Actually, it depends on the number of cores:
> fun1 <- function(c){function(i){c*i}}
> fun2 <- function(f) f(2)
> sapply(mclapply(1:4, fun1, mc.cores=1L), fun2)
[1] 8 8 8 8
> sapply(mclapply(1:4, fun1, mc.cores=2L), fun2)
[1] 6 8 6 8
> sapply(mclapply(1:4, fun1, mc.cores=4L), fun2)
[1] 2 4 6 8
> >/ On Feb 24, 2015, at 10:50 AM, <luke-tierney at uiowa.edu
> ><https://stat.ethz.ch/mailman/listinfo/r-devel>> wrote:
> />/
> />/ The documentation is not specific enough on the indented semantics in
> />/ this situation to consider this a bug. The original R-level
> />/ implementation of lapply was
> />/
> />/ lapply <- function(X, FUN, ...) {
> />/ FUN <- match.fun(FUN)
> />/ if (!is.list(X))
> />/ X <- as.list(X)
> />/ rval <- vector("list", length(X))
> />/ for(i in seq(along = X))
> />/ rval[i] <- list(FUN(X[[i]], ...))
> />/ names(rval) <- names(X) # keep `names' !
> />/ return(rval)
> />/ }
> />/
> />/ and the current internal implementation is consistent with this. With
> />/ a loop like this lazy evaluation and binding assignment interact in
> />/ this way; the force() function was introduced to help with this.
> />/
> />/ That said, the expression FUN(X[[i]], ...) could be replaced by
> />/
> />/ local({
> />/ i <- i
> />/ list(FUN(X[[i]], ...)
> />/ })
> />/
> />/ which would produce the more desirable result
> />/
> />/ > sapply(test, function(myfn) myfn(2))
> />/ [1] 2 4 6 8
> />/
> /
> Would the same semantics be applied to parallel::mclapply and friends?
>
> sapply(lapply(1:4, function(c){function(i){c*i}}), function(f) f(2))
>
> # [1] 8 8 8 8
>
> sapply(mclapply(1:4, function(c){function(i){c*i}}), function(f) f(2))
>
> # [1] 6 8 6 8
>
> I understand why they differ, but making mclapply easier for 'drop-in'
> parallelism might be a good thing.
>
> Michael
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