How to use tidy eval NSE to expand a `an expression`

Question

I want to expand the !!! expression just like they do in dplyr-verbs e.g.

aggregate_expressions <- list(n = quote(n()))

do_something(iris, !!!(aggregate_expressions))

and say I want do_something to perform

do_something <- function(...) {
  iris %>%
    some_function( # expand the ... # ) # some_function(n = n())
}

which will do this but the n = n() is dynamic

do_something <- function(...) {
  iris %>%
    some_function(n = n())
}

I tried to trace the code for dplyr::summarise and I see that enquos(...) which converts the ... to a list of quosure, but then how do I apply the quosures? I think I am meant to create the code summarise(n = n()) from the quosure and then evaluate i using eval_tidy, but I can't figure out how to generate the code. I know that pass ... summarise works but the actual use case is to pass it to summarise.disk.frame which means I can't just reuse dplyr::summarise

The actual case i not

For example in dplyr, the below works by expanding the aggregate_expression using !!!

  aggregate_expressions <- list(n = quote(n()))

  iris %>%    
    group_by(Species) %>%
    summarise(!!!(aggregate_expressions))

do_something <- function(...) { iris %>% summarise( # expand the ... # ) # summarise(n = n()) } — xiaodai, Mar 01 '20 at 01:30
But what do you expect to get when you call `do_something(iris, !!!(aggregate_expressions))`? — camille, Mar 01 '20 at 01:35
Shouldn't you call only `do_something(!!!aggregate_expressions)`? You are calling `iris` inside the function, so no need to pass it as an argument. — Vitali Avagyan, Mar 01 '20 at 01:51
BTW, your function should look like something like this: `do_something <- function(...) { groupvars <- quos(...) iris %>% summarise(!!!groupvars) # summarise(n = n()) }` — Vitali Avagyan, Mar 01 '20 at 01:52
summarise is just an example. I mentioned in the post that dplyr:: summarise would just work but I wanted something more general, what if I want to do something other than summarise — xiaodai, Mar 01 '20 at 01:54
Is this what you are looking for: https://stackoverflow.com/a/60046162/300187 — Artem Sokolov, Mar 04 '20 at 04:21

score 1 · Answer 1 · answered Mar 01 '20 at 02:34

1

Modify it like this:

do_something <- function(x) {
  iris %>%
    summarise(!!!x)
}

aggregate_expressions <- list(n = quote(n()))
do_something(aggregate_expressions)
##     n
## 1 150

answered Mar 01 '20 at 02:34

G. Grothendieck

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(Just a note that for robustness it's better to create the list of expressions as quosures, e.g. with `vars()`.) – Lionel Henry Mar 02 '20 at 12:54
This is missing the point, because summarise(!!!x) works because summarise is designed to handle !!! but it won't work for some other function, – xiaodai Mar 03 '20 at 02:42
There was no example given for `do_something` other than the code which I took straight from the question. Please fix the question and provide a complete example. – G. Grothendieck Mar 03 '20 at 02:58

How to use tidy eval NSE to expand a `an expression`

1 Answers1