title: perform calculation across columns of type list
body: <p>I have a dataframe with columns of type list:</p>
  <pre><code>       h1      h2      h3
  1 6, 5.25 66, 4.2  4, 4.2
  2   5, 11   7, 10   7, 10
  3   6, 11  16, 11  16, 11
  4 6, 0.25 7, 2.50 7, 7.77
  </code></pre>
  <p>I want to multiply the first value in each column with the second value, so in <code>h1</code>this would be <code>6*5.25</code>, <code>5*11</code>, <code>6*11</code>, etc.</p>
  <p>I've tried this code in <code>dplyr</code> but it gives an error:</p>
  <pre><code>library(dplyr)
  df0 %&gt;%
    mutate(across(c(everything()), 
                  ~ as.numeric(.x)[1]*12 + as.numeric(x.)[2]))  
  Error: Problem with `mutate()` input `..1`.
  x 'list' object cannot be coerced to type 'double'
  ℹ Input `..1` is `(function (.cols = everything(), .fns = NULL, ..., .names = NULL) ...`.
  </code></pre>
  <p>Reproducible data:</p>
  <pre><code>structure(list(h1 = list(c(&quot;6&quot;, &quot;5.25&quot;), c(&quot;5&quot;, &quot;11&quot;), c(&quot;6&quot;, 
  &quot;11&quot;), c(&quot;6&quot;, &quot;0.25&quot;)), h2 = list(c(&quot;66&quot;, &quot;4.2&quot;), c(&quot;7&quot;, &quot;10&quot;
  ), c(&quot;16&quot;, &quot;11&quot;), c(&quot;7&quot;, &quot;2.50&quot;)), h3 = list(c(&quot;4&quot;, &quot;4.2&quot;), c(&quot;7&quot;, 
  &quot;10&quot;), c(&quot;16&quot;, &quot;11&quot;), c(&quot;7&quot;, &quot;7.77&quot;))), class = &quot;data.frame&quot;, row.names = c(NA, 
  -4L))
  </code></pre>