R devel - Mar 2023 - Multiple Assignment built into the R Interpreter?

On 11/03/2023 11:57 a.m., Ivan Krylov wrote:
> On Sat, 11 Mar 2023 11:11:06 -0500
> Duncan Murdoch <murdoch.duncan at 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

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, 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 at 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 at 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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