Export S3 method for a 'function' class object

Question

Function objects seems to work well with dispatching of S3 methods.
But for some reason they cannot be exported in NAMESPACE file.

Below code works with dispatching to *.function method:

as.abc = function(x, ...){
    UseMethod("as.abc")
}
as.abc.list = function(x, ...){
    stopifnot(is.list(x))
    structure(x, class="abc")
}
as.abc.function = function(x, ...){
    stopifnot(is.function(x))
    structure(x, class="abc")
}
# list
l = as.abc(list(1))
str(l)
#List of 1
# $ : num 1
# - attr(*, "class")= chr "abc"

# function
f = as.abc(function(x) x)
str(f)
#function (x)  
# - attr(*, "srcref")=Class 'srcref'  atomic [1:8] 1 12 1 24 12 24 1 1
# .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x3dbb848> 
# - attr(*, "class")= chr "abc"

But when I try to define export in NAMESPACE as:

export(as.abc)
S3method(as.abc, list)
S3method(as.abc, function)

It throws an error while R CMD check:

* checking package namespace information ... ERROR
Invalid NAMESPACE file, parsing gives:
Error in parse(nsFile, keep.source = FALSE, srcfile = NULL): 29:26: unexpected ')'
28: S3method(as.abc, list)
29: S3method(as.abc, function)

I tried also wrap into ` but it didn't help too

S3method(as.abc, `function`)

What is the proper way to export method for a function class?

Answer 1

According to Writing R Extensions section 1.5.2, you have to use regular quotes when registering S3 methods that work on objects of class function:

(Note that function and class names may be quoted, and reserved words and non-standard names such as [<- and function must be.)