[Rd] Multiple Assignment built into the R Interpreter?
Duncan Murdoch
murdoch@dunc@n @end|ng |rom gm@||@com
Sun Mar 12 12:18:20 CET 2023
On 12/03/2023 6:07 a.m., Sebastian Martin Krantz wrote:
> Thinking more about this, and seeing Kevins examples at
> https://github.com/kevinushey/dotty
> <https://github.com/kevinushey/dotty>, I think this is the most R-like
> way of doing it,
> with an additional benefit as it would allow to introduce the useful
> data.table semantics DT[, .(a = b, c, d)] to more general R. So I would
> propose to
> introduce a new primitive function . <- function(...) .Primitive(".") in
> R with an assignment method and the following features:
I think that proposal is very unlikely to be accepted. If it was a
primitive function, it could only be maintained by R Core. They are
justifiably very reluctant to take on extra work for themselves.
Kevin's package demonstrates that this can be done entirely in a
contributed package, which means there's no need for R Core to be
involved. I don't know if he has plans to turn his prototype into a
CRAN package. If he doesn't, then it will be up to some other
interested maintainer to step up and take on the task, or it will just
fade away.
I haven't checked whether your proposals below represent changes from
the current version of dotty, but if they do, the way to proceed is to
fork that project, implement your changes, and offer to contribute them
back to the main branch.
Duncan Murdoch
>
> * Positional assignment e.g. .[nr, nc] <- dim(x), and named assignment
> e.g. .[new = carb] <- mtcars or .[new = log(carb)] <- mtcars. All
> the functionality proposed by Kevin at
> https://github.com/kevinushey/dotty
> <https://github.com/kevinushey/dotty> is useful, unambiguous and
> feasible.
> * Silent dropping of RHS values e.g. .[mpg_new, cyl_new] <- mtcars.
> * Mixing of positional and named assignment e.g .[mpg_new, carb_new =
> carb, cyl_new] <- mtcars. The inputs not assigned by name are simply
> the elements of RHS in the order they occur, regardless of whether
> they have been used previously e.g. .[mpg_new, cyl_new = cyl,
> log_cyl = log(cyl), cyl_new2] <- mtcars is feasible. RHS here could
> be any named vector type.
> * Conventional use of the function as lazy version of of list(), as in
> data.table: .(A = B, C, D) is the same as list(A = B, C = C, D = D).
> This would also be useful, allowing more parsimonious code, and
> avoid the need to assign names to all return values in a function
> return, e.g. if I already have matrices A, C, Q and R as internal
> objects in my function, I can simply end by return(.(A, C, Q, R))
> instead of return(list(A = A, C = C, Q = Q, R = R)) if I wanted the
> list to be named with the object names.
>
> The implementation of this in R and C should be pretty straightforward.
> It would just require a modification to R CMD Check to recognize .[<- as
> assignment.
>
> Best regards,
>
> Sebastian
> -
> 2.)
>
> On Sun, 12 Mar 2023 at 09:42, Sebastian Martin Krantz
> <sebastian.krantz using graduateinstitute.ch
> <mailto:sebastian.krantz using graduateinstitute.ch>> wrote:
>
> Thanks Gabriel and Kevin for your inputs,
>
> regarding your points Gabriel, I think Python and Julia do allow
> multiple sub-assignment, but in-line with my earlier suggestion in
> response to Duncan to make multiple assignment an environment-level
> operation (like collapse::%=% currently works), this would not be
> possible in R.
>
> Regarding the [a] <- coolest_function() syntax, yeah it would mean
> do multiple assignment and set a equal to the first element dropping
> all other elements. Multiple assignment should be positional loke in
> other languages, enabling flexible renaming of objects on the fly.
> So it should be irrelevant whether the function returns a named or
> unnamed list or vector.
>
> Thanks also Kevin for this contribution. I think it’s a remarkable
> effort, and I wouldn’t mind such semantics e.g. making it a function
> call to ‘.[‘ or any other one-letter function, as long as it’s coded
> in C and recognized by the interpreter as an assignment operation.
