[Rd] the pipe |> and line breaks in pipelines

[Duncan Murdoch]

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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