title: Get the index of multiples elements in a haskell list
body: <p>I have a list like <code>[B,B,N,B,N]</code> and I want to retrieve all the index of the <code>N</code>. So, in this example it would be <code>[2,4]</code>.</p>
  
  <p>I don't really know how to do it... I tried with <code>elemIndex</code> but in fact I don't think that in my case it's allowed, because that's all the point of the exercise.</p>
  
  <p>I did this for now but I do know that it doesn't work:</p>
  
  <pre><code>indice :: [Case] -&gt; [Int]
  indice [] = [0]
  indice (x:xs)
      | x == N = [1 + head(indice(xs))] ++ indice(xs)
      | x == B = [1]
  </code></pre>