Provides a backward-pipe operator for 'magrittr' (%<%) or 'pipeR' (%<<%) that allows for a performing operations from right-to-left. This indispensable for writing clear code where there is natural right-to-left ordering common with nested structures and hierarchies such as trees/directories or markup languages such as HTML and XML.
The backpipe package provides a single 'backpipe" operator (
%<%) that allows
the order of operands in a pipe statuement to be inverted. In some
situations this promotes more legible, interpretable and debuggable code.
Popular packages magrittr and pipeR do not provide a backward pipe operator.
This package files the void by providing a
%<% for use with magrittr and
%<<% for use with pipeR.
The package also 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)
backpipe can be used to:
write clearer, more debuggable shiny UI code such that the order of code matches the HTML output.
write test and assertions where the test condition is listed first.
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 almost as ugly as the 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 the
backpipe code for more details.
Update README.md, DESCRIPTION and vignette
Update README.md and Vignettes with shiny example
Initial CRAN Version