the pipe |> and line breaks in pipelines

I agree with Duncan that the right solution is to wrap the pipe
expression with parentheses. Having the parser treat newlines
differently based on whether the session is interactive, or on what
type of operator happens to follow a newline, feels like a pretty big
can of worms.

I think this (or something similar) would accomplish what you want
while still retaining the nice aesthetics of the pipe expression, with
a minimal amount of syntax "noise":

result <- (
   data
     |> op1()
     |> op2()
)

For interactive sessions where you wanted to execute only parts of the
pipeline at a time, I could see that being accomplished by the editor
-- it could transform the expression so that it could be handled by R,
either by hoisting the pipe operator(s) up a line, or by wrapping the
to-be-executed expression in parentheses for you. If such a style of
coding became popular enough, I'm sure the developers of such editors
would be interested and willing to support this ...

Perhaps more importantly, it would be much easier to accomplish than a
change to the behavior of the R parser, and it would be work that
wouldn't have to be maintained by the R Core team.

Best,
Kevin

On Wed, Dec 9, 2020 at 11:34 AM Timothy Goodman <timsgoodman at gmail.com> wrote:

If I type my_data_frame_1 and press Enter (or Ctrl+Enter to execute the
command in the Notebook environment I'm using) I certainly *would* expect R
to treat it as a complete statement.

But what I'm talking about is a different case, where I highlight a
multi-line statement in my notebook:

     my_data_frame1
         |> filter(some_conditions_1)

and then press Ctrl+Enter.  Or, I suppose the equivalent would be to run an
R script containing those two lines of code, or to run a multi-line
statement like that from the console (which in RStudio I can do by pressing
Shift+Enter between the lines.)

In those cases, R could either (1) Give an error message [the current
behavior], or (2) understand that the first line is meant to be piped to
the second.  The second option would be significantly more useful, and is
almost certainly what the user intended.

(For what it's worth, there are some languages, such as Javascript, that
consider the first token of the next line when determining if the previous
line was complete.  JavaScript's rules around this are overly complicated,
but a rule like "a pipe following a line break is treated as continuing the
previous line" would be much simpler.  And while it might be objectionable
to treat the operator %>% different from other operators, the addition of
|>, which isn't truly an operator at all, seems like the right time to
consider it.)

-Tim

On Wed, Dec 9, 2020 at 3:12 AM Duncan Murdoch <murdoch.duncan at gmail.com>
wrote:

The requirement for operators at the end of the line comes from the
interactive nature of R.  If you type

      my_data_frame_1

how could R know that you are not done, and are planning to type the
rest of the expression

        %>% filter(some_conditions_1)
        ...

before it should consider the expression complete?  The way languages
like C do this is by requiring a statement terminator at the end.  You
can also do it by wrapping the entire thing in parentheses ().

However, be careful: Don't use braces:  they don't work.  And parens
have the side effect of removing invisibility from the result (which is
a design flaw or bonus, depending on your point of view).  So I actually
wouldn't advise this workaround.

Duncan Murdoch


On 09/12/2020 12:45 a.m., Timothy Goodman wrote:

Hi,

I'm a data scientist who routinely uses R in my day-to-day work, for

tasks

such as cleaning and transforming data, exploratory data analysis, etc.
This includes frequent use of the pipe operator from the magrittr and

dplyr

libraries, %>%.  So, I was pleased to hear about the recent work on a
native pipe operator, |>.

This seems like a good time to bring up the main pain point I encounter
when using pipes in R, and some suggestions on what could be done about
it.  The issue is that the pipe operator can't be placed at the start of

a

line of code (except in parentheses).  That's no different than any

binary

operator in R, but I find it's a source of difficulty for the pipe

because

of how pipes are often used.

[I'm assuming here that my usage is fairly typical of a lot of users; at
any rate, I don't think I'm *too* unusual.]

