title: Can't write a type declaration in playground of Haskell IDE
body: <p>I'm writing Haskell with the <a href="https://apps.apple.com/jp/app/haskell/id841285201?l=en&amp;mt=12" rel="nofollow noreferrer">‎Haskell on the Mac App Store</a> tool.</p>
  <p>I can't write a type declaration with the playground of <a href="https://apps.apple.com/jp/app/haskell/id841285201?l=en&amp;mt=12" rel="nofollow noreferrer">‎Haskell on the Mac App Store</a>.</p>
  <p>The following syntax, without type declarations, is OK:</p>
  <pre><code>data Foo = A | B
  data Bar = C | D
  data FooBar = FooBar Foo Bar
  
  x = FooBar A C
  </code></pre>
  <p>The following writing style is not acceptable:</p>
  <pre><code>data Foo = A | B
  data Bar = C | D
  data FooBar = FooBar Foo Bar
  
  x :: FooBar
  x = FooBar A C
  </code></pre>
  <p>The error message</p>
  <pre><code>&lt;playground&gt;:5:1: error: Variable not in scope: x :: FooBar
  </code></pre>
  <p>Why can't I write the type declarations?</p>
  <p><a href="https://i.stack.imgur.com/wnFPW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wnFPW.png" alt="enter image description here" /></a></p>