[Rd] as.list fails on functions with S3 classes

[brodie gaslam]

> On Wednesday, April 28, 2021, 5:16:20 PM EDT, Gabriel Becker <gabembecker using gmail.com> wrote:
>
> Hi Antoine,
>
> I would say this is the correct behavior. S3 dispatch is solely (so far as
> I know?) concerned with the "actual classes" on the object. This is because
> S3 classes act as labels that inform dispatch what, and in what order,
> methods should be applied. You took the function class (ie label) off of
> your object, which means that in the S3 sense, that object is no longer a
> function and dispatching to function methods for it would be incorrect.
> This is independent of whether the object is still callable "as a function".
>
> The analogous case for non-closures to what you are describing would be for
> S3 to check mode(x) after striking out with class(x) to find relevant
> methods. I don't think that would be appropriate.

I would think of the general case to be to check `class(unclass(x))` on
strike-out.  This would then include things such as "matrix", etc.
Dispatching on the implicit class as fallback seems like a natural thing
to do in a language that dispatches on implicit class when there is none.
After all, once you've struck out of your explicit classes, you have
none left!

This does happen naturally in some places (e.g. interacting with a
data.frame as a list), and is quite delightful (usually).  I won't get
into an argument of what the documentation states or whether any changes
should be made, but to me that dispatch doesn't end with the implicit
class seems feels like a logical wrinkle.  Yes, I can twist my brain to
see how it can be made to make sense, but I don't like it.

A fun past conversation on this very topic:

https://stat.ethz.ch/pipermail/r-devel/2019-March/077457.html

Best,

B.

> Also, as an aside, if you want your class to override methods that exist
> for function you would want to set the class to c("foo", "function"), not
> c("function", "foo"), as you had it in your example.
>
> Best,
> ~G
>
> On Wed, Apr 28, 2021 at 1:45 PM Antoine Fabri <antoine.fabri using gmail.com>
> wrote:
>
>> Dear R devel,
>>
>> as.list() can be used on functions, but not if they have a S3 class that
>> doesn't include "function".
>>
>> See below :
>>
>> ```r
>> add1 <- function(x) x+1
>>
>> as.list(add1)
>> #> $x
>> #>
>> #>
>> #> [[2]]
>> #> x + 1
>>
>> class(add1) <- c("function", "foo")
>>
>> as.list(add1)
>> #> $x
>> #>
>> #>
>> #> [[2]]
>> #> x + 1
>>
>> class(add1) <- "foo"
>>
>> as.list(add1)
>> #> Error in as.vector(x, "list"): cannot coerce type 'closure' to vector of
>> type 'list'
>>
>> as.list.function(add1)
>> #> $x
>> #>
>> #>
>> #> [[2]]
>> #> x + 1
>> ```
>>
>> In failing case the argument is dispatched to as.list.default instead of
>> as.list.function.
>>
>> (1) Shouldn't it be dispatched to as.list.function ?
>>
>> (2) Shouldn't all generics when applied on an object of type closure fall
>> back to the `fun.function` method  before falling back to the `fun.default`
>> method ?
>>
>> Best regards,
>>
>> Antoine