eval doesn't work for `.` in `by`

[eantonya]

d = data.table(a = 1)
q = quote(.(a))

d[, eval(q)]
#   a
#1: 1

d[, 1, by = .(a)]
#   a V1
#1: 1  1

# the problem
d[, 1, by = eval(q)]
#Error in eval(expr, envir, enclos) : could not find function "."

[franknarf1]

Somehow this is happening again in this SO example: https://stackoverflow.com/q/57253159/1191259

library(data.table)
dt_mtcars <- as.data.table(mtcars)
group = ".(cyl, gear)"
dt_mtcars[, .(mean_mpg = mean(mpg), median_mpg = median(mpg)), by=eval(parse(text=group))]
# Error in .(cyl, gear) : could not find function "."

[jangorecki]

parse return an expression call, it will work by adding [[1L]]

dt_mtcars[, .(mean_mpg = mean(mpg), median_mpg = median(mpg)), by=eval(parse(text=group)[[1L]])]

I don't think it qualify for re-opening, it is easy to investigate what is being passed to eval.