[Rd] New pipe operator

[Deepayan Sarkar]

On Fri, Dec 4, 2020 at 7:35 PM Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
>
> On 04/12/2020 8:13 a.m., Hiroaki Yutani wrote:
> >>   Error: function '::' not supported in RHS call of a pipe
> >
> > To me, this error looks much more friendly than magrittr's error.
> > Some of them got too used to specify functions without (). This
> > is OK until they use `::`, but when they need to use it, it takes
> > hours to figure out why
> >
> > mtcars %>% base::head
> > #> Error in .::base : unused argument (head)
> >
> > won't work but
> >
> > mtcars %>% head
> >
> > works. I think this is a too harsh lesson for ordinary R users to
> > learn `::` is a function. I've been wanting for magrittr to drop the
> > support for a function name without () to avoid this confusion,
> > so I would very much welcome the new pipe operator's behavior.
> > Thank you all the developers who implemented this!
>
> I agree, it's an improvement on the corresponding magrittr error.
>
> I think the semantics of not evaluating the RHS, but treating the pipe
> as purely syntactical is a good decision.
>
> I'm not sure I like the recommended way to pipe into a particular argument:
>
>    mtcars |> subset(cyl == 4) |> \(d) lm(mpg ~ disp, data = d)
>
> or
>
>    mtcars |> subset(cyl == 4) |> function(d) lm(mpg ~ disp, data = d)
>
> both of which are equivalent to
>
>    mtcars |> subset(cyl == 4) |> (function(d) lm(mpg ~ disp, data = d))()
>
> It's tempting to suggest it should allow something like
>
>    mtcars |> subset(cyl == 4) |> lm(mpg ~ disp, data = .)

Which is really not that far off from

mtcars |> subset(cyl == 4) |> \(.) lm(mpg ~ disp, data = .)

once you get used to it.

One consequence of the implementation is that it's not clear how
multiple occurrences of the placeholder would be interpreted. With
magrittr,

sort(runif(10)) %>% ecdf(.)(.)
## [1] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

This is probably what you would expect, if you expect it to work at all, and not

ecdf(sort(runif(10)))(sort(runif(10)))

There would be no such ambiguity with anonymous functions

sort(runif(10)) |> \(.) ecdf(.)(.)

-Deepayan

> which would be expanded to something equivalent to the other versions:
> but that makes it quite a bit more complicated.  (Maybe _ or \. should
> be used instead of ., since those are not legal variable names.)
>
> I don't think there should be an attempt to copy magrittr's special
> casing of how . is used in determining whether to also include the
> previous value as first argument.
>
> Duncan Murdoch
>
>
> >
> > Best,
> > Hiroaki Yutani
> >
> > 2020年12月4日(金) 20:51 Duncan Murdoch <murdoch.duncan using gmail.com>:
> >>
> >> Just saw this on the R-devel news:
> >>
> >>
> >> R now provides a simple native pipe syntax ‘|>’ as well as a shorthand
> >> notation for creating functions, e.g. ‘\(x) x + 1’ is parsed as
> >> ‘function(x) x + 1’. The pipe implementation as a syntax transformation
> >> was motivated by suggestions from Jim Hester and Lionel Henry. These
> >> features are experimental and may change prior to release.
> >>
> >>
> >> This is a good addition; by using "|>" instead of "%>%" there should be
> >> a chance to get operator precedence right.  That said, the ?Syntax help
> >> topic hasn't been updated, so I'm not sure where it fits in.
> >>
> >> There are some choices that take a little getting used to:
> >>
> >>   > mtcars |> head
> >> Error: The pipe operator requires a function call or an anonymous
> >> function expression as RHS
> >>
> >> (I need to say mtcars |> head() instead.)  This sometimes leads to error
> >> messages that are somewhat confusing:
> >>
> >>   > mtcars |> magrittr::debug_pipe |> head
> >> Error: function '::' not supported in RHS call of a pipe
> >>
> >> but
> >>
> >> mtcars |> magrittr::debug_pipe() |> head()
> >>
> >> works.
> >>
> >> Overall, I think this is a great addition, though it's going to be
> >> disruptive for a while.
> >>
> >> Duncan Murdoch
> >>
> >> ______________________________________________
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> >
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