[External] Re: New pipe operator

The following gives an error.

  1 |> `+`(2)
  ## Error: function '+' is not supported in RHS call of a pipe

  1 |> `+`()
  ## Error: function '+' is not supported in RHS call of a pipe

but this does work:

  1 |> (`+`)(2)
  ## [1] 3

  1 |> (`+`)()
  ## [1] 1

The error message suggests that this was intentional.
It isn't mentioned in ?"|>"

On Sat, Dec 5, 2020 at 1:19 PM <luke-tierney at uiowa.edu> wrote:

We went back and forth on this several times. The key advantage of
requiring parentheses is to keep things simple and consistent.  Let's
get some experience with that. If experience shows requiring
parentheses creates too many issues then we can add the option of
dropping them later (with special handling of :: and :::). It's easier
to add flexibility and complexity than to restrict it after the fact.

Best,

luke

On Sat, 5 Dec 2020, Hugh Parsonage wrote:

I'm surprised by the aversion to

mtcars |> nrow

over

mtcars |> nrow()

and I think the decision to disallow the former should be
reconsidered.  The pipe operator is only going to be used when the rhs
is a function, so there is no ambiguity with omitting the parentheses.
If it's disallowed, it becomes inconsistent with other treatments like
sapply(mtcars, typeof) where sapply(mtcars, typeof()) would just be
noise.  I'm not sure why this decision was taken

If the only issue is with the double (and triple) colon operator, then
ideally `mtcars |> base::head` should resolve to `base::head(mtcars)`
-- in other words, demote the precedence of |>

Obviously (looking at the R-Syntax branch) this decision was
considered, put into place, then dropped, but I can't see why
precisely.

Best,


Hugh.







On Sat, 5 Dec 2020 at 04:07, Deepayan Sarkar <deepayan.sarkar at gmail.com> wrote:

On Fri, Dec 4, 2020 at 7:35 PM Duncan Murdoch <murdoch.duncan at 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?12?4?(?) 20:51 Duncan Murdoch <murdoch.duncan at 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

--
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
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