title: Argument not realized in a function with filter but there is no error message
body: <p>This is a problem I have encountered before and always chose a workaround, but no more!
  I want to bring an argument into my function for <code>filter</code>, but it does not work correctly. I suspect it has something to do with quotation marks but I do not get it right. Let me explain the problem in an example:</p>
  
  <pre class="lang-r prettyprint-override"><code>library(dplyr)
  data(mtcars)
  
  mtcars %&gt;% filter(am &gt; 0)
  </code></pre>
  
  <p>Filters as intended.
  Let's make a corresponding function.</p>
  
  <pre><code>fun_a &lt;- function(b) {
    mtcars %&gt;%
    filter(b &gt; 0)
  }
  </code></pre>
  
  <p>And test it.</p>
  
  <pre class="lang-r prettyprint-override"><code>fun_a(am)
  </code></pre>
  
  <p>Throws error:</p>
  
  <pre class="lang-r prettyprint-override"><code>Error: object 'am' not found
  </code></pre>
  
  <p>Another try.</p>
  
  <pre class="lang-r prettyprint-override"><code>fun_a("am")
  </code></pre>
  
  <p>No error, but does not filter either. <br>
  What is happening here?<br><br><br><br><br><br></p>
  
  <p>I found an example that illustrates very well <em>what is happening here</em>:</p>
  
  <pre class="lang-r prettyprint-override"><code>mtcars %&gt;% mutate(ex = am)
  </code></pre>
  
  <p>This gets you a column named ex that is a copy of <code>mtcars$am</code>:</p>
  
  <pre class="lang-r prettyprint-override"><code>    mpg cyl  disp  hp drat    wt  qsec vs am gear carb ex
  1  21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4  1
  2  21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4  1
  3  22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1  1
  4  21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1  0
  5  18.7   8 360.0 175 3.15 3.440 17.02  0  0    3    2  0
  6  18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1  0
  </code></pre>
  
  <pre class="lang-r prettyprint-override"><code>fun_f &lt;- function(f){
    mtcars %&gt;% mutate(ex = f)
  }
  
  fun_f("am")
  </code></pre>
  
  <p>And from this you get a new column ex filled with <em>am</em>, literally:</p>
  
  <pre class="lang-r prettyprint-override"><code>   mpg cyl disp  hp drat    wt  qsec vs am gear carb ex
  1 21.0   6  160 110 3.90 2.620 16.46  0  1    4    4 am
  2 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4 am
  3 22.8   4  108  93 3.85 2.320 18.61  1  1    4    1 am
  4 21.4   6  258 110 3.08 3.215 19.44  1  0    3    1 am
  5 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2 am
  6 18.1   6  225 105 2.76 3.460 20.22  1  0    3    1 am
  </code></pre>