R 向`NextMethod()`提供'；泛型'；参数有什么目的效果

我在试着理解NextMethod()的generic论点的目的是什么.

例如

# example data
x <- as.Date(c("2022-01-01", "2023-01-01", "1900-01-01", "2024-01-01")) 

# gives cumulative maximum as expected
cummax.Date <- function(x, ...) .Date(NextMethod(), cl = oldClass(x))
cummax(x)
#> [1] "2022-01-01" "2023-01-01" "2023-01-01" "2024-01-01"

# gives cumulative minimum as expected
cummin.Date <- function(x, ...) .Date(NextMethod(), cl = oldClass(x))
cummin(x)
#> [1] "2022-01-01" "2022-01-01" "1900-01-01" "1900-01-01"

最佳实践

Quite often、泛型被显式调用.

cummin.Date <- function(x, ...) .Date(NextMethod(generic = "cummin"), cl = oldClass(x))
cummin(x)
#> [1] "2022-01-01" "2022-01-01" "1900-01-01" "1900-01-01"

糟糕的做法--没有区别

但是，显式调用泛型不会影响结果，无论它是否存在.

# still returns the cumulative minimum
cummin.Date <- function(x, ...) .Date(NextMethod(generic = "cummax"), cl = oldClass(x))
cummin(x)
#> [1] "2022-01-01" "2022-01-01" "1900-01-01" "1900-01-01"

# still returns the cumulative minimum
cummin.Date <- function(x, ...) .Date(NextMethod(generic = "foobar"), cl = oldClass(x))
cummin(x)
#> [1] "2022-01-01" "2022-01-01" "1900-01-01" "1900-01-01"