Create S3 generic return_()

[Bisaloo]

As discussed in #130.

Feel free to edit anything in any way you like before merging.

[Bisaloo]

Maybe it should not be exported 🤔

[nikosbosse]

Thanks a lot, this is really cool! 2 questions / comments

1. Pure learning-question: I haven't used S3-methods myself yet. Are they superior in some sense? Or would it be the same to have a function

return_ <- function(x) {
  if (data.table %in% class(x)) {
    return(x[])
  } else {
    return(x)
  }
}

2. There is an issue with the code as is: I think right now return_ doesn't cause function execution to end immediately. I think tests fail because sometimes, if there is a return statement in a function rather than at the end it will still execute the remainder of the code and then return NULL. Is there a way to fix this?

[Bisaloo]

Pure learning-question: I haven't used S3-methods myself yet. Are they superior in some sense? Or would it be the same to have a function

For the case at hand, I'd say it's equivalent. The main benefits (as far as I know) of using S3 in the general case:

• extensibility: once use defined the generic return_(), anybody can define their own method for their custom object. A good example for this is knitr::knit_print(). data.table, tibble, or really any other package can define how the objects they are creating will be printed in a Rmd document. And as you see with this example, methods can be defined in a different package than the generic, that's not an issue.
• automatic dispatch based on order of elements in the class attribute. Let's complexify a little the example code you posted in the previous comment with another method (on an unrelated note, it's better to use inherits() to test class inheritance):

f <- function(x) {
  if (inherits(x, "class1") {
    do1(x)
  } else if (inherits(x, "class2") {
    do2(x)
  } else {
    do3(x)
  }
}

Now what happens when you have an object x1 with class c("class1", "class2")? Since you start your if/else chain with class1, you would get action do1. And what about x2 with c("class2", "class1")? Well, exactly the same.

If we use a S3 system in this case, x1 will trigger do1 and x2 will trigger do2 because it's based on which element appears first. (Note that if we only define a method for class1, both x1 and x2 will trigger do1.)

[Bisaloo]

For the other point, I'll think about it. It might be the thing that I overlooked and that makes this impossible (hence why nobody suggested this idea).

[nikosbosse]

I'm closing this for now as we're not sure what to do with the return statements. Please feel free to open again!