How do you specify of possible arguments states in functions using non-standard evaluation?

Question

I am learning about programming with tidy evaluation and non-standard evaluation and have been trying to work out how to constrain the possible states of an argument in a function.

For instance given a data set:

set.seed(123)
data <- data_frame(GROUP_ONE = sample(LETTERS[1:3], 10, replace = TRUE), 
                   GROUP_TWO = sample(letters[4:6], 10, replace = TRUE), 
                   result = rnorm(10))

I can create a function which has an argument I use to group the data using a quosure like so:

my_function <- function(data, group = GROUP_ONE){

  require(dplyr)
  require(magrittr)

  group <- enquo(group)

  result <- data %>% 
    group_by(!!group) %>% 
    summarise(mean=mean(result))

  return(result)
}

and this does what I want

my_function(data)

# A tibble: 3 x 2
  GROUP_ONE       mean
      <chr>      <dbl>
1         A  1.5054975
2         B  0.2817966
3         C -0.5129904

and I can supply a different group:

my_function(data, group = GROUP_TWO)

# A tibble: 3 x 2
  GROUP_TWO       mean
      <chr>      <dbl>
1         d -0.3308130
2         e  0.2352483
3         f  0.7347437

However, I cannot group by a column for which is not present in the data.

e.g.

 my_function(data, group = GROUP_THREE)

Error in grouped_df_impl(data, unname(vars), drop) : Column GROUP_THREE is unknown

I would like to add a step at the beginning of the function so that the function stops with a custom error message if the group argument is not GROUP_ONE or GROUP_TWO

something like:

if(!group %in% c(~GROUP_ONE, ~GROUP_TWO)) stop("CUSTOM ERROR MESSAGE")

except this does not work as you apparently you can't put quosures in a vector. It should be possible to convert the quosure to a string somehow and have a vector of strings but I can't figure out how.

How is this done?

Answer 1

I think you need quo_name (from dplyr or rlang), which transforms a quoted symbol to a string:

my_function <- function(data, group = GROUP_ONE){

    require(dplyr)
    require(magrittr)

    group <- enquo(group)

    if(!quo_name(group) %in% names(data)) stop("CUSTOM ERROR MESSAGE")

    result <- data %>% 
        group_by(!!group) %>% 
        summarise(mean=mean(result))

    return(result)
}

# > my_function(data, GROUP_THREE)
# Error in my_function(data, GROUP_THREE) : CUSTOM ERROR MESSAGE

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Edit

As noted by lionel in comment: except for quo_name, there are many other alternatives including base R as.character and as_string from rlang.