Why does `substitute` work in multiple lines, but not in a single line?

Question

I was attempting to answer this nice question about creating a non-standard evaluating function for a data.table object, doing a grouped sum. Akrun came up with a lovely answer which I'll simplify here:

akrun <- function(data, var, group){
 var <- substitute(var)
 group <- substitute(group)
 data[, sum(eval(var)), by = group]
}

library(data.table)
mt = as.data.table(mtcars)
akrun(mt, cyl, mpg)
#    group    V1
# 1:     6 138.2
# 2:     4 293.3
# 3:     8 211.4

I was also working on an answer, and had close to the same answer, but with the substitutes inline with the rest. Mine results in an error:

gregor = function(data, var, group) {
  data[, sum(eval(substitute(var))), by = substitute(group)]
} 

gregor(mt, mpg, cyl)
# Error in `[.data.table`(data, , sum(eval(substitute(var))), by = substitute(group)) : 
#  'by' or 'keyby' must evaluate to vector or list of vectors 
#  (where 'list' includes data.table and data.frame which are lists, too) 

At its face, my function is a simple substitution of Akrun's. Why doesn't it work?

---

Note that both substitutions cause problems, as shown here:

gregor_1 = function(data, var, group) {
  var = substitute(var)
  data[,sum(eval(var)), 
       by = substitute(group)]
} 
gregor_1(mt, mpg, cyl)
# Same error as above


gregor_2 = function(data, var, group) {
  group = substitute(group)
  data[,sum(eval(substitute(var))), 
       by = group]
} 
gregor_2(mt, mpg, cyl)
# Error in eval(substitute(var)) : object 'mpg' not found 

Answer 1

In substitute's documentation you can read how it decides what to substitute, and the fact that, by default, it searches the environment where it is called. If you call substitute inside the data.table frame (i.e. inside []) it won't be able to find the symbols because they are not present inside the data.table evaluation environment, they are in the environment where [ was called.

You can "invert" the order in which the functions are called in order to get the behavior you want:

library(data.table)

foo <- function(dt, group, var) {
    eval(substitute(dt[, sum(var), by = group]))
}

foo(as.data.table(mtcars), cyl, mpg)
   cyl    V1
1:   6 138.2
2:   4 293.3
3:   8 211.4