title: Replace NA on numeric columns with mutate_if and replace_na
body: <p>I would like to replace NAs in numeric columns using some variation of <code>mutate_if</code> and <code>replace_na</code> if possible, but can't figure out the syntax.</p>
  
  <pre><code>df &lt;-tibble(
      first = c("a", NA, "b"),
      second = c(NA, 2, NA),
      third = c(10, NA, NA)
    )
  
  #&gt; # A tibble: 3 x 3
  #&gt;   first second third
  #&gt;   &lt;chr&gt;  &lt;dbl&gt; &lt;dbl&gt;
  #&gt; 1 a      NA     10.0
  #&gt; 2 &lt;NA&gt;    2.00  NA  
  #&gt; 3 b      NA     NA
  </code></pre>
  
  <p>Final result should be:</p>
  
  <pre><code>#&gt; # A tibble: 3 x 3
  #&gt;   first second third
  #&gt;   &lt;chr&gt;  &lt;dbl&gt; &lt;dbl&gt;
  #&gt; 1 a       0     10.0
  #&gt; 2 &lt;NA&gt;    2.00   0  
  #&gt; 3 b       0      0
  </code></pre>
  
  <p>My attempts look like: </p>
  
  <pre><code>df %&gt;% mutate_if(is.numeric , replace_na(., 0) )
  #&gt;Error: is_list(replace) is not TRUE
  </code></pre>