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

[Gabor Grothendieck]

I think a rule is needed that would require that a class not succeed any of
its parents on the method list.  That way "array" could never come
after "vector" and, in general, a parent would never be found prior
to any of its descendents.

Of course that still does not address the problem that the order
of the setMethod's rather than the relationships among the classes
can influence the search order but it would eliminate the situation
we saw here.

On 4/13/06, John Chambers <jmc at r-project.org> wrote:
> What a delicious example!  (I'm taking the liberty of sharing it with
> r-devel, since it raises some good issues.)
>
> 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 at gmail.com>
> To: Paul Roebuck <roebuck at mdanderson.org> Cc: R Help Mailing List
> <r-help at stat.math.ethz.ch> Subject: Re: [R] S4 method dispatch matrixOrArray
> On 4/12/06, Paul Roebuck <roebuck at mdanderson.org> wrote:
>
>
> On Wed, 12 Apr 2006, Gabor Grothendieck wrote:


> On 4/12/06, Paul Roebuck <roebuck at 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.



>