title: R `scales::comma` works yet `scales::comma()` does not - why?
body: <p>When I am using a function in a package I can often write it in the form <code>function()</code> or just <code>function</code> without parenthesis. This doesn't seem to be the case with <code>scales::comma</code>. Why does line 7 work below, yet line 8 does not.</p>
  
  <pre><code>library(tidyverse)
  mtcars %&gt;% 
    count(cyl) %&gt;% 
    ungroup() %&gt;% 
    mutate(n = n * 1000) %&gt;% 
    ggplot(aes(cyl, n)) + 
    scale_y_continuous(labels = scales::comma) +  # line 7
    # scale_y_continuous(labels = scales::comma()) +  # line 8
    geom_line()
  </code></pre>
  
  <hr>
  
  <p><em>LINE 8 Error</em></p>
  
  <pre><code>Error in number(x = x, accuracy = accuracy, scale = scale, prefix = prefix,  : 
    argument "x" is missing, with no default
  </code></pre>