New pipe operator

Hi Gabor,

On Sun, Dec 6, 2020 at 3:22 PM Gabor Grothendieck <ggrothendieck at gmail.com> wrote:

I understand very well that it is implemented at the syntax level;
however, in any case the implementation is irrelevant to the principles.

Here a similar example to the one I gave before but this time written out:

This works:

  3 |> function(x) x + 1

but this does not:

  foo <- function(x) x + 1
  3 |> foo

so it breaks the principle of functions being first class objects.  foo and its
definition are not interchangeable.

I understood what you meant as well.

The issue is that neither foo nor its definition are being operated on, or even exist within the scope of what |> is defined to do. You are used to magrittr's %>% where arguably what you are saying would be true. But its not here, in my view.

Again, I think the issue is that |>, in as much as it "operates" on anything at all (it not being a function, regardless of appearances), operates on call expression objects, NOT on functions, ever.

function(x) x parses to a call expression as does RHSfun(), while RHSfun does not, it parses to a name, regardless of whether that symbol will eventually evaluate to a closure or not.

So in fact, it seems to me that, technically, all name symbols are being treated exactly the same (none are allowed, including those which will lookup to functions during evaluation), while all* call expressions are also being treated the same. And again, there are no functions anywhere in either case.

* except those that include that the parser flags as syntactically special.

You have
to write 3 |> foo() but don't have to write 3 |> (function(x) x + 1)().

I think you should probably be careful what you wish for here. I'm not involved with this work and do not speak for any of those who were, but the principled way to make that consistent while remaining entirely in the parser seems very likely to be to require the latter, rather than not require the former.

This isn't just a matter of notation, i.e. foo vs foo(), but is a
matter of breaking
the way R works as a functional language with first class functions.

I don't agree. Consider `+`

Having

foo <- get("+") ## note no `` here
foo(x,y)

parse and work correctly while

+(x,y)

 does not does not mean + isn't a function or that it is a "second class citizen", it simply means that the parser has constraints on the syntax for writing code that calls it that calling other functions are not subject to. The fact that such syntactic constraints can exist proves that there is not some overarching inviolable principle being violated here, I think. Now you may say "well thats just the parser, it has to parse + specially because its an operator with specific precedence etc". Well, the same exact thing is true of |> I think.

Best,
~G

On Sun, Dec 6, 2020 at 4:06 PM Gabriel Becker <gabembecker at gmail.com> wrote:

Hi Gabor,

On Sun, Dec 6, 2020 at 12:52 PM Gabor Grothendieck <ggrothendieck at gmail.com> wrote:

I think the real issue here is that functions are supposed to be
first class objects in R
or are supposed to be and |> would break that if if is possible
to write function(x) x + 1 on the RHS but not foo (assuming foo
was defined as that function).

I don't think getting experience with using it can change that
inconsistency which seems serious to me and needs to
be addressed even if it complicates the implementation
since it drives to the heart of what R is.

With respect I think this is a misunderstanding of what is happening here.

Functions are first class citizens. |> is, for all intents and purposes, a macro.

LHS |> RHS(arg2=5)

parses to

RHS(LHS, arg2 = 5)

There are no functions at the point in time when the pipe transformation happens, because no code has been evaluated. To know if a symbol is going to evaluate to a function requires evaluation which is a step entirely after the one where the |> pipe is implemented.

Another way to think about it is that

LHS |> RHS(arg2 = 5)

is another way of writing RHS(LHS, arg2 = 5), NOT R code that is (or even can be) evaluated.


Now this is a subtle point that only really has implications in as much as it is not the case for magrittr pipes, but its relevant for discussions like this, I think.

~G

On Sat, Dec 5, 2020 at 1:08 PM Gabor Grothendieck
<ggrothendieck at gmail.com> wrote:

The construct utils::head  is not that common but bare functions are
very common and to make it harder to use the common case so that
the uncommon case is slightly easier is not desirable.

Also it is trivial to write this which does work:

mtcars %>% (utils::head)

On Sat, Dec 5, 2020 at 11:59 AM Hugh Parsonage <hugh.parsonage at gmail.com> 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

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Statistics & Software Consulting
GKX Group, GKX Associates Inc.
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email: ggrothendieck at gmail.com

--
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com

--
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com