R function force

When to use force function?

R function arguments are Lazy Evaluated. It causes confuse when function return a function in for a loop.

f <- function(x) function() x

The function f will evaluate arguments in the end of the loop.

lf <- vector("list", 5)
for (i in seq(lf)) lf[[i]] <- f(i)
lf[[1]]()

## [1] 5

The force function make sure the argument evaluated in each loop.

f1 <- function(x) {
    force(x)
    function()x
}
for (i in seq(lf)) lf[[i]] <- f1(i)
lf[[1]]()

## [1] 1

The lapply function seems not lazy evaluated.

lr <- lapply(seq(5), f)
lr[[1]]()

## [1] 1

Ref: https://gist.github.com/peterhurford/d86331d8acdc99910de9