title: Haskell handling negative parameters
body: <p>Trying to sum two values, with only one of them negative, such as <code>-1</code> and <code>2</code>:</p>
  
  <pre><code>soma :: Float -&gt; Float -&gt; Float
  soma x1 x2 = x1 + x2
  </code></pre>
  
  <p>The result in an error; WHY?</p>
  
  <pre><code>&lt;interactive&gt;:10:6:
  No instance for (Num (Float -&gt; Float -&gt; Float))
    arising from a use of `-'
  Possible fix:
    add an instance declaration for (Num (Float -&gt; Float -&gt; Float))
  In the expression: soma - 1 2
  In an equation for `it': it = soma - 1 2
  </code></pre>