From zhengda1936 at gmail.com  Sat Mar  4 21:36:28 2017
From: zhengda1936 at gmail.com (Da Zheng)
Date: Sat, 4 Mar 2017 15:36:28 -0500
Subject: [Rd] can we override "if" in R?
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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?

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, G?bor Cs?rdi <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, G?bor Cs?rdi <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
>
>



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