title: GHCi error - "Not in scope: `isUpper'"
body: <p>I'm learning haskell and I'm trying to write some simple functions. Everything worked well until I've used function <code>isUpper</code>. I'm unable to compile the project because of this error:</p>
  
  <pre><code>[1 of 1] Compiling Main             ( C:\Users\...\src\Main.hs, interpreted )
  C:\Users\...\src\Main.hs:147:25:
      Not in scope: `isUpper'
  Failed, modules loaded: none.
  Prelude&gt;
  </code></pre>
  
  <p>My code:</p>
  
  <pre><code>module Main where
  main::IO()
  main = undefined
  stringIsUpper [] = True
  stringIsUpper (x:ys) = (isUpper x) &amp;&amp; (stringIsUpper(ys))  
  </code></pre>
  
  <p>The goal of this code should be just to check if the inserted string consists of the uppercase letters.
  I'm using EclipseFP for development
  Thank you for your help</p>