[datatable-help] Problem with FAQ 2.8

[Gabor Grothendieck]

FAQ 2.8 says:
2.8 What are the scoping rules for j expressions?
Think of the subset as an environment where all the column names are
variables. When a variable
foo is used in the j of a query such as X[Y,sum(foo)], foo is looked
for in the following order :
1. The scope of X's subset; i.e., X's column names.
2. The scope of each row of Y; i.e., Y's column names (join inherited scope)
...
but consider the following (which is modified from this example:
https://r-forge.r-project.org/tracker/?func=detail&atid=975&aid=1663&group_id=240):

> d1 <- data.table(id1 = c(1L, 2L, 2L, 3L), val = 1:4, key = "id1")
> d2 <- data.table(id2 = c(1L, 2L, 4L), val2 = c(11, 12, 14),key = "id2")
>
> d1[d2, sum(id2 * val)]
   id1 V1
1:   1  1
2:   2 10
3:   4 NA
>
> d1[d2, sum(id1 * val)]
Error in `[.data.table`(d1, d2, sum(id1 * val)) : object 'id1' not found

Note that column id1 of d1 is not in scope contrary to point 1.

Even stranger is that d1[, id1] works but d1[d2, id1] does not.

Is the FAQ describing how its supposed to work and the actual behavior
is wrong or is the behavior as intended and the FAQ wrong?



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