How can I define an S3 method for an unevaluated assignment expression?

tl;dr: R CMD check complains when I implement a generic for the S3 class <-, because it thinks that the function is a replacement function with incorrect arguments.

I need to define a set of S3 generics to walk an AST of unevaluated R expressions.

For demonstration purposes, consider the following S3 generic and its methods:

walk = function (x) UseMethod('walk')

walk.default = function (x) message('default')

walk.name = function (x) message('name')

walk.call = function (x) message('call')

This works fine:

tests = alist('2', c, f(1))

invisible(lapply(tests, walk))

default
name
call

However, there are quite a few call expressions whose S3 class isn’t call; for example:

tests2 = alist(for (x in y) ., if (.) ., x <- .)
invisible(lapply(tests2, walk))

default
default
default

… oops. What I want for these expressions to be treated as calls. I can do this by adding more methods:

walk.for = function (x) message('for')

walk.if = function (x) message('if')

`walk.<-` = function (x) message('<-')

# … and so on for other syntax constructs.

And now I’m getting the expected result when calling walk:

for
if
<-

However, this code is part of a package, and R CMD check complains about the definition of walk.<- because it thinks that the function is a replacement function:

W  checking replacement functions ...
    ‘walk.<-’
  The argument of a replacement function which corresponds to the right
  hand side must be named ‘value’.

And I understand why I’m getting the warning (indeed, this looks like a botched attempt at defining a replacement function for walk.). But of course this isn’t a replacement function, so the warning is a false positive. So how am I supposed to implement an S3 generic for the class <- unambiguously? Or is this a bug in R CMD check? Can I get rid of the warning? I can’t add additional arguments to the functions.

Writing R Extensions, section 1.5.2 Registering S3 methods provides a workaround solution:

It is possible to specify a third argument to S3method, the function to be used as the method, for example

S3method(print, check_so_symbols, .print.via.format)

when print.check_so_symbols is not needed.

This means that, instead of defining `walk.<-`, we do the following:

Implementation in R/‹some-file›.r:

walk_assign = function (x) message('<-')

(Unlike for other S3 methods, the function name is unimportant.)

NAMESPACE declaration:

S3method(walk, '<-', walk_assign)

This effectively does the same as calling .S3method('walk', '<-', \(x) message('<-')) inside a regular R script. Interestingly, .S3method() also seems to work inside a package (instead of the above NAMESPACE-based solution), but the function’s documentation says not to use it in packages so this might break in the future.

---

Of course an arguably simpler solution, as commented by Allan Cameron, is to fuse the <- case with default and using if inside walk.default to disambiguate:

walk.default = function (x) {
    if (inherits(x, '<-')) {
        message('<-')
    } else {
        message('default')
    }

}

However, personally I prefer not to mix S3 dispatch with if.