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

[Joshua Ulrich]

On Sun, May 26, 2019 at 4:06 AM Michael Chirico
<michaelchirico4 using gmail.com> wrote:
>
> 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?
>
Your rbind(DF, DF) call dispatches to base::rbind.data.frame().  The
`[<-` call is this line:
value[[jj]][ri] <- if (is.factor(xij)) as.vector(xij) else xij

That's where the storage.mode changes from integer to double.

debug: value[[jj]][ri] <- if (is.factor(xij)) as.vector(xij) else xij
Browse[2]>
debug: xij
Browse[2]> storage.mode(xij)
[1] "integer"
Browse[2]> value[[jj]][ri]
[1] "2019-05-26"
Browse[2]> storage.mode(value[[jj]][ri])
[1] "integer"
Browse[2]>
debug: if (!is.null(nm <- names(xij))) names(value[[jj]])[ri] <- nm
Browse[2]> storage.mode(value[[jj]][ri])
[1] "double"


> 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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>
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