The backpipe package provides a single 'backpipe' operator (
%<%) that allows
the order of operands in a pipe statuement to be reversed. In some
situations this promotes more legible and debuggable code.
Popular packages magrittr and pipeR do not provide a backward pipe operator.
This package fills the void by providing a
%<% for use with magrittr and
%<<% for use with pipeR.
An RStudio addin (
insert_backpipe _addin) provides for inserting the
backpipe operator at the cursor location. It is recommended that this be bound
CTRL + SHIFT + < keyboard shortcut. This can be accomplished in RStudio
from the Tools > Modify Keyboard Shortcuts menu.
The package provides the
backpipefunction for defining backpipe operators
for any forward pipe implementation.
# BASIC USAGE mean %<% 1:5 # magrittr mean %<<% 1:5 # pipeR # MULTIPLE STEPS mean %<% range %<% 1:5 # WITH FORWARD PIPE: # Although technically possible, don't do this add(1) %<% 1:5 %>% multiply_by(2) # same as 1:5 %>% add(1) %>% multiply_by(2)
A rstudio addin is included with the package, 'insert_backpipe_addin()` which can be mapped to an rstudio shortcut. It is recommended that this be bound to CTRL + SHIFT + <
Common Use Cases
backpipe can be used to:
write clearer, more debuggable shinyUI code such that the order of code matches the HTML output.
div() %<% p("This is some text")
write test and assertions where the test condition is listed first.
assert_equal(1) %<% 1
pipeR and other pipes allow the developer to create left-to-right
operation flows. However, some code is better expressed with a right-to-left
syntax and is more common than one might expect. As an example, consider how
shiny has the developer write HTML-producing code.Let's say you wished to
produce the following HTML:
<div class="outer-outer"> <div class="outer"> <div class="inner"> <h1 role="heading">content</h1> </div> </div> </div>
HTML is ugly, but works. To generate this code using shiny, you'd write:
div( class="outer-outer", div( class="outer", div( class="inner", h1( "content", role="heading" ) ) ) )
Yuck!!! This is uglier than HTML. magrittr or pipeR allows this can to be cleaner:
h1( "content", role="heading" ) %>% div( class="inner") %>% div( class="outer") %>% div( class="outer-outer")
That's a little better; it produces the same HTML and is much cleaner, but still has a big problems. The code does not match the output. The ordering of the HTML output is outside-in while the code is ordered inside-out. This disconnect reduces clarity and makes debugging difficult.
With the backpipe operator, the same output can be generated with:
div( class="outer-outer") %<% div( class="outer") %<% div( class="inner") %<% h1( "content", role="heading" )
Notice how that a) the order of the code now matches the output HTML and b) the indentatoin aligns with the hierachal nature of HTML.
While there is no question about the utility of the forward pipe, it does not always promote the most expressive code. The backpipe solves this problem. In fact, writing cleaner shiny code was the impetus for this package. Though it, can be used to clarify code in other common operations. I am looking at you, testthat and assertthat.
The backpipe operators are implemented as a simple reording of arguments. See
backpipe code for more details.