NextMethod: Call an Inherited Method

Description Usage Arguments Details Value Ambiguous Selection References See Also Examples

Description

A call to callNextMethod can only appear inside a method definition. It then results in a call to the first inherited method after the current method, with the arguments to the current method passed down to the next method. The value of that method call is the value of callNextMethod.

Usage

1

Arguments

...

Optionally, the arguments to the function in its next call (but note that the dispatch is as in the detailed description below; the arguments have no effect on selecting the next method.)

If no arguments are included in the call to callNextMethod, the effect is to call the method with the current arguments. See the detailed description for what this really means.

Calling with no arguments is often the natural way to use callNextMethod; see the examples.

Details

The ‘next’ method (i.e., the first inherited method) is defined to be that method which would have been called if the current method did not exist. This is more-or-less literally what happens: The current method (to be precise, the method with signature given by the defined slot of the method from which callNextMethod is called) is deleted from a copy of the methods for the current generic, and selectMethod is called to find the next method (the result is cached in the method object where the call occurred, so the search typically happens only once per session per combination of argument classes).

The next method is defined from the signature of the current method, not from the actual classes of the arguments. In particular, modifying any of the arguments has no effect on the selection. As a result, the selected next method can be called with invalid arguments if the calling function assigns objects of a different class before the callNextMethod() call. Be careful of any assignments to such arguments.

It is possible for the selection of the next method to be ambiguous, even though the original set of methods was consistent. See the section “Ambiguous Selection”.

The statement that the method is called with the current arguments is more precisely as follows. Arguments that were missing in the current call are still missing (remember that "missing" is a valid class in a method signature). For a formal argument, say x, that appears in the original call, there is a corresponding argument in the next method call equivalent to x = x. In effect, this means that the next method sees the same actual arguments, but arguments are evaluated only once.

Value

The value returned by the selected method.

Ambiguous Selection

There are two fairly common situations in which the choice of a next method is ambiguous, even when the original set of methods uniquely defines all method selection unambiguously. In these situations, callNextMethod() should be replaced, either by a call to a specific function or by recalling the generic with different arguments.

The most likely situation arises with methods for binary operators, typically through one of the group generic functions. See the example for class "rnum" below. Examples of this sort usually require three methods: two for the case that the first or the second argument comes from the class, and a third for the case that both arguments come from the class. If that last method uses callNextMethod, the other two methods are equally valid. The ambiguity is exactly the same that required defining the two-argument method in the first place.

In fact, the two possibilities are equally valid conceptually as well as formally. As in the example below, the logic of the application usually requires selecting a computation explicitly or else calling the generic function with modified arguments to select an appropriate method.

The other likely source of ambiguity arises from a class that inherits directly from more than one other class (a “mixin” in standard terminology). If the generic has methods corresponding to both superclasses, a method for the current class is again needed to resolve ambiguity. Using callNextMethod will again reimpose the ambiguity. Again, some explicit choice has to be made in the calling method instead.

These ambiguities are not the result of bad design, but they do require workarounds. Other ambiguities usually reflect inconsistencies in the tree of inheritances, such as a class appearing in more than one place among the superclasses. Such cases should be rare, but with the independent definition of classes in multiple packages, they can't be ruled out.

References

Chambers, John M. (2016) Extending R, Chapman & Hall. (Chapters 9 and 10.)

See Also

callGeneric to call the generic function with the current dispatch rules (typically for a group generic function); Methods_Details for the general behavior of method dispatch.

Examples

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## callNextMethod() used for the Math, Math2 group generic functions

## A class to automatically round numeric results to "d" digits

rnum <- setClass("rnum", slots = c(d = "integer"), contains = "numeric")

## Math functions operate on the rounded numbers, return a plain
## vector.  The next method will always be the default, usually a primitive.
setMethod("Math", "rnum",
          function(x)
              callNextMethod(round(as.numeric(x), x@d)))
setMethod("Math2", "rnum",
          function(x, digits)
              callNextMethod(round(as.numeric(x), x@d), digits))

## Examples of callNextMethod with two arguments in the signature.

## For arithmetic and one rnum with anything, callNextMethod with no arguments
## round the full accuracy result, and return as plain vector
setMethod("Arith", c(e1 ="rnum"),
          function(e1, e2)
              as.numeric(round(callNextMethod(), e1@d)))
setMethod("Arith", c(e2 ="rnum"),
          function(e1, e2)
              as.numeric(round(callNextMethod(), e2@d)))

## A method for BOTH arguments from "rnum" would be ambiguous
## for callNextMethod(): the two methods above are equally valid.
## The method chooses the smaller number of digits,
## and then calls the generic function, postponing the method selection
## until it's not ambiguous.
setMethod("Arith", c(e1 ="rnum", e2 = "rnum"),
          function(e1, e2) {
              if(e1@d <= e2@d)
                  callGeneric(e1, as.numeric(e2))
              else
                  callGeneric(as.numeric(e1), e2)
          })

## For comparisons, callNextMethod with the rounded arguments
setMethod("Compare", c(e1 = "rnum"),
          function(e1, e2)
              callNextMethod(round(e1, e1@d), round(e2, e1@d)))
setMethod("Compare", c(e2 = "rnum"),
          function(e1, e2)
              callNextMethod(round(e1, e2@d), round(e2, e2@d)))

## similarly to the Arith case, the method for two "rnum" objects
## can not unambiguously use callNextMethod().  Instead, we rely on
## The rnum() method inhertited from Math2 to return plain vectors.
setMethod("Compare", c(e1 ="rnum", e2 = "rnum"),
          function(e1, e2) {
              d <- min(e1@d, e2@d)
              callGeneric(round(e1, d), round(e2, d))
          })




set.seed(867)

x1 <- rnum(10*runif(5), d=1L)
x2 <- rnum(10*runif(5), d=2L)

x1+1
x2*2
x1-x2

## Simple examples to illustrate callNextMethod with and without arguments
B0 <- setClass("B0", slots = c(s0 = "numeric"))

## and a function to illustrate callNextMethod

f <- function(x, text = "default") {
    str(x) # print a summary
    paste(text, ":", class(x))
}

setGeneric("f")
setMethod("f", "B0", function(x, text = "B0") {
    cat("B0 method called with s0 =", x@s0, "\n")
    callNextMethod()
})

b0 <- B0(s0 = 1)

## call f() with 2 arguments: callNextMethod passes both to the default method
f(b0, "first test")

## call f() with 1 argument:  the default "B0" is not passed by callNextMethod
f(b0)

## Now, a class that extends B0, with no methods for f()
B1 <- setClass("B1", slots = c(s1 = "character"), contains = "B0")
b1 <- B1(s0 = 2, s1 = "Testing B1")

## the two cases work as before, by inheriting the "B0" method

f(b1, b1@s1)

f(b1)

B2 <- setClass("B2", contains = "B1")

## And, a method for "B2" that calls with explicit arguments.
## Note that the method selection in callNextMethod
## uses the class of the *argument* to consistently select the "B0" method

setMethod("f", "B2", function(x, text = "B1 method") {
    y <- B1(s0 = -x@s0, s1 ="Modified x")
    callNextMethod(y, text)
})

b2 <- B2(s1 = "Testing B2", s0 = 10)

f(b2, b2@s1)

f(b2)


## Be careful:  the argument passed must be legal for the method selected
## Although the argument here is numeric, it's still the "B0" method that's called
setMethod("f", "B2", function(x, text = "B1 method") {
    callNextMethod(x@s0, text)
})

##  Now the call will cause an error:

tryCatch(f(b2), error = function(e) cat(e$message,"\n"))

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