title: type error in func declaration
body: <p>I do:</p>
  
  <pre><code>Prelude&gt; "sone" ++ "otehr"
  "soneotehr"
  </code></pre>
  
  <p>But such code:</p>
  
  <pre><code>addOneToElement :: [a] -&gt; [a]
  addOneToElement element = element ++ "next"
  
  main = do
     let s = addOneToElement("some")
     putStrLn s
  </code></pre>
  
  <p>produces this output:</p>
  
  <pre><code>all_possible_combinations.hs:22:37:
      Couldn't match expected type `a' against inferred type `Char'
        `a' is a rigid type variable bound by
            the type signature for `addOneToElement'
              at all_possible_combinations.hs:21:20
        Expected type: [a]
        Inferred type: [Char]
      In the second argument of `(++)', namely `"next"'
      In the expression: element ++ "next"
  </code></pre>
  
  <p>Why I get this error and how I can fix it?</p>