Lag argument from variable fails when grouped

[dakvid]

Using a variable to control the lag (or lead) shift fails when in a group:

library(data.table)
library(dplyr)
library(dtplyr)

LAG_ROWS <- 2L

data.table(g = rep(0,4), a = 1:4) |>
  lazy_dt() |>
  mutate(b = lag(a, n = LAG_ROWS))
# Source: local data table [4x3]
# Call:   copy(`_DT1`)[, `:=`(b = shift(a, n = ..LAG_ROWS, type = "lag"))]
#
#       g     a     b
#   <dbl> <int> <int>
# 1     0     1    NA
# 2     0     2    NA
# 3     0     3     1
# 4     0     4     2

data.table(g = rep(0,4), a = 1:4) |>
  lazy_dt() |>
  group_by(g) |>
  mutate(b = lag(a, n = LAG_ROWS))
# Error in eval(q[[i]], parent.frame(2L)) : object '..LAG_ROWS' not found

I think I understand what's happening in as far as dtplyr is translating to

data.table(g = rep(0,4), a = 1:4)[, `:=`(b = shift(a, n = ..LAG_ROWS, type = "lag")), by = .(g)]
# Error in eval(q[[i]], parent.frame(2L)) : object '..LAG_ROWS' not found

But if I drop the .. it works:

data.table(g = rep(0,4), a = 1:4)[, `:=`(b = shift(a, n = LAG_ROWS, type = "lag")), by = .(g)] |> print()
#        g     a     b
#    <num> <int> <int>
# 1:     0     1    NA
# 2:     0     2    NA
# 3:     0     3     1
# 4:     0     4     2

I can work around it by using the injection operator, but obviously it would be nice to have it working without that. :)

data.table(g = rep(0,4), a = 1:4) |>
  lazy_dt() |>
  group_by(g) |>
   mutate(b = lag(a, n = !!LAG_ROWS))
# Source: local data table [4 x 3]
# Groups: g
# Call:   copy(`_DT3`)[, `:=`(b = shift(a, n = 2L, type = "lag")), by = .(g)]
# 
#       g     a     b
#   <dbl> <int> <int>
# 1     0     1    NA
# 2     0     2    NA
# 3     0     3     1
# 4     0     4     2

sessionInfo()
# R version 4.5.1 (2025-06-13)
# Platform: aarch64-apple-darwin20
# Running under: macOS Sequoia 15.6.1
# 
# Matrix products: default
# BLAS:   /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib 
# LAPACK: /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/lib/libRlapack.dylib;  LAPACK version 3.12.1
# 
# locale:
# [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
# 
# time zone: Pacific/Auckland
# tzcode source: internal
# 
# attached base packages:
# [1] stats     graphics  grDevices utils     datasets  methods   base     
# 
# other attached packages:
# [1] dplyr_1.1.4       dtplyr_1.3.2      data.table_1.17.8
# 
# loaded via a namespace (and not attached):
#  [1] R6_2.6.1          tidyselect_1.2.1  magrittr_2.0.4    glue_1.8.0        tibble_3.3.0      pkgconfig_2.0.3   generics_0.1.4    lifecycle_1.0.4   cli_3.6.5         vctrs_0.6.5      
# [11] withr_3.0.2       compiler_4.5.1    rstudioapi_0.17.1 tools_4.5.1       pillar_1.11.1     rlang_1.1.6