An Object Oriented System Meant to Become a Successor to S3 and S4.

S7

The S7 package is a new OOP system designed to be a successor to S3 and S4. It has been designed and implemented collaboratively by the R Consortium Object-Oriented Programming Working Group, which includes representatives from R-Core, Bioconductor, the tidyverse/Posit, and the wider R community.

S7 is somewhat experimental; we are confident in the design but it has relatively little usage in the wild currently. We hope to avoid any major breaking changes, but reserve the right if we discover major problems.

Installation

The long-term goal of this project is to merge S7 in to base R. For now, you can experiment by installing it from CRAN:

install.packages("S7")

Usage

This section gives a very brief overview of the entirety of S7. Learn more of the basics in vignette("S7"), generics and methods in vignette("generics-methods"), classes and objects in vignette("classes-objects"), and compatibility with S3 and S4 in vignette("compatibility").

library(S7)

Classes and objects

S7 classes have a formal definition, which includes a list of properties and an optional validator. Use new_class() to define a class:

range <- new_class("range",
  properties = list(
    start = class_double,
    end = class_double
  ),
  validator = function(self) {
    if (length(self@start) != 1) {
      "@start must be length 1"
    } else if (length(self@end) != 1) {
      "@end must be length 1"
    } else if (self@end < self@start) {
      "@end must be greater than or equal to @start"
    }
  }
)

new_class() returns the class object, which is also the constructor you use to create instances of the class:

x <- range(start = 1, end = 10)
x
#> <range>
#>  @ start: num 1
#>  @ end  : num 10

Properties

The data possessed by an object is called its properties. Use @ to get and set properties:

x@start
#> [1] 1
x@end <- 20
x
#> <range>
#>  @ start: num 1
#>  @ end  : num 20

Properties are automatically validated against the type declared in new_class() (double in this case), and with the class validator:

x@end <- "x"
#> Error: <range>@end must be <double>, not <character>
x@end <- -1
#> Error: <range> object is invalid:
#> - @end must be greater than or equal to @start

Generics and methods

Like S3 and S4, S7 uses functional OOP where methods belong to generic functions, and method calls look like all other function calls: generic(object, arg2, arg3). This style is called functional because from the outside it looks like a regular function call, and internally the components are also functions.

Use new_generic() to create a new generic: the first argument is the generic name (used in error messages) and the second gives the arguments used for dispatch. The third, and optional argument, supplies the body of the generic. This is only needed if your generic has additional arguments that aren’t used for method dispatch.

inside <- new_generic("inside", "x")

Once you have a generic, you can define a method for a specific class with method<-:

# Add a method for our class
method(inside, range) <- function(x, y) {
  y >= x@start & y <= x@end
}

inside(x, c(0, 5, 10, 15))
#> [1] FALSE  TRUE  TRUE  TRUE

You can use method<- to register methods for base types on S7 generics, and S7 classes on S3 or S4 generics. See vignette("compatibility") for more details.