Oh, I had missed that that code path is now enabled by default. It’s worth noting that the commented-out test in that commit also still succeeds if invoked via `getS3method`. So at the very least there’s now an inconsistency in the lookup performed by R internally (via `UseMethod`) and `getS3method`, which is probably unintentional.
I see how the change is beneficial by preventing surprising behaviour in a corner case. Unfortunately it also breaks at least one published package [1], and if I understand correctly it no longer conforms to the documented behaviour (quoted in my initial message), which even explicitly mentions non-namespace environments. [1] https://github.com/klmr/modules/issues/147 On Wed, Oct 9, 2019 at 11:23 PM Duncan Murdoch <murdoch.dun...@gmail.com> wrote: > On 09/10/2019 3:22 p.m., Konrad Rudolph wrote: > > tl;dr: S3 lookup no longer works in custom non-namespace environments as > of > > R 3.6.1. Is this a bug? > > I don't know whether this was intentional or not, but a binary search > through the svn commits finds that the errors started in this one: > > ------------------------------------------------------------------------ > r75127 | hornik | 2018-08-13 09:58:47 -0400 (Mon, 13 Aug 2018) | 2 lines > Changed paths: > M /trunk/src/main/objects.c > M /trunk/tests/reg-tests-1a.R > > Have S3 methods lookup by default look for the S3 registry in the topenv > of the generic. > ------------------------------------------------------------------------ > > Duncan Murdoch > > > > > I am implementing S3 dispatch for generic methods in environments that > are > > not > > packages. I am trying to emulate the R package namespace mechanism by > > having a > > “namespace” environment that defines generics and methods, but only > exposes > > the > > generics themselves, not the methods. > > > > To make S3 lookup work when using the generics, I am using > > `registerS3method`. > > While this method itself has no extensive documentation, the > documentation > > of > > `UseMethod` contains this relevant passage: > > > >> Namespaces can register methods for generic functions. To support this, > >> ‘UseMethod’ and ‘NextMethod’ search for methods in two places: in the > >> environment in which the generic function is called, and in the > > registration > >> data base for the environment in which the generic is defined > (typically a > >> namespace). So methods for a generic function need to be available in > the > >> environment of the call to the generic, or they must be registered. (It > > does > >> not matter whether they are visible in the environment in which the > > generic is > >> defined.) As from R 3.5.0, the registration data base is searched after > > the > >> top level environment (see ‘topenv’) of the calling environment (but > > before > >> the parents of the top level environment). > > > > This used to work but it stopped working in R 3.6.1 and I cannot figure > out > > (a) > > why, and (b) how to fix it. Unfortunately I am unable to find the > relevant > > information by reading the R source code, even when “diff”ing what seem > to > > be > > the only even remotely relevant changes [1]. > > > > The R NEWS merely list the following change for R 3.6.0: > > > >> * S3method() directives in ‘NAMESPACE’ can now also be used to perform > > delayed > >> S3 method registration. > >> […] > >> * Method dispatch uses more relevant environments when looking up class > >> definitions. > > > > Unfortunately it is not clear to me what exactly this means. > > > > Here’s a minimal example code that works under R 3.5.3 but breaks under > > R 3.6.1 > > (I don’t know about 3.6.0). > > > > ``` > > # Define “package namespace”: > > ns = new.env(parent = .BaseNamespaceEnv) > > local(envir = ns, { > > test = function (x) UseMethod('test') > > test.default = function (x) message('test.default') > > test.foo = function (x) message('test.foo') > > > > .__S3MethodsTable__. = new.env(parent = .BaseNamespaceEnv) > > .__S3MethodsTable__.$test.default = test.default > > .__S3MethodsTable__.$test.foo = test.foo > > > > # Or, equivalently: > > # registerS3method('test', 'default', test.default) > > # registerS3method('test', 'foo', test.foo) > > }) > > > > # Expose generic publicly: > > test = ns$test > > > > # Usage: > > test(1) > > test(structure(1, class = 'foo')) > > ``` > > > > Output in R up to 3.5.3: > > > > ``` > > test.default > > test.foo > > ``` > > > > Output in R 3.6.1: > > > > ``` > > Error in UseMethod("test") : > > no applicable method for 'test' applied to an object of class > > "c('double', 'numeric')" > > ``` > > > > It’s worth noting that the output of `.S3methods` is the same for all R > > versions, and from my understanding of its output, this *should* indicate > > that > > S3 lookup should behave identically, too. Furthermore, lookup via > > `getS3method` > > succeeds in all R versions, and (again, in my understanding) the logic of > > this > > function should be identical to the logic of R’s internal S3 dispatch: > > > > ``` > > getS3method('test', 'default')(1) > > getS3method('test', 'foo')(1) > > ``` > > > > Conversely, specialising an existing generic from a loaded package works. > > E.g.: > > > > ``` > > local(envir = ns, { > > print.foo = function (x) message('print.foo') > > registerS3method('print', 'foo', print.foo) > > }) > > > > print(structure(1, class = 'foo')) > > ``` > > > > This prints “print.foo” in all R versions as expected. > > > > So my question is: Why do the `test(…)` calls in R 3.6.1 no longer > trigger > > S3 > > method lookup in the generic function’s environment? Is this behaviour by > > design > > or is it a bug? If it’s by design, why does `getS3method` still use the > old > > behaviour? And, most importantly, how can I fix my definition of `ns` to > > make > > S3 dispatch for non-exposed methods work again? > > > > … actually I just found a workaround: > > > > ``` > > ns$.packageName = 'not important' > > ``` > > > > This marks `ns` as a package namespace. To me, the documentation seems to > > imply > > that this shouldn’t be necessary (and it previously wasn’t). Furthermore, > > the > > code for `registerS3method` explicitly supports non-package namespace > > environments. Unfortunately this workaround is not satisfactory because > > pretending that the environment is a package namespace, when it really > > isn’t, > > might break other things. > > > > [1] See r75273; there’s also r74625, which changes the actual lookup > > mechanism > > used by `UseMethod`, but that seems even less relevant, because it > is > > disabled unless a specific environment variable is set. > > > > -- Konrad Rudolph [[alternative HTML version deleted]] ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
