From murdoch@dunc@n @end|ng |rom gm@||@com  Sat Mar 11 17:11:06 2023
From: murdoch@dunc@n @end|ng |rom gm@||@com (Duncan Murdoch)
Date: Sat, 11 Mar 2023 11:11:06 -0500
Subject: [Rd] Multiple Assignment built into the R Interpreter?
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On 11/03/2023 9:54 a.m., Sebastian Martin Krantz wrote:
> Thanks Duncan,
> 
> I know about list2env(), in fact a previous version of collapse::`%=%` 
> was coded as
> 
> "%=%" <- function(lhs, rhs) {
>  ?? if(!is.character(lhs)) stop("lhs needs to be character")
>  ?? if(!is.list(rhs)) rhs <- as.vector(rhs, "list")
>  ?? if(length(lhs) != length(rhs)) stop("length(lhs) not equal to 
> length(rhs)")
>  ?? list2env(`names<-`(rhs, lhs), envir = parent.frame())
>  ?? invisible()
> }
> 
> but as you say, the input needs to be converted to a list, and it calls 
> several R functions, which led me to end up writing `%=%` in C:
> https://github.com/SebKrantz/collapse/blob/master/src/small_helper.c#L162 <https://github.com/SebKrantz/collapse/blob/master/src/small_helper.c#L162>.
> This implementation works in the way you describe, i.e. it has separate 
> methods for all the standard vector types, and coerces to list otherwise.
> 
> That being said, all implementations in packages falls short of being 
> very useful, because R CMD Check it will still require global bindings 
> for variables,
> unless this becomes a standard feature of the language. So I cannot use 
> this in packages, and there is still a performance cost to it, in my 
> case a call to
> .Call() and parent.frame(), which is quite low, but still high compared 
> to the cost of `<-` or `=`.

Another R way to do what you're doing would be to stay within a list the 
whole time, i.e. code it as

   mats <- init_matrices(X, Y, Z)
   with(mats, ... do things with A, C, Q, and R ... )

This won't give warnings about globals, and it makes very clear that 
those 4 matrices are all closely related, and it allows you to work with 
multiple 4-tuples of matrices, etc.

> So what I am requesting is indeed nothing less than to consider making 
> this a permanent feature of the language itself.

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. 
What's the value of

    1 + (A, C = init_matrices())

?  I think you would disallow the above (though you didn't address it 
when I raised it the first time), which means there would now be two 
kinds of statements:  ones that are expressions and therefore can be 
used as function arguments, and ones that aren't.

> Given that the other 3 major scientific computing languages (Matlab, 
> Python and Julia) have implemented it very successfully,
> I don't think the general practicality of it should be an issue. 
> Regarding implementation in other languages, Julia works as follows:
> 
> function init_matrices()
>  ??? A = 1; C = 2; Q = 3; R = 4
>  ??? return A, C, Q, R
> end
> 
> res = init_matrices()???????????? # gives a Julia Tuple (A, C, Q, R)
> A, C = init_matrices()?????????? # Works, A is 1, C is 2, the others are 
> dropped

That's pretty ugly having a singular LHS handled so much differently 
from a plural LHS.

> A, C, Q, R = init_matrices()? # Standard
> 
> I think as far as R is concerned multiple return values are not really 
> necessary given that one can always,
> return(list(A, C, Q, R)), although of course there is also a cost to 
> list(). I also wouldn't mind being strict about it and
> not allowing A, C = init_matrices(), but others might disagree.

Another ambiguity:  suppose f() returns list(A = 1, B = 2) and I do

   B, A <- f()

Should assignment be by position or by name?

Honestly, given that this is simply syntactic sugar, I don't think I 
would support it.

