title: merge string list in a comma separated string
body: <p>I need to write a piece of software that, given a list of strings, returns a novel string made of the comma separated concatenation of the input list elements.</p>
  
  <pre><code>(comma-con (list "a" "b" "c"))  ;=&gt; "a,b,c"
  </code></pre>
  
  <p>I would love to write Scheme idiomatic code.</p>
  
  <p>So far, I wrote this:</p>
  
  <pre><code>(define (comma-con l)
     (foldr
           (λ (x y) (if (string? y) (string-append x "," y) x))
           '()
           l))
  </code></pre>
  
  <p>That works, but it seems a bit dirty. Is there a way to write it in a better way, e.g. avoiding the "if"?</p>