Passing dots arguments through a target factory

[petrbouchal]

I am trying to build a target factory which is a combination of a file target and a target running a read function - it should take a path and a function as arguments, and return two targets: a file target and a the result of reading file using function.

e.g.

tar_read_data(data, "data.csv", read_csv, skip = 5)

where skip is passed via .... But I am completely stumped with the metaprogramming of passing ... into the expression for the second target. Is there a function in tarchetypes or elsewhere that I could learn from? I looked at tar_render() but was not able to make sense of how it works with ....

More generally, something along these lines might be a good addition in tarchetypes? I feel I use the pattern of these two targets quite often.

[wlandau]

rlang has nice functions for this, especially enexprs(). rlang::call2() almost fits, but base::as.call() may be a little better for the situation.

library(targets)

tar_script({
  tar_read_data <- function(name, file, reader, ...) {
    name_read <- deparse(rlang::enexpr(name))
    name_file <- paste0(name_read, "_file")
    sym_file <- rlang::sym(name_file)
    expr_read <- as.call(rlang::enexprs(reader, sym_file, ...))
    list(
      targets::tar_target_raw(name_file, file, format = "file"),
      targets::tar_target_raw(name_read, expr_read)
    )
  }

  tar_read_data(data, "data.csv", read_csv, skip = 5, col_types = cols())
})

tar_manifest(fields = all_of(c("command", "format")))
#> # A tibble: 2 × 3
#>   name      command                                             format
#>   <chr>     <chr>                                               <chr> 
#> 1 data_file "\"data.csv\""                                      file  
#> 2 data      "read_csv(data_file, skip = 5, col_types = cols())" rds

^{Created on 2022-01-18 by the reprex package (v2.0.1)}

Could be a nice addition to tarchetypes, with a clearer name and some refinement to accept other tar_target() arguments like storage and memory.

[petrbouchal]

Excellent, thank you!

For tarchetypes, a nice complement to this would be the analogue write factory, which would just return a single file target, but unlike tar_file would also take a fun and a … argument - but the gain in brevity would be less in this case.

[petrbouchal]

Which I now realise would be just a wrapper

[wlandau]

I just implemented tar_file_read(). Let's see how that goes for now.

[petrbouchal]

Thank you, that looks great.

Very much appreciate your responsiveness!