knitr::opts_chunk$set( collapse = TRUE, comment = "#>" )
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 # CHAINING mean %<% range %<% 1:5 # CHAINING WITH MIXED PIPES # Chained with mixed pipes are performed left-to-right # Although technically possible, this is probably a bad idea. add(1) %<% 1:5 %>% multiply_by(2) 1:5 % add(1) % multiply_by(2) # same
insert_backpipe function can be bound to a keyboard shortcut. We
CTRL + SHIFT + <
This can be done through RStudio's Tools > Modify Keyboard Shortcuts menu. Future versions of this package may do this automatically.
backpipe can be used to:
div() %<% p %<% "Hello World"
warn %<% "there is a problem here" # OR expect_true() %<% (ans==1) # OR log_warn %<% "This didn't work"
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
backpipe code for more details.
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