=== Why this is a problem ===

It's very common (for me, and I suspect for many users of dplyr) to write
multi-step pipelines and put each step on its own line for readability.
Something like this:

    ### Example 1 ###
    my_data_frame_1 %>%
      filter(some_conditions_1) %>%
      inner_join(my_data_frame_2, by = some_columns_1) %>%
      group_by(some_columns_2) %>%
      summarize(some_aggregate_functions_1) %>%
      filter(some_conditions_2) %>%
      left_join(my_data_frame_3, by = some_columns_3) %>%
      group_by(some_columns_4) %>%
      summarize(some_aggregate_functions_2) %>%
      arrange(some_columns_5)

[I guess some might consider this an overly long pipeline; for me it's
pretty typical.  I *could* split it up by assigning intermediate results

to

variables, but much of the value I get from the pipe is that it lets my
code communicate which results are temporary, and which will be used

again

later.  Assigning variables for single-use results would remove that
expressiveness.]

I would prefer (for reasons I'll explain) to be able to write the above
example like this, which isn't valid R:

    ### Example 2 (not valid R) ###
    my_data_frame_1
      %>% filter(some_conditions_1)
      %>% inner_join(my_data_frame_2, by = some_columns_1)
      %>% group_by(some_columns_2)
      %>% summarize(some_aggregate_functions_1)
      %>% filter(some_conditions_2)
      %>% left_join(my_data_frame_3, by = some_columns_3)
      %>% group_by(some_columns_4)
      %>% summarize(some_aggregate_functions_2)
      %>% arrange(some_columns_5)

One (minor) advantage is obvious: It lets you easily line up the pipes,
which means that you can see at a glance that the whole block is a single
pipeline, and you'd immediately notice if you inadvertently omitted a

pipe,

which otherwise can lead to confusing output.  [It's also aesthetically
pleasing, especially when %>% is replaced with |>, but that's

subjective.]

But the bigger issue happens when I want to re-run just *part* of the
pipeline.  I do this often when debugging: if the output of the pipeline
seems wrong, I re-run the first few steps and check the output, then
include a little more and re-run again, etc., until I locate my mistake.
Working in an interactive notebook environment, this involves using the
cursor to select just the part of the code I want to re-run.

It's fast and easy to select *entire* lines of code, but unfortunately

with

the pipes placed at the end of the line I must instead select everything
*except* the last three characters of the line (the last two characters

for

the new pipe).  Then when I want to re-run the same partial pipeline with
the next line of code included, I can't just press SHIFT+Down to select

it

as I otherwise would, but instead must move the cursor horizontally to a
position three characters before the end of *that* line (which is

generally

different due to varying line lengths).  And so forth each time I want to
include an additional line.

Moreover, with the staggered positions of the pipes at the end of each
line, it's very easy to accidentally select the final pipe on a line, and
then sit there for a moment wondering if the environment has stopped
responding before realizing it's just waiting for further input (i.e.,

for

the right-hand side).  These small delays and disruptions add up over the
course of a day.

This desire to select and re-run the first part of a pipeline is also the
reason why it doesn't suffice to achieve syntax like my "Example 2" by
wrapping the entire pipeline in parentheses.  That's of no use if I want

to

re-run a selection that doesn't include the final close-paren.

=== Possible Solutions ===

I can think of two, but maybe there are others.  The first would make
"Example 2" into valid code, and the second would allow you to run a
selection that included a trailing pipe.

    Solution 1: Add a special case to how R is parsed, so if the first
(non-whitespace) token after an end-line is a pipe, that pipe gets moved

to

before the end-line.
      - Argument for: This lets you write code like example 2, which
addresses the pain point around re-running part of a pipeline, and has
advantages for readability.  Also, since starting a line with a pipe
operator is currently invalid, the change wouldn't break any working

code.

      - Argument against: It would make the behavior of %>% inconsistent

with

that of other binary operators in R.  (However, this objection might not
apply to the new pipe, |>, which I understand is being implemented as a
syntax transformation rather than a binary operator.)

    Solution 2: Ignore the pipe operator if it occurs as the final token

of

the code being executed.
      - Argument for: This would mean the user could select and re-run the
first few lines of a longer pipeline (selecting *entire* lines), avoiding
the difficulties described above.
      - Argument against: This means that %>% would be valid even if it
occurred without a right-hand side, which is inconsistent with other
operators in R.  (But, as above, this objection might not apply to |>.)
Also, this solution still doesn't enable the syntax of "Example 2", with
its readability benefit.

Thanks for reading this and considering it.

- Tim Goodman

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