compact1 <- function(x) Filter(length, x)
compact2 <- function(x) x[as.logical(lengths(x))]
compact3 <- function(x) x[lengths(x) > 0L]

x <- list(1:100, NULL, NULL, NULL, 1:100)
bench::mark(
  compact1(x),
  compact2(x),
  compact3(x),
  mlr3misc::discard(x, is.null),
  iterations = 100
)
#> # A tibble: 4 × 6
#>   expression                         min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>                    <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 compact1(x)                     2.83µs   3.08µs    14038.        0B        0
#> 2 compact2(x)                   492.03ns 573.99ns   117142.        0B        0
#> 3 compact3(x)                   491.97ns 573.99ns   143186.        0B        0
#> 4 mlr3misc::discard(x, is.null)   3.77µs   4.08µs   230836.    1.75MB        0