Forcing specific data types as arguments to a function

Question

I was just wondering if there was a way to force a function to only accept certain data types, without having to check for it within the function; or, is this not possible because R's type-checking is done at runtime (as opposed to those programming languages, such as Java, where type-checking is done during compilation)?

For example, in Java, you have to specify a data type:

class t2 {
    public int addone (int n) {
        return n+1;
    }
}

In R, a similar function might be

addone <- function(n)
{
    return(n+1)
}

but if a vector is supplied, a vector will (obviously) be returned. If you only want a single integer to be accepted, then is the only way to do to have a condition within the function, along the lines of

addone <- function(n)
{
  if(is.vector(n) && length(n)==1)
  {
    return(n+1)
  } else
  {
    return ("You must enter a single integer")
  }
}

Thanks,
Chris

Answer 1

This is entirely possible using S3 classes. Your example is somewhat contrived in the context or R, since I can't think of a practical reason why one would want to create a class of a single value. Nonetheless, this is possible. As an added bonus, I demonstrate how the function addone can be used to add the value of one to numeric vectors (trivial) and character vectors (so A turns to B, etc.):

Start by creating a generic S3 method for addone, utlising the S3 despatch mechanism UseMethod:

addone <- function(x){
  UseMethod("addone", x)
}

Next, create the contrived class single, defined as the first element of whatever is passed to it:

as.single <- function(x){
  ret <- unlist(x)[1]
  class(ret) <- "single"
  ret
}

Now create methods to handle the various classes. The default method will be called unless a specific class is defined:

addone.default <- function(x) x + 1
addone.character <- function(x)rawToChar(as.raw(as.numeric(charToRaw(x))+1))
addone.single <- function(x)x + 1

Finally, test it with some sample data:

addone(1:5)
[1] 2 3 4 5 6

addone(as.single(1:5))
[1] 2
attr(,"class")
[1] "single"

addone("abc")
[1] "bcd"

---

Some additional information:

1. Hadley's devtools wiki is a valuable source of information on all things, including the S3 object system.

2. The S3 method doesn't provide strict typing. It can quite easily be abused. For stricter object orientation, have a look at S4 classes, reference based classesor the proto package for Prototype object-based programming.