title: What does it mean to pass `_` (i.e., underscore) as the sole parameter to a Dart language function?
body: <p>I'm learning Dart and see the following idiom a lot:</p>
  <p><code>someFuture.then((_) =&gt; someFunc());</code></p>
  <p>I have also seen code like:</p>
  <p><code>someOtherFuture.then(() =&gt; someOtherFunc());</code></p>
  <p>Is there a functional difference between these two examples?
  A.k.a., What does passing <code>_</code> as a parameter to a Dart function do?</p>
  <p>This is particularly confusing given Dart's use of <code>_</code> as a prefix for declaring private functions.</p>