[Rd] rbind has confusing result for custom sub-class (possible bug?)

[Michael Chirico]

Have finally managed to come up with a fix after checking out sys.calls()
from within the as.Date.IDate debugger, which shows something like:

[[1]] rbind(DF, DF)
[[2]] rbind(deparse.level, ...)
[[3]] `[<-`(`*tmp*`, ri, value = 18042L)
[[4]] `[<-.Date`(`*tmp*`, ri, value = 18042L)
[[5]] as.Date(value)
[[6]] as.Date.IDate(value)

I'm not sure why [<- is called, I guess the implementation is to assign to
the output block by block? Anyway, we didn't have a [<- method. And
[<-.Date looks like:

value <- unclass(as.Date(value)) # <- converts to double
.Date(NextMethod(.Generic), oldClass(x)) # <- restores 'IDate' class

So we can fix our bug by defining a [<- class; the question that I still
don't see answered in documentation or source code is, why/where is [<-
called, exactly?

Mike C

On Sun, May 26, 2019 at 1:16 PM Michael Chirico <michaelchirico4 using gmail.com>
wrote:

> Debugging this issue:
>
> https://github.com/Rdatatable/data.table/issues/2008
>
> We have custom class 'IDate' which inherits from 'Date' (it just forces
> integer storage for efficiency, hence, I).
>
> The concatenation done by rbind, however, breaks this and returns a double:
>
> library(data.table)
> DF = data.frame(date = as.IDate(Sys.Date()))
> storage.mode(rbind(DF, DF)$date)
> # [1] "double"
>
> This is specific to base::rbind (data.table's rbind returns an integer as
> expected); in ?rbind we see:
>
> The method dispatching is not done via UseMethod(), but by C-internal
> dispatching. Therefore there is no need for, e.g., rbind.default.
> The dispatch algorithm is described in the source file
> (‘.../src/main/bind.c’) as
> 1. For each argument we get the list of possible class memberships from
> the class attribute.
> 2. *We inspect each class in turn to see if there is an applicable
> method.*
> 3. If we find an applicable method we make sure that it is identical to
> any method determined for prior arguments. If it is identical, we proceed,
> otherwise we immediately drop through to the default code.
>
> It's not clear what #2 means -- an applicable method *for what*? Glancing
> at the source code would suggest it's looking for rbind.IDate:
>
> https://github.com/wch/r-source/blob/trunk/src/main/bind.c#L1051-L1063
>
> const char *generic = ((PRIMVAL(op) == 1) ? "cbind" : "rbind"); // should
> be rbind here
> const char *s = translateChar(STRING_ELT(classlist, i)); // iterating over
> the classes, should get to IDate first
> sprintf(buf, "%s.%s", generic, s); // should be rbind.IDate
>
> but adding this method (or even exporting it) is no help [ simply defining
> rbind.IDate = function(...) as.IDate(NextMethod()) ]
>
> Lastly, it appears that as.Date.IDate is called, which is causing the type
> conversion:
>
> debug(data.table:::as.Date.IDate)
> rbind(DF, DF) # launches debugger
> x
> # [1] "2019-05-26" <-- singleton, so apparently applied to DF$date, not
> c(DF$date, DF$date)
> undebug(data.table:::as.Date.IDate)
>
> I can't really wrap my head around why as.Date is being called here, and
> even allowing that, why the end result is still the original class [
> class(rbind(DF, DF)$date) == c('IDate', 'Date') ]
>
> So, I'm beginning to think this might be a bug. Am I missing something?
>

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