Multiple inheritance for R6 classes

Question

Actual question

What are my options to workaround the fact that R6 does not support multiple inheritance?

Disclaimer

I know that R is primarily a fun

Answer 1

For those interested:

I gave it a second thought and realized that's it's not really multiple inheritance per se that I want/need, but rather some sort of better mimicking the use of interfaces/abstract classes without giving up inherit for that.

So I tried tweaking R6 a bit so it would allow me to distinguish between inherit and implement in a call to R6Class.

Probably tons of reasons why this is a bad idea, but for now, it gets the job done ;-)

You can install the tweaked version from my forked branch.

Example

devtools::install_github("rappster/R6", ref = "feat_interface")
library(R6)

Correct implementation of interface and "standard inheritance":

IFoo <- R6Class("IFoo",
  public = list(foo = function() stop("I'm the inferace method"))
)
BaseClass <- R6Class("BaseClass",
  public = list(foo = function(n = 1) private$x[1:n])
)
Foo <- R6Class("Foo", implement = IFoo, inherit = BaseClass,
  private = list(x = letters)
)

> Foo$new()

  Implements interface: 
  Inherits from: 
  Public:
    clone: function (deep = FALSE) 
    foo: function (n = 1) 
  Private:
    x: a b c d e f g h i j k l m n o p q r s t u v w x y z

When an interface is not implemented correctly (i.e. method not implemented):

 Bar <- R6Class("Bar", implement = IFoo,
    private = list(x = letters)
  )
> Bar$new()
Error in Bar$new() : 

Non-implemented interface method: foo

Proof of concept for dependency injection

This is a little draft that elaborates a bit on the motivation and possible implementation approaches for interfaces and inversion of dependency in R6.