Where shall one put the ellipsis in S3 function overloads?

user_653152 0
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We have an S3 class for which we define plot and other generic functions. We are not sure where the ... has to go. There are two options:

  1. plot.hadronacf(x, col = "black", ...)
  2. plot.hadronacf(x, ..., col = "black")

Similarly also for print. and summary..

In the usage of summary it seems to be inconsistent:

summary(object, ...)

## Default S3 method:
summary(object, ..., digits)
## S3 method for class 'data.frame'
summary(object, maxsum = 7,
       digits = max(3, getOption("digits")-3), ...)

## S3 method for class 'factor'
summary(object, maxsum = 100, ...)

## S3 method for class 'matrix'
summary(object, ...)

## S3 method for class 'summaryDefault'
format(x, digits = max(3L, getOption("digits") - 3L), ...)
 ## S3 method for class 'summaryDefault'
print(x, digits = max(3L, getOption("digits") - 3L), ...)

For print it seems that the ellipsis goes to the end:

print(x, ...)

## S3 method for class 'factor'
print(x, quote = FALSE, max.levels = NULL,
      width = getOption("width"), ...)

## S3 method for class 'table'
print(x, digits = getOption("digits"), quote = FALSE,
      na.print = "", zero.print = "0",
      right = is.numeric(x) || is.complex(x),
      justify = "none", ...)

## S3 method for class 'function'
print(x, useSource = TRUE, ...)

It seems that ellipsis at the end is used in the majority. Is there some guideline for this?

r-s3 r
1 Answers

There's no "right" way to do this. It's a matter of preference or a design decision based on what you think the function should do with "extra parameters". For example with the two variations A and B

summary(object, maxsum = 100, ...)  # A
summary(object, ..., maxsum = 100)  # B

The only way you can pass a maxsum to version B is by a named parameter in the function call. Whereas, version A would take the second unnamed parameter and pass it to maxsum. They differ in how "important" that parameter is to the function call.

user_23720640
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