>
> Best regards,
>
> Sebastian
>
>
>
>
>
> On Sun 12. Mar 2023 at 01:00, Kevin Ushey <kevinushey using gmail.com
> <mailto:kevinushey using gmail.com>> wrote:
>
> FWIW, it's possible to get fairly close to your proposed semantics
> using the existing metaprogramming facilities in R. I put together a
> prototype package here to demonstrate:
>
> https://github.com/kevinushey/dotty
> <https://github.com/kevinushey/dotty>
>
> The package exports an object called `.`, with a special
> `[<-.dot` S3
> method which enables destructuring assignments. This means you can
> write code like:
>
> .[nr, nc] <- dim(mtcars)
>
> and that will define 'nr' and 'nc' as you expect.
>
> As for R CMD check warnings, you can suppress those through the
> use of
> globalVariables(), and that can also be automated within the
> package.
> The 'dotty' package includes a function 'dotify()' which automates
> looking for such usages in your package, and calling
> globalVariables()
> so that R CMD check doesn't warn. In theory, a similar technique
> would
> be applicable to other packages defining similar operators (zeallot,
> collapse).
>
> Obviously, globalVariables() is a very heavy hammer to swing for
> this
> issue, but you might consider the benefits worth the tradeoffs.
>
> Best,
> Kevin
>
> On Sat, Mar 11, 2023 at 2:53 PM Duncan Murdoch
> <murdoch.duncan using gmail.com <mailto:murdoch.duncan using gmail.com>> wrote:
> >
> > On 11/03/2023 4:42 p.m., Sebastian Martin Krantz wrote:
> > > Thanks Duncan and Ivan for the careful thoughts. I'm not
> sure I can
> > > follow all aspects you raised, but to give my limited take
> on a few:
> > >
> > >> your proposal violates a very basic property of the
> language, i.e. that all statements are expressions and have a
> value. > What's the value of 1 + (A, C = init_matrices()).
> > >
> > > I'm not sure I see the point here. I evaluated 1 + (d =
> dim(mtcars); nr
> > > = d[1]; nc = d[2]; rm(d)), which simply gives a syntax error,
> >
> >
> > d = dim(mtcars); nr = d[1]; nc = d[2]; rm(d)
> >
> > is not a statement, it is a sequence of 4 statements.
> >
> > Duncan Murdoch
> >
> > as the
> > > above expression should. `%=%` assigns to
> > > environments, so 1 + (c("A", "C") %=% init_matrices()) returns
> > > numeric(0), with A and C having their values assigned.
> > >
> > >> suppose f() returns list(A = 1, B = 2) and I do > B, A <-
> f() > Should assignment be by position or by name?
> > >
> > > In other languages this is by position. The feature is not
> meant to
> > > replace list2env(), and being able to rename objects in the
> assignment
> > > is a vital feature of codes
> > > using multi input and output functions e.g. in Matlab or Julia.
> > >
> > >> Honestly, given that this is simply syntactic sugar, I
> don't think I would support it.
> > >
> > > You can call it that, but it would be used by almost every
> R user almost
> > > every day. Simple things like nr, nc = dim(x); values,
> vectors =
> > > eigen(x) etc. where the creation of intermediate objects
> > > is cumbersome and redundant.
> > >
> > >> I see you've already mentioned it ("JavaScript-like"). I
> think it would fulfil Sebastian's requirements too, as long as
> it is considered "true assignment" by the rest of the language.
> > >
> > > I don't have strong opinions about how the issue is phrased or
> > > implemented. Something like [t, n] = dim(x) might even be
> more clear.
> > > It's important though that assignment remains by position,
> > > so even if some output gets thrown away that should also be
> positional.
> > >
> > >> A <- 0 > [A, B = A + 10] <- list(1, A = 2)
> > >
> > > I also fail to see the use of allowing this. something like
> this is an
> > > error.
> > >
> > >> A = 2
> > >> (B = A + 1) <- 1
> > > Error in (B = A + 1) <- 1 : could not find function "(<-"
> > >
> > > Regarding the practical implementation, I think
> `collapse::%=%` is a
> > > good starting point. It could be introduced in R as a
> separate function,
> > > or `=` could be modified to accommodate its capability. It
> should be
> > > clear that
> > > with more than one LHS variables the assignment is an
> environment level
> > > operation and the results can only be used in computations
> once assigned
> > > to the environment, e.g. as in 1 + (c("A", "C") %=%
> init_matrices()),
> > > A and C are not available for the addition in this
> statement. The
> > > interpretor then needs to be modified to read something
> like nr, nc =
> > > dim(x) or [nr, nc] = dim(x). as an environment-level
> multiple assignment
> > > operation with no
> > > immediate value. Appears very feasible to my limited
> understanding, but
> > > I guess there are other things to consider still.
