Backpipe

Popular packages magrittr and pipeR packages do not provide a backward piping operator. This package provides operators %<% for use with magrittr and %<<% for use with pipeR. The package also provides the backpipefunction for defining backpipe operators

Motivation

Some structure are simply better expressed with right-to-left operations. This more common than one might expect. For example, right-to-left structures are better at representing trees, directories and nested structures.

As an example, consider how shiny has the developer write code that produces HTML. Let's say you wished to produce the HTML:

<div class="outer-outer">
  <div class="outer">
    <div class="inner">
      <h1 role="heading">content</h1>
    </div>
  </div>
</div> 

HTML is ugly, but works. With shiny, you'd write:

div( class="outer-outer", 
  div( class="outer",
    div( class="inner",
      h1( "content", role="heading" )
    )
  )
)

Yuck!!! This is almost as ugly as the HTML. Using magrittr or pipeR this can be a little neater:

h1( "content", role="heading" )        %>%
  div( class="inner")                  %>%
    div( class="outer")                %>% 
      div( class="outer-outer")      
  

That's a little better and it produces the same HTML. It can still be better. The above example illustrates two incongruities between code and output. The first is a reversal of order between code and HTML output. HTML is ordered from outside-in. shiny has the developer write code inside-out. This makes debugging difficult expecially with complex code. If you haven't done anything complex with shiny yet, you will. Don't worry. The second problem is that code indentation no longer illustrate the nested structure of the HTML.

The backpipe package and backpipe operators solve both of these dilemmas. In fact, writing cleaner shiny code was the impetus for this package, though its utility can be found when working with other things too. Using the backpipe operator the same code can be written as:

div( class="outer-outer")              %<%
  div( class="outer")                  %<% 
    div( class="inner")                %<% 
      h1( "content", role="heading" ) 

Now, both the order of the tags and the nesting matches the outputed HTML. Your code is cleaner, more legible and debugging is easy. Your overall coding time is lessened. With the time you save, you can join me at the pub and buy me a beer. I'm already there. I used backpipe. My shiny code was done hours ago.

Usage

The back-piping operators

mean %<%  1:3   # magrittr
mean %<<% 1:3   # pipeR
 
mean %<% range %<% 1:5

Technical Implementation

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

References

• Relevant magrittr issue

• Stackoverflow question