quo-quo-quo-your-boat.R

library(rlang)
library(purrr, warn.conflicts = FALSE)

a <- 2
b <- 5

fn_dots <- function(...) {
  # capture `...` without evaluating
  # (also captures the environment so we know how to 
  # evaluate them when the time comes)
  dots <- enquos(...)
  
  # `as_label()` can turn a quosure into a pretty string
  # `eval_tidy()` will evaluate each one of your dots
  list(
    strings = map_chr(dots, as_label),
    values = map_dbl(dots, eval_tidy)
  )
}

# you have to pass in each element separately
fn_dots(0, a, (a + b) / 2, b, a + b)
#> $strings
#>                                                             
#>         "0"         "a" "(a + b)/2"         "b"     "a + b" 
#> 
#> $values
#>                     
#> 0.0 2.0 3.5 5.0 7.0

# If you want to use this outside a function you can play around with it
# with quos()
captured <- quos(0, a, (a + b) / 2, b, a + b)
captured
#> <list_of<quosure>>
#> 
#> [[1]]
#> <quosure>
#> expr: ^0
#> env:  empty
#> 
#> [[2]]
#> <quosure>
#> expr: ^a
#> env:  global
#> 
#> [[3]]
#> <quosure>
#> expr: ^(a + b) / 2
#> env:  global
#> 
#> [[4]]
#> <quosure>
#> expr: ^b
#> env:  global
#> 
#> [[5]]
#> <quosure>
#> expr: ^a + b
#> env:  global

map_dbl(captured, eval_tidy)
#>                     
#> 0.0 2.0 3.5 5.0 7.0

# there is no way to make the variable first
# and then try and capture the information
# because once you assign it to `steps` it has been evaluated!
steps <- c(0, a, (a + b) / 2, b, a + b)

# i.e. no way to recover a / b symbols at this point
steps
#> [1] 0.0 2.0 3.5 5.0 7.0



# the original example also had you going from 
# c(0, a, (a + b) / 2, b, a + b) -> string version
# this is rather hard to do

fn_dots2 <- function(x) {
  x <- enquo(x)
  as_label(x)
}

# you can capture and label the whole thing
whole_string <- fn_dots2(c(0, a, (a + b) / 2, b, a + b))

# but then you'd have to use regex to split it apart :/
library(stringr)
str_split(str_sub(whole_string, 3, -2), ", ")
#> [[1]]
#> [1] "0"         "a"         "(a + b)/2" "b"         "a + b"