[Rd] RFC: (in-principle) native unquoting for standard evaluation

[Jonathan Carroll]

Firstly, credit where due: the lazyeval NSE vignette [1] covers so many of
the angles that this proposal needs to address and is extremely well
written (even if it has been superseded). The @ prefix I'm proposing is a
drop-in replacement for `uq()` (as used in that vignette) but for which the
`f_eval()` and `~` steps would not be required by the author/user.

This is proposed as an admittedly naive suggestion which fails to account
for the subtleties raised in [1] such as unquoting of multiple arguments
and scope selection. I am hoping that the discussion can cover how best to
address those matters.

The significant hurdles (apart from implementation which I cannot speak to)
that are dealt with in lazyeval (and presumably tidyeval) seem to be:

- a prefix can be attached to only a single object, so the extra_args
example from [1] would not be possible. I'm not certain why the unquoting
of the variable would not still be possible with the form

    variable = "x"
    mean(@variable, na.rm = TRUE, trim = 0.9)

since I'm proposing that the call need not be a formula (I may be way off
on this interpretation).

- I am proposing that the new syntax be able to achieve the example

    f <- function(col1, col2, new_col_name) {
        mtcars %>% mutate(@new_col_name = @col1 + @col2)
    }

but this is ambiguous if there is, say, an object "mpg" within that
function scope. [1] handles this with .env and .data pronouns but this
doesn't seem possible with just a prefix. One solution may be to have @@
and @ representing these two options.

I appreciate the significant work that has gone into the tidyverse packages
which use NSE and my intention is not to downplay any of that. I would just
like to be able to use the language more efficiently, so native access to
the unquoting seems like a step forward.

Kindest regards,

- Jonathan.

[1] https://cran.r-project.org/web/packages/lazyeval/vignettes/lazyeval.html

On Sun, Mar 19, 2017 at 1:09 PM, Hadley Wickham <h.wickham at gmail.com> wrote:

> Would this return a quosure? (i.e. a single sided formula that captures
> both expression and environment). That's the data structure we've adopted
> in tidyeval as it already has some built in support.
>
> Hadley
>
> On Friday, March 17, 2017, Michael Lawrence <lawrence.michael at gene.com>
> wrote:
>
>> Interesting idea. Lazy and non-standard evaluation is going to happen; the
>> language needs a way to contain it.
>>
>> I'll extend the proposal so that prefixing a formal argument with @ in
>> function() marks the argument as auto-quoting, so it arrives as a language
>> object without use of substitute(). Kind of like how '*' in C declares a
>> pointer and dereferences one.
>>
>> subset <- function(x, @subset, ...) { }
>>
>> This should make it easier to implement such functions, simplify
>> compilation, and allow detection of potential quoting errors through
>> static
>> analysis.
>>
>> Michael
>>
>> On Thu, Mar 16, 2017 at 5:03 PM, Jonathan Carroll <jono at jcarroll.com.au>
>> wrote:
>>
>> > (please be gentle, it's my first time)
>> >
>> > I am interested in discussions (possibly reiterating past threads --
>> > searching didn't turn up much) on the possibility of supporting standard
>> > evaluation unquoting at the language level. This has been brought up in
>> a
>> > recent similar thread here [1] and on Twitter [2] where I proposed the
>> > following desired (in-principle) syntax
>> >
>> >     f <- function(col1, col2, new_col_name) {
>> >         mtcars %>% mutate(@new_col_name = @col1 + @col2)
>> >     }
>> >
>> > or closer to home
>> >
>> >     x <- 1:10; y <- "x"
>> >     data.frame(z = @y)
>> >
>> > where @ would be defined as a unary prefix operator which substitutes
>> the
>> > quoted variable name in-place, to allow more flexibility of NSE
>> functions
>> > within a programming context. This mechanism exists within MySQL [3]
>> (and
>> > likely other languages) and could potentially be extremely useful.
>> Several
>> > alternatives have been incorporated into packages (most recently work
>> > on tidyeval) none of which appear to fully match the simplicity of the
>> > above, and some of which cut a forceful path through the syntax tree.
>> >
>> > The exact syntax isn't my concern at the moment (@ vs unquote() or
>> other,
>> > though the first requires user-supplied native prefix support within the
>> > language, as per [1]) and neither is the exact way in which this would
>> be
>> > achieved (well above my pay grade). The practicality of @ being on the
>> LHS
>> > of `=` is also of a lesser concern (likely greater complexity) than the
>> > RHS.
>> >
>> > I hear there exists (justified) reluctance to add new syntax to the
>> > language, but I think this has sufficient merit (and a growing number of
>> > workarounds) to warrant continued discussion.
>> >
>> > With kindest regards,
>> >
>> > - Jonathan.
>> >
>> > [1] https://stat.ethz.ch/pipermail/r-devel/2017-March/073894.html
>> > [2] https://twitter.com/carroll_jono/status/842142292253196290
>> > [3] https://dev.mysql.com/doc/refman/5.7/en/user-variables.html
>> >
>> >         [[alternative HTML version deleted]]
>> >
>> > ______________________________________________
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>> > https://stat.ethz.ch/mailman/listinfo/r-devel
>> >
>>
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>>
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>
>
> --
> http://hadley.nz
>

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