[Rd] can we override "if" in R?

[Michael Lawrence]

On Sat, Mar 4, 2017 at 12:36 PM, Da Zheng <zhengda1936 at gmail.com> wrote:

> In my case, I create a new type of matrices and override matrix
> operations in R for these matrices.
> My goal is to make the system as transparent as possible, which means
> my system should execute the existing R code without modification.
> The problem is that when data is in my own vectors or matrices, "if"
> or "while" can't access their values unless we explicitly convert them
> into R objects. But this means users need to modify the existing code.
> So I hope I can override "if", "while", etc to access data in my own
> vectors and matrices directly.
> Does this sound reasonable?
>
>
Would you really need the alternate representation for scalar logicals?

I can see a case in the deferred evaluation context, although it would be
problematic wrt side effects unless the deferral is complete.




> Best,
> Da
>
> On Sat, Mar 4, 2017 at 3:22 PM, Michael Lawrence
> <lawrence.michael at gene.com> wrote:
> > I'm curious as to precisely why someone would want to do this.
> >
> > On Sat, Mar 4, 2017 at 11:49 AM, Da Zheng <zhengda1936 at gmail.com> wrote:
> >>
> >> I'm just curious. Why making "if" generic is even more dangerous?
> >>
> >> Best,
> >> Da
> >>
> >> On Sat, Mar 4, 2017 at 1:22 PM, Gábor Csárdi <csardi.gabor at gmail.com>
> >> wrote:
> >> > `!` is a generic, `if` is not. You can define an `if` that is generic,
> >> > but this might be even more dangerous....
> >> >
> >> > ❯ `if` <- function(a, b, c) UseMethod("if")
> >> > ❯ `if.default` <- function(a,b,c) base::`if`(a, b, c)
> >> > ❯ `if.foo` <- function(a, b, c) FALSE
> >> > ❯ a <- structure(42, class = "foo")
> >> >
> >> > ❯ if (a) TRUE else FALSE
> >> > [1] FALSE
> >> >
> >> > ❯ if (1) TRUE else FALSE
> >> > [1] TRUE
> >> >
> >> > Gabor
> >> >
> >> > On Sat, Mar 4, 2017 at 5:47 PM, Da Zheng <zhengda1936 at gmail.com>
> wrote:
> >> >> Thanks.
> >> >> Can I override it for a specific class?
> >> >> I can do that for operators such as "!". For example, "!.fm" works
> for
> >> >> objects of the class "fm".
> >> >> It seems I can't do the same for "if".
> >> >>
> >> >> Best,
> >> >> Da
> >> >>
> >> >> On Sat, Mar 4, 2017 at 12:41 PM, Gábor Csárdi <
> csardi.gabor at gmail.com>
> >> >> wrote:
> >> >>> You can. Perhaps needless to say, be careful with this.
> >> >>>
> >> >>> ❯ `if` <- function(...) FALSE
> >> >>> ❯ if (TRUE) TRUE else FALSE
> >> >>> [1] FALSE
> >> >>>
> >> >>> G.
> >> >>>
> >> >>> On Sat, Mar 4, 2017 at 5:36 PM, Da Zheng <zhengda1936 at gmail.com>
> >> >>> wrote:
> >> >>>> Hello,
> >> >>>>
> >> >>>> I heard we can override almost everything in R. Is it possible to
> >> >>>> override "if" keyword in R to evaluate my own object instead of a
> >> >>>> logical value?
> >> >>>>
> >> >>>> Thanks,
> >> >>>> Da
> >> >>>>
> >> >>>> ______________________________________________
> >> >>>> R-devel at r-project.org mailing list
> >> >>>> https://stat.ethz.ch/mailman/listinfo/r-devel
> >>
> >> ______________________________________________
> >> R-devel at r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-devel
> >
> >
>

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