title: How I can migrate from `MGLConstantStyleValue` to `NSExpression`
body: <p>I'm trying to migrate from <code>mapbox-sdk-3.7</code> to <code>mapbox-sdk-4.0</code>.</p>
  
  <p>I can't understand how to convert this syntax with <code>MGLConstantStyleValue</code> to <code>NSExpression</code>.</p>
  
  <pre><code>let layer = style.layer(withIdentifier: "milestones") as? MGLSymbolStyleLayer
  let styledValues: [String: MGLStyleValