s4 dispatching

dispatch.R

bin <- function(x)x

setGeneric("bin", function(x) standardGeneric("bin"))
setMethod("bin", "matrix", function(x)x)

i <- 1:10
m <- matrix(1:10, nrow=2)

showMethods("bin")
# Function: bin (package .GlobalEnv)
# x="ANY"
# x="matrix"

bin2 <- function(x)x

f1 <- function(x) { for(i in x)bin(x) }
f2 <- function(x) { for(i in x)bin2(x) }
f3 <- function(x) { .bin <- selectMethod("bin", "ANY"); for(i in x).bin(x) }

x <- 1:1e5

library("microbenchmark")
options(digits=4)
microbenchmark(always=f1(x), never=f2(x), once=f3(x))
# Unit: milliseconds
#    expr   min    lq   mean median     uq    max neval
#  always 91.89 99.10 106.79 100.23 115.85 146.54   100
#   never 25.22 27.12  28.94  27.59  30.13  48.34   100
#    once 24.30 25.89  27.37  26.14  28.61  36.89   100