---
title: "Backpipe Operations"
author: "Christopher Brown"
date: "`r Sys.Date()`"
output:
  rmarkdown::html_vignette:
    toc: true
vignette: >
  %\VignetteIndexEntry{Backpipe Operations}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r setup, include = FALSE}
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 `backpipe`function for defining backpipe operators 
for any forward pipe implementation


## Installation

### CRAN (release)

    install.packages('backpipe')


### Github (devel)

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


## Usage

    # 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


## Keyboard Shortcut

The `insert_backpipe` function can be bound to a keyboard shortcut. We 
recommend: `CTRL + SHIFT + <` 

This can be done through RStudio's **Tools > Modify Keyboard Shortcuts** menu. 
Future versions of this package may do this automatically.


## 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 %<% "Hello World"
````

* write tests, assertions, logging statements, etc. where the command is listed
first
````
    warn %<% "there is a problem here"  # OR 
    expect_true() %<% (ans==1)          # OR  
    log_warn %<% "This didn't work"
````


## 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 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**.    
   

## Technical Implementation

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


## References

 * [Relevant magrittr issue #26](https://github.com/smbache/magrittr/issues/26)

 * [Stackoverflow: Is Right-to-left Operator Associativity in R Possible ](http://stackoverflow.com/questions/31305342/is-right-to-left-operator-associativity-in-r-possible)
