Hi,
there seem to be some (small) bugs in the mclapply function in parallel.
I discovered this in the current R release version, and I checked that it is
still present in R-devel.
I think it only occurs in the part of the code corresponding to argument option
mc.preschedule = FALSE.
Here are two examples:
a)
library(parallel)
mclapply(list(), identity, mc.preschedule=FALSE)
Error in sum(sapply(res, inherits, "try-error")) :
invalid 'type' (list) of argument
Possible reason / fixes:
The relevant portion of the code is this:
if (!mc.preschedule) {
FUN <- match.fun(FUN)
if (length(X) <= cores) {
jobs <- lapply(seq_along(X), function(i) mcparallel(FUN(X[[i]],
...), name = names(X)[i], mc.set.seed = mc.set.seed,
silent = mc.silent))
res <- mccollect(jobs)
if (length(res) == length(X))
names(res) <- names(X)
has.errors <- sum(sapply(res, inherits, "try-error"))
}
If there are 0 jobs, mccollect returns NULL, sapply on this returns the empty
list, leads to an error in sum.
Possible fix might be to use vapply with FUN.VALUE instead.
b)
library(parallel)
foo <- function(i) NULL
mclapply(1, foo, mc.preschedule=FALSE)
Error in sum(sapply(res, inherits, "try-error")) :
invalid 'type' (list) of argument
mclapply(1:2, foo, mc.preschedule=FALSE)
$`17222`
NULL
# should be list(NULL, NULL)
Possible reason / fixes:
The bug here is more complicated I think, and occurs in mccollect.
By using debug, I ended up in this line in mccollect:
res[[which(pid == pids)]] <- unserialize(r)
Here, the result of foo (=NULL) is not entered in the result list "res", because
"<- NULL" is sementically interpreted as "delete list element".
(which has btw trapped me quite often, too...)
Therefore mccollect does not return a list of NULLs as it should, but only
list(), which leads to the same error as in a).
In the case mclapply(1:2, foo, mc.preschedule=FALSE) something even more
complicated happens, as on.exit we jump into a "cleanup" function which again
calls mccollect. Anyway, the final result is wrong, too.
Possible fix:
Actually I am not sure what the best way here is, except for an extra if-case to
avoid this when the assigment happens?
Best,
Bernd
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