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

[Michael Lawrence]

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
>
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