Backpipe

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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 to the 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.

Installation

CRAN

install.packages('backpipe')

Github

library(devtools)
install_github("decisionpatterns/backpipe")

Usage

# 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)

Keyboard Binding

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

Motivation

magrittr, 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.

Technical Implementation

The backpipe operators are implemented as a simple reording of arguments. See the backpipe code for more details.