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

[Mark.Bravington at data61.csiro.au]

I can't comment for Da, but one example where the ability to make 'if' generic would have been desirable:

A couple of years ago I wrote S3 classes and methods for 1-byte integers and logicals stored as raw vectors, in order to handle massive amounts of genetic data (by the standards of the day). Everything worked pretty nicely, ie I could "methodize" just about everything I needed--- except if-statements, which would fail to respect eg my definitions of NA. [ The precise details elude me, but if() was untrustworthy. ]  To use 'if()',  I had to remember to "typecast", which was prone to "user error".

Whether this kind of thing is worth the "risk", is another matter. 

cheers
Mark

Mark Bravington
CSIRO Marine Lab
Hobart
Australia

________________________________________
From: R-devel [r-devel-bounces at r-project.org] on behalf of Gabriel Becker [gmbecker at ucdavis.edu]
Sent: 06 March 2017 11:43
To: Da Zheng
Cc: r-devel at r-project.org
Subject: Re: [Rd] can we override "if" in R?

Da,

I've been following this thread and I'm still confused as to exactly what
you want/why you want it.

I'm probably just missing some context here, but, If() doesn't operate on
matrices, generally. Can you give an example of the type of code you want
to have continue to run that requires if operation *directly* on one of
your matrix objects, as opposed, say, to a value pulled out from it, or the
dot-product of two vectors in your system, both of which would be values
(scalars) not matrices.

Now ifelse(), is of course, a different beast altogether, and would need to
be overloaded within your system, I imagine.

Best,
~G

On Sun, Mar 5, 2017 at 12:52 PM, Da Zheng <zhengda1936 at gmail.com> wrote:

> On Sun, Mar 5, 2017 at 2:50 PM, Michael Lawrence
> <lawrence.michael at gene.com> wrote:
> >
> >
> > 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.
> This is exactly why I want to use my own matrix objects and redefine
> "if" for the matrices. In my framework, all matrices are read-only, so
> there isn't side effect.
>
> Best,
> Da
> >
> >
> >
> >>
> >> 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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>



--
Gabriel Becker, PhD
Associate Scientist (Bioinformatics)
Genentech Research

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