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

[Gabriel Becker]

Mark,

I do understand this desire, though notably, if selecting a single element
from one of these custom vectors gave you a normal value, if would be safe,
right? e.g. if the [ method, etc checked if the index length is one, and
returned the non-compressed value if it was, as an off-the-cuff possibility.

Also, I do feel compelled to say that I'm working with R-core (Luke Tierney
primarily) on low-level support for this kind of data-storage abstraction
that would let you create custom vector/matrix storage mechanisms that
still behave themselves as atomic vectors. See
https://www.r-project.org/dsc/2016/slides/customvectors.html for my initial
pitch and https://svn.r-project.org/R/branches/ALTREP/
<https://svn.r-project.org/R/branches/ALTREP/> for the work-in-progress
branch. We hope to merge that in after the release of 3.4, but there's no
firm date as some work on our part and discussion amongst r-core remains
before it can happen.

So in the future, I hope you'll be able to do this in ways that R sees as
actual atomic vectors. They do have to be define in C-code currently,
though.

~G

On Sun, Mar 5, 2017 at 5:09 PM, <Mark.Bravington at data61.csiro.au> wrote:

> 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
> > >> >
> > >> >
> > >
> > >
> >
> > ______________________________________________
> > R-devel at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
>
>
>
> --
> Gabriel Becker, PhD
> Associate Scientist (Bioinformatics)
> Genentech Research
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>



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

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