Using setDT after loading

Reproduce the supplied R script using setDT, setalloccol, and := after save/load, starting from the documented behavior of these entry points. Determine whether the observed mutation is expected or a data.table defect, then add a focused regression test or clarify the behavior in the project’s documentation.

Thanks for the data.table package. I'm a new user and I've found it useful so far, but one thing I'm having trouble with is using setDT or setalloccol after loading from disk, in order to be able to use :=. I use those functions but := still appears not to work. I may be misunderstanding how those functions are meant to be used.

Example:

library(data.table)

L <- vector("list", length = 2)
names(L) <- c("A1", "A2")

for (A in c("A1", "A2")) {
  L[[A]] <- vector("list", length = 2)
  names(L[[A]]) <- c("B1", "B2")
}

for (A in c("A1", "A2")) {
  for (B in c("B1", "B2")) {
    L[[A]][[B]] <- data.table(u = 1:3, v = 2:4)
  }
}

L[["A1"]][["B1"]][, w := 0]  # this works

L

## $A1
## $A1$B1
##    u v w
## 1: 1 2 0
## 2: 2 3 0
## 3: 3 4 0

## $A1$B2
##    u v
## 1: 1 2
## 2: 2 3
## 3: 3 4


## $A2
## $A2$B1
##    u v
## 1: 1 2
## 2: 2 3
## 3: 3 4

## $A2$B2
##    u v
## 1: 1 2
## 2: 2 3
## 3: 3 4

save(L, file = "~/L.RData")

rm(L)

load("~/L.RData")

for (A in c("A1", "A2")) {
  for (B in c("B1", "B2")) {
    setDT(L[[A]][[B]])
  }
}

L[["A1"]][["B1"]][, x := 99]

L  # x doesn't appear

## $A1
## $A1$B1
##    u v w
## 1: 1 2 0
## 2: 2 3 0
## 3: 3 4 0

## $A1$B2
##    u v
## 1: 1 2
## 2: 2 3
## 3: 3 4


## $A2
## $A2$B1
##    u v
## 1: 1 2
## 2: 2 3
## 3: 3 4

## $A2$B2
##    u v
## 1: 1 2
## 2: 2 3
## 3: 3 4

# Try using setalloccol instead of setDT

for (A in c("A1", "A2")) {
  for (B in c("B1", "B2")) {
    setalloccol(L[[A]][[B]])
  }
}

L[["A1"]][["B1"]][, x := 99]

L  # x doesn't appear

## $A1
## $A1$B1
##    u v w
## 1: 1 2 0
## 2: 2 3 0
## 3: 3 4 0

## $A1$B2
##    u v
## 1: 1 2
## 2: 2 3
## 3: 3 4


## $A2
## $A2$B1
##    u v
## 1: 1 2
## 2: 2 3
## 3: 3 4

## $A2$B2
##    u v
## 1: 1 2
## 2: 2 3
## 3: 3 4

sessionInfo()

## R version 4.0.3 (2020-10-10)
## Platform: x86_64-w64-mingw32/x64 (64-bit)
## Running under: Windows 10 x64 (build 18363)

## Matrix products: default

## locale:
## [1] LC_COLLATE=English_Australia.1252  LC_CTYPE=English_Australia.1252
## [3] LC_MONETARY=English_Australia.1252 LC_NUMERIC=C
## [5] LC_TIME=English_Australia.1252

## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base

## other attached packages:
## [1] data.table_1.13.4

## loaded via a namespace (and not attached):
## [1] compiler_4.0.3

Please let me know if I've misunderstood something. Thank you.