with, before trying to go the
> > >>classUnion route. Matter of fact, the vector method can
> > >>retain use of callGeneric.
> > >>
> > >>The solution has to do with the order in which calls to
> > >>setMethod are made. Adding foo-vector after foo-array
> > >>works fine; the other way around causes infinite recursion.
> > >>
> > >>
> > >
> > >This is surprising. I would have thought that the
> > >parent/child relationships determine the order that
> > >dispatched methods are invoked, not the order that
> > >the setMethod commands are issued in.
> > >
> > >
> > >
> > >
> >
>
> ----------------------------------------------------------
> SIGSIG -- signature too long (core dumped)
>
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https://stat.ethz.ch/mailman/listinfo/r-devel Received on Sun Apr 16 00:58:44 2006

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