Dot dot prefix with expression instead of symbol

[christianhomberg]

I like the .. notation (new feature #18 in https://github.com/Rdatatable/data.table/blob/master/NEWS.md) and the idea of eventually replacing with = F. For now, I can't find a way to get around with = F when using expressions in j, e.g.:

require(magrittr)
require(data.table)

iris_dt = iris %>% setDT
iris_cols = iris_dt %>% colnames
iris_dt[, iris_cols %>% first, with = F]

I tried unsuccessfully and would suggest one of the following syntax:

iris_dt[, ..(iris_cols %>% first)]
iris_dt[, ..{iris_cols %>% first}]

Output of sessionInfo():

``` R version 3.4.4 (2018-03-15) Platform: x86_64-pc-linux-gnu (64-bit) Running under: Ubuntu 18.04 LTS

Matrix products: default
BLAS: /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.7.1
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.7.1

locale:
[1] LC_CTYPE=en_GB.UTF-8 LC_NUMERIC=C LC_TIME=en_GB.UTF-8
[4] LC_COLLATE=en_GB.UTF-8 LC_MONETARY=en_GB.UTF-8 LC_MESSAGES=en_GB.UTF-8
[7] LC_PAPER=en_GB.UTF-8 LC_NAME=C LC_ADDRESS=C
[10] LC_TELEPHONE=C LC_MEASUREMENT=en_GB.UTF-8 LC_IDENTIFICATION=C

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

other attached packages:
[1] markovchain_0.6.9.8-1 shiny_1.0.5 data.table_1.11.0 magrittr_1.5
[5] ggplot2_2.2.1

loaded via a namespace (and not attached):
[1] Rcpp_0.12.16 compiler_3.4.4 pillar_1.2.2 later_0.7.2
[5] plyr_1.8.4 tools_3.4.4 digest_0.6.15 packrat_0.4.9-2
[9] lattice_0.20-35 jsonlite_1.5 evaluate_0.10.1 tibble_1.4.2
[13] gtable_0.2.0 pkgconfig_2.0.1 rlang_0.2.0.9001 Matrix_1.2-14
[17] igraph_1.2.1 parallel_3.4.4 yaml_2.1.19 expm_0.999-2
[21] stringr_1.3.0 knitr_1.20 stats4_3.4.4 rprojroot_1.3-2
[25] grid_3.4.4 flexdashboard_0.5.1 R6_2.2.2 rmarkdown_1.9
[29] matlab_1.0.2 purrr_0.2.4 codetools_0.2-15 backports_1.1.2
[33] scales_0.5.0 promises_1.0.1 htmltools_0.3.6 mime_0.5
[37] xtable_1.8-2 colorspace_1.3-2 httpuv_1.4.2 stringi_1.2.2
[41] RcppParallel_4.4.0 lazyeval_0.2.1 munsell_0.4.3

</details>

[jangorecki]

Related #2826

[MichaelChirico]

I really like the way that looks, but unfortunately it won't parse (so AFAICT it's impossible to work)

Comparse substitute(..var_name) (fine) with substitute(..{expression}) (errors: "unexpected {")

..(expression) will parse... might be easier to test whether to evaluate through is.call as well. Doesn't look as natural as ..{} IMO

[jangorecki]

Using the api proposed in #4304 this operation could be generalized into

iris_dt[, v, env = list(v = I(first(iris_cols)))]

# then it will work as
iris_dt[, "Sepal.Length"]
# if you will skip `I()` call above then it will get translated into
iris_dt[, Sepal.Length]

As there is no way to handle ..{} I think we can close your request now, eventually ..() could be supported in future.