Writing a decorator for R functions

The trace function in R does that. See ?trace.

my github package tag attempts to tackle this issue.

Your example could be solved as follows :

# remotes::install_github("moodymudskipper/tag")
library(tag)
deco <- tag(args = list(.first = NULL, .last = NULL), pattern = {
  t_args <- T_ARGS()                       # fetch arguments fed to tag
  eval.parent(t_args[[".first"]])          # run .first arg
  on.exit(eval.parent(t_args[[".last"]]))  # run .last arg on exit
  CALL()                                   # run main call
})


foo <- function(x, y) {Sys.sleep(1); x + y} # sleep 1 sec to highlight expected behavior

deco(quote(message("calling foo")), quote(message("done")))$foo(1, 2)
#> calling foo
#> done
#> [1] 3

foo2 <- deco(quote(message("calling foo")), quote(message("done")))$foo
foo2(1, 2)
#> calling foo
#> done
#> [1] 3

deco2 <- deco(quote(message("calling foo")), quote(message("done")))
deco2$foo(1, 2)
#> calling foo
#> done
#> [1] 3

^{Created on 2020-01-30 by the reprex package (v0.3.0)}

tags are function operator factories (or adverb factories), here deco is a tag, and deco(quote(message("calling foo")), quote(message("done"))) is an adverb, with a method for $. It means you could run deco(quote(message("calling foo")), quote(message("done")))(foo)(1,2), but the dollar notation makes it friendlier.

The tag definition features defaults arguments (default value is mandatory, dots aren't supported), and a pattern which is a bit like your new body, using special functions T_ARGS(), F_ARGS(), F_ARGS(), F_FORMALS() and CALL() to access the tag or function's arguments or formals and the call itself (see ?tag::CALL).

Some more magic is implemented so the tag's argument can be given to the tagged function itself, so the following can be done too :

deco$foo(1, 2, quote(message("calling foo")), quote(message("done")))
#> calling foo
#> done
#> [1] 3
foo2 <- deco$foo
foo2(1, 2, quote(message("calling foo")), quote(message("done")))
#> calling foo
#> done
#> [1] 3

In these cases you can enjoy the autocomplete in RStudio :

More info : https://github.com/moodymudskipper/tag The package tags contains a collection of such "decorators"