[R-Forge #2619] Scoping for LHS in `:=`

Submitted by: michael neslon; Assigned to: Nobody; R-Forge link

In trying to work a reshape to wide format procedure using data.table and :=

I've stumbled as the LHS of

DT[, c(a) :=b]

does not evaluate in the same scope as the rest of j (including the RHS)
and it doesn't recognize data.table constants like .N or .BY

For example

DT <- data.table(a = letters[1:4], b = letters[1:2], d = 1:8)

DTw <- DT[,.N,by=list(a,b)][, ab := paste0(a,b)]

I'd like to be reshape this to wide using something like

DTw[, c(ab) := N, by = ab]

or

DTw[, .BY := N, by = ab]

both of which will search the global environment for ab and .BY

Currently I can do something like

setkey(DT, ab)

for(nn in DTw[['ab']]){
pp <- DTw[list(nn), list(N)]
DTw[list(nn), c(nn) := pp[['N']]]
}

Start with the DT[, c(a) := b] and DTw[, c(ab) := N, by = ab] examples in the issue, then trace how := evaluates its left-hand side versus j and the right-hand side. Compare the desired column and .BY scoping with the current global-environment lookup; done means the proposed reshape expressions resolve those names in the intended data.table scope.