unlist ellipsis list into variables (when unquoted) and arguments

Question

I have the following function which works:

library(cowplot)
library(tidyverse)

hist_fun <- function(data, ...) {
  
  g <- map(
    list(data %>% 
           select(...) %>% 
           map2(., names(.), 
                ~{ggplot(data.frame(var = .x), aes(var)) + 
                    geom_histogram(aes(y = after_stat(density))) + 
                    labs(x = .y) 
                })),
    ~cowplot::plot_grid(plotlist = .x, ncol = 1))
  g
}

#works
hist_fun(mtcars, mpg, cyl, disp)

However, I would like to be able to pass it geom_histogram arguments too so something like:

hist_fun(mtcars, mpg, cyl, disp, bins = 20, color = "white")

I'm not sure of the best way to approach this. Should I collect everything in the ellipsis in a list and then try to separate out the list into a) variables and b) geom_histogram arguments, something like:

hist_fun <- function(data, ...) {
  everything <- list(...)
  #unlist and extract variables/geom_histogram arguments?
  #just_vars
  #just_geom_histogram
  
  g <- map(
    list(data %>%
           select(just_vars) %>%
           map2(., names(.),
                ~{ggplot(data.frame(var = .x), aes(var)) +
                    geom_histogram(aes(y = after_stat(density)), just_geom_histogram) +
                    labs(x = .y)
                })),
    ~cowplot::plot_grid(plotlist = .x, ncol = 1))
  g
}

That seems a bit messy so would it better to separate the variables at the start and keep geom_histogram arguments to the ellipsis, something like:

hist_fun(mtcars, just_vars = list(), ...)

Any suggestions and the best way to achieve it, particularly when I want to keep the variables unquoted?

Thanks

Answer 1

There are only a small number of arguments that a user might pass to geom_histogram, so I would just explicitly name these in the function definition. Ensure they go after the ellipsis though.

library(cowplot)
library(tidyverse)

hist_fun <- function(data, ..., color = NA, bins = 30) {
  
  g <- map(
    list(data %>% 
           select(...) %>% 
           map2(., names(.), 
                ~{ggplot(data.frame(var = .x), aes(var)) + 
                    geom_histogram(aes(y = after_stat(density)),
                                   bins = bins, color = color) + 
                    labs(x = .y) 
                })),
    ~cowplot::plot_grid(plotlist = .x, ncol = 1))
  g
}

Allowing

hist_fun(mtcars, mpg, cyl, disp, bins = 10, color = "red")

If you didn't want to limit the user to the named arguments you decided upon, you could indeed do it by sorting through the ellipsis arguments, but it's a bit more complex:

hist_fun <- function(data, ...) {
  args <- as.list(match.call())[-c(1:2)]
  named_args <- args[which(nzchar(names(args)))]
  unnamed_args <- args[which(!nzchar(names(args)))]
  hist_args <- c(list(mapping = aes(y = after_stat(density))), named_args)
  select_args <- c(list(quote(data)), unnamed_args)
  g <- map(
    list(
           map2(do.call('select', select_args), 
                names(do.call('select', select_args)), 
                ~{ggplot(data.frame(var = .x), aes(var)) + 
                    do.call('geom_histogram', hist_args) + 
                    labs(x = .y) 
                })),
    ~cowplot::plot_grid(plotlist = .x, ncol = 1))
  g
}

For example

hist_fun(mtcars, mpg, cyl, disp, bins = 15, fill = 'red')