title: How to set main class in build?
body: <p>Upon <code>sbt run</code> I have multiple choices of main class.</p>
  
  <p>I would like to set a main class so I've writen in <code>build.sbt</code>:</p>
  
  <pre><code>mainClass := Some("aMainClass")
  </code></pre>
  
  <p>But sbt fails with:</p>
  
  <pre><code>build.sbt:1: error: not found: value aMainClass
  </code></pre>
  
  <p>I've also tried with <code>project/Project.scala</code> file :</p>
  
  <pre><code>import sbt._
    class ExecutableProject(info: ProjectInfo) extends DefaultProject(info)  {
    override def mainClass = Some("aMainClass")
  }
  </code></pre>
  
  <p>error :</p>
  
  <pre><code> project/Project.scala:3: not found: type aMainClass
  </code></pre>
  
  <p>How to set the main class in a build?</p>