> Definitely appreciate
> > > the responses so far though.
> > >
> > > Best regards,
> > >
> > > Sebastian
> > >
> > >
> > >
> > >
> > >
> > > On Sat, 11 Mar 2023 at 20:38, Duncan Murdoch
> <murdoch.duncan using gmail.com <mailto:murdoch.duncan using gmail.com>
> > > <mailto:murdoch.duncan using gmail.com
> <mailto:murdoch.duncan using gmail.com>>> wrote:
> > >
> > > On 11/03/2023 11:57 a.m., Ivan Krylov wrote:
> > > > On Sat, 11 Mar 2023 11:11:06 -0500
> > > > Duncan Murdoch <murdoch.duncan using gmail.com
> <mailto:murdoch.duncan using gmail.com>
> > > <mailto:murdoch.duncan using gmail.com
> <mailto:murdoch.duncan using gmail.com>>> wrote:
> > > >
> > > >> That's clear, but your proposal violates a very
> basic property
> > > of the
> > > >> language, i.e. that all statements are expressions
> and have a value.
> > > >
> > > > How about reframing this feature request from
> multiple assignment
> > > > (which does go contrary to "everything has only one
> value, even
> > > if it's
> > > > sometimes invisible(NULL)") to "structured binding"
> / "destructuring
> > > > assignment" [*], which takes this single single
> value returned by the
> > > > expression and subsets it subject to certain rules?
> It may be
> > > easier to
> > > > make a decision on the semantics for destructuring
> assignment (e.g.
> > > > languages which have this feature typically allow
> throwing unneeded
> > > > parts of the return value away), and it doesn't seem
> to break as much
> > > > of the rest of the language if implemented.
> > > >
> > > > I see you've already mentioned it
> ("JavaScript-like"). I think it
> > > would
> > > > fulfil Sebastian's requirements too, as long as it
> is considered
> > > "true
> > > > assignment" by the rest of the language.
> > > >
> > > > The hard part is to propose the actual grammar of
> the new feature (in
> > > > terms of src/main/gram.y, preferably without introducing
> > > conflicts) and
> > > > its semantics (including the corner cases, some of
> which you have
> > > > already mentioned). I'm not sure I'm up to the task.
> > > >
> > >
> > > If I were doing it, here's what I'd propose:
> > >
> > > '[' formlist ']' LEFT_ASSIGN expr
> > > '[' formlist ']' EQ_ASSIGN expr
> > > expr RIGHT_ASSIGN '[' formlist ']'
> > >
> > > where `formlist` has the syntax of the formals list for
> a function
> > > definition. This would have the following semantics:
> > >
> > > {
> > > *tmp* <- expr
> > >
> > > # For arguments with no "default" expression,
> > >
> > > argname1 <- *tmp*[[1]]
> > > argname2 <- *tmp*[[2]]
> > > ...
> > >
> > > # For arguments with a default listed
> > >
> > > argname3 <- with(*tmp*, default3)
> > > }
> > >
> > >
> > > The value of the whole thing would therefore be
> (invisibly) the
> > > value of
> > > the last item in the assignment.
> > >
> > > Two examples:
> > >
> > > [A, B, C] <- expr # assign the first three
> elements of expr to A,
> > > B, and C
> > >
> > > [A, B, C = a + b] <- expr # assign the first two
> elements of expr
> > > # to A and B,
> > > # assign with(expr, a +
> b) to C.
> > >
> > > Unfortunately, I don't think this could be done entirely by
> > > transforming
> > > the expression (which is the way |> was done), and that
> makes it a lot
> > > harder to write and to reason about. E.g. what does
> this do?
> > >
> > > A <- 0
> > > [A, B = A + 10] <- list(1, A = 2)
> > >
> > > According to the recipe above, I think it sets A to 1
> and B to 12, but
> > > maybe a user would expect B to be 10 or 11. And
> according to that
> > > recipe this is an error:
> > >
> > > [A, B = A + 10] <- c(1, A = 2)
> > >
> > > which probably isn't what a user would expect, given
> that this is fine:
> > >
> > > [A, B] <- c(1, 2)
> > >
> > > Duncan Murdoch
> > >
> >
> > ______________________________________________
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