Duncan Murdoch

> 
> Best regards,
> 
> Sebastian
> 
> 
> On Sat, 11 Mar 2023 at 15:37, Duncan Murdoch <murdoch.duncan at gmail.com 
> <mailto:murdoch.duncan at gmail.com>> wrote:
> 
>     I think the standard way to do this in R is given by list2env(), as
>     described in a couple of answers on the SO page you linked.
> 
>     The syntax you proposed would be likely to be confusing in complex
>     expressions, e.g.
> 
>      ? ?f(A, C, Q, R = init_matrices(X, Y, Z))
> 
>     would obviously not work but wouldn't trigger a syntax error, and
> 
>      ? ?f((A, C, Q, R = init_matrices(X, Y, Z)))
> 
>     could work, but looks too much like the previous one.? So I think R
>     would want Javascript-like
> 
>      ? ?[A, C, Q, R] <- init_matrices(X, Y, Z)
> 
>     instead.? But then the question would come up about how to handle the
>     RHS.? Does the function have to return a list?? What if the length of
>     the list is not 4?? Or is it just guaranteed to be equivalent to
> 
>      ? ?temp <- init_matrices(X, Y, Z)
>      ? ?A <- temp[[1]]
>      ? ?C <- temp[[2]]
>      ? ?Q <- temp[[3]]
>      ? ?R <- temp[[4]]
> 
>     which would work for other vector types besides lists?
> 
>     BTW, here's a little hack that almost works:
> 
>     `vals<-` <- function(x, ..., value) {
>      ? ? others <- substitute(list(...))
>      ? ? if (length(others) > 1)
>      ? ? ? for (i in seq_along(others)[-1])
>      ? ? ? ? assign(as.character(others[[i]]), value[[i]], envir =
>     parent.frame())
>      ? ? value[[1]]
>     }
> 
>     You call it as
> 
>      ? vals(a, b, c) <- 1:3
> 
>     and it assigns 1 to a, 2 to b, and 3 to c.? It doesn't quite do what
>     you
>     want because it requires that a exists already, but b and c don't
>     have to.
> 
>     Duncan Murdoch
> 
>     On 11/03/2023 4:04 a.m., Sebastian Martin Krantz wrote:
>      > Dear R Core,
>      >
>      > working on my dynamic factor modelling package, which requires
>     several
>      > subroutines to create and update several system matrices, I come
>     back to
>      > the issue of being annoyed by R not supporting multiple
>     assignment out of
>      > the box like Matlab, Python and julia. e.g. something like
>      >
>      > A, C, Q, R = init_matrices(X, Y, Z)
>      >
>      > would be a great addition to the language. I know there are several
>      > workarounds such as the %<-% operator in the zeallot package or
>     my own %=%
>      > operator in collapse, but these don't work well for package
>     development as
>      > R CMD Check warns about missing global bindings for the created
>     variables,
>      > e.g. I would have to use
>      >
>      > A <- C <- Q <- R <- NULL
>      > .c(A, C, Q, R) %=% init_matrices(X, Y, Z)
>      >
>      > in a package, which is simply annoying. Of course the standard way of
>      >
>      > init <- init_matrices(X, Y, Z)
>      >? ?A <- init$A; C <- init$C; Q <- init$Q; R <- init$R
>      > rm(init)
>      >
>      > is also super cumbersome compared to Python or Julia. Another
>     reason is of
>      > course performance, even my %=% operator written in C has a
>     non-negligible
>      > performance cost for very tight loops, compared to a solution at the
>      > interpretor level or in a primitive function such as `=`.
>      >
>      > So my conclusion at this point is that it is just significantly
>     easier to
>      > implement such codes in Julia, in addition to the greater
>     performance it
>      > offers. There are obvious reasons why I am still coding in R and
>     C, thanks
>      > to the robust API and great ecosystem of packages, but adding
>     this could be
>      > a presumably low-hanging fruit to make my life a bit easier.
>     Several issues
>      > for this have been filed on Stackoverflow, the most popular one (
>      >
>     https://stackoverflow.com/questions/7519790/assign-multiple-new-variables-on-lhs-in-a-single-line <https://stackoverflow.com/questions/7519790/assign-multiple-new-variables-on-lhs-in-a-single-line>)
>      > has been viewed 77 thousand times.
>      >
>      > But maybe this has already been discussed here and already
>     decided against.
>      > In that case, a way to browse R-devel archives to find out would
>     be nice.
>      >
>      > Best regards,
>      >
>      > Sebastian
>      >
>      >? ? ? ?[[alternative HTML version deleted]]
>      >
>      > ______________________________________________
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>



[Rd] Multiple Assignment built into the R Interpreter?
Duncan Murdoch