S4 method for `[` not found in SNOW parallel processes

[jorainer]

Dear all,

I had an issue with S4 [ methods and parallel processing with SnowParam: the S4 method is not found if the respective package is not specifically loaded first. Details and a description are given in this issue I opened in S4Vectors.

[Jiefei-Wang]

Thanks for reporting, it looks like this problem is related to R lazy evaluation, it is not a BiocParallel issue nor S4Vector. Here is a simple reprex:

library(S4Vectors)
## Just a simple list is not enough, you must bury it deeply.
x <- list(list(X=DataFrame(a = 1:4, b = "a")))
saveRDS(x, "test.rds")

## Open in a fresh R session without loading S4Vector
## There is no package load at this point
x <- readRDS("test.rds")

## First try, package loaded, but subset fails
x[[1]]$X[1,]

## Second try, the package has been loaded from the previous line, subset succeeds
x[[1]]$X[1,]

While I believe we can fix this issue on our side(by adding force?), do you think we should send this issue to R developer @mtmorgan ?