In this lab, we will learn how to write methods for S3 generics.

Goal: by the end of this lab, you should be able to write your own S3 generic methods and functions.

Finding out about S3 methods

First, we’ll load the sloop package to get some information about the S3 system.

library(sloop)

Since we’ve already loaded the tidyverse, there are lots of S3 generic function and their methods already available to us.

For example, there are many methods for the print() generic available:

s3_methods_generic("print")

In fact, there are so many that we might as well restrict our attention to those relevant to tibbles and data.frames.

s3_methods_generic("print") %>%
  filter(str_detect(class, "tbl|data.frame"))
## Filtering a tbl_df object with 385 rows...

Other common S3 generics include summary() and plot():

s3_methods_generic("summary")
s3_methods_generic("plot")
  1. Try to think of another generic function that you’ve worked with in the past. Use s3_methods_generic() to find its available methods. [Note that you might have to load other packages to find the methods or the generic that you recall.]

In addition to listing the methods for a particular generic, we can find all the methods that are defined for a particular class.

s3_methods_class("tbl_df")

Note that while the methods for a particular generic are most likely written by many different developers in many different packages, the methods for a particular class are most likely written by the same set of developers of a single package.

Method dispath

If you are ever unsure of which method is actually being run when you invoke a generic, use s3_dispatch() to figure out what is going on.

s3_dispatch(print(starwars))
##    print.tbl_df
## => print.tbl
##  * print.data.frame
##  * print.default
s3_dispatch(print(mtcars))
## => print.data.frame
##  * print.default

  1. Why do the previous two examples show a different list of available methods?

  2. Why doesn’t print.data.frame() get called on starwars?

Defining a method

First, we’ll learn about two fun packages: emoji and crayon.

emoji

The emoji package allows you print emoji via the emoji() function.

install.packages("emoji")

To display an emoji, call it by name:

library(emoji)
emoji("star")
## [1] "⭐"
emoji("dog")
## [1] "🐶"
emoji("basketball")
## [1] "🏀"

You can see the full list of available emojis in the tibble returned by emojis.

emojis %>% View()
  1. Find an emoji that you like and use emoji() to print it to the console.

crayon

The crayon package allows you to print in color in the console. For a complicated example of what you can do, consider the tidyverse logo:

tidyverse_logo()
## ⬢ __  _    __   .    ⬡           ⬢  . 
##  / /_(_)__/ /_ ___  _____ _______ ___ 
## / __/ / _  / // / |/ / -_) __(_-</ -_)
## \__/_/\_,_/\_, /|___/\__/_/ /___/\__/ 
##      ⬢  . /___/      ⬡      .       ⬢

For a simpler, example, you can write your name in blue. Be sure to wrap the text in cat() (or message()) to send it to the console.

cat(crayon::blue("Ben"))
## Ben
  1. Pick a color from the crayon package and write something to the console in that color.

Defining a method

Now we will define a simple print() method that will only be invoked for objects of class beanumber, which is my GitHub username. This method will simply print a colorful message to the console, and then invoke the NextMethod() available.

print.beanumber <- function(x, ...) {
  cat(
    crayon::magenta(
      "Ben", 
      emoji::emoji("heart"), 
      "s programming in R!\n"
    )
  )
  NextMethod()
}

To use it, we just have to create an object of type beanumber, and then call print(). Remember that in R you can simply modify the class attribute of any object!

class(starwars) <- c("beanumber", class(starwars))
starwars

Note that this method works on objects of any underlying class!!

class(Titanic)
## [1] "table"
class(Titanic) <- c("beanumber", class(Titanic))
Titanic
## Ben ❤️ s programming in R!
## , , Age = Child, Survived = No
## 
##       Sex
## Class  Male Female
##   1st     0      0
##   2nd     0      0
##   3rd    35     17
##   Crew    0      0
## 
## , , Age = Adult, Survived = No
## 
##       Sex
## Class  Male Female
##   1st   118      4
##   2nd   154     13
##   3rd   387     89
##   Crew  670      3
## 
## , , Age = Child, Survived = Yes
## 
##       Sex
## Class  Male Female
##   1st     5      1
##   2nd    11     13
##   3rd    13     14
##   Crew    0      0
## 
## , , Age = Adult, Survived = Yes
## 
##       Sex
## Class  Male Female
##   1st    57    140
##   2nd    14     80
##   3rd    75     76
##   Crew  192     20
  1. Define your own print() method for objects having class [your GitHub username].

Defining a new generic

In the previous section, we defined a method for an existing generic function. Now, we will define a new generic function.

print_emoji <- function(x, ...) {
  UseMethod("print_emoji")
}

Since we haven’t defined any methods yet, the function won’t actually work!

print_emoji(starwars)
## Error in UseMethod("print_emoji"): no applicable method for 'print_emoji' applied to an object of class "c('beanumber', 'tbl_df', 'tbl', 'data.frame')"

However, we can start by writing a default method that will simply call the existing print() generic.

print_emoji.default <- function(x, ...) {
  print(x, ...)
}

Now our print_emoji() function will work.

print_emoji(starwars)
## Ben ❤️ s programming in R!
## # A tibble: 87 × 15
##    name        height  mass hair_…¹ skin_…² eye_c…³ birth…⁴ sex   gender homew…⁵
##    <chr>        <int> <dbl> <chr>   <chr>   <chr>     <dbl> <chr> <chr>  <chr>  
##  1 Luke Skywa…    172    77 blond   fair    blue       19   male  mascu… Tatooi…
##  2 C-3PO          167    75 <NA>    gold    yellow    112   none  mascu… Tatooi…
##  3 R2-D2           96    32 <NA>    white,… red        33   none  mascu… Naboo  
##  4 Darth Vader    202   136 none    white   yellow     41.9 male  mascu… Tatooi…
##  5 Leia Organa    150    49 brown   light   brown      19   fema… femin… Aldera…
##  6 Owen Lars      178   120 brown,… light   blue       52   male  mascu… Tatooi…
##  7 Beru White…    165    75 brown   light   blue       47   fema… femin… Tatooi…
##  8 R5-D4           97    32 <NA>    white,… red        NA   none  mascu… Tatooi…
##  9 Biggs Dark…    183    84 black   light   brown      24   male  mascu… Tatooi…
## 10 Obi-Wan Ke…    182    77 auburn… fair    blue-g…    57   male  mascu… Stewjon
## # … with 77 more rows, 5 more variables: species <chr>, films <list>,
## #   vehicles <list>, starships <list>, is_bald <lgl>, and abbreviated variable
## #   names ¹​hair_color, ²​skin_color, ³​eye_color, ⁴​birth_year, ⁵​homeworld

  1. In the default method for print_emoji(), can we use NextMethod() instead of print()? Why or why not?

  2. Write a print_emoji() method for data.frames.

Engagement

Prompt: Paste the code from your print_emoji() method to the #emojis channel. What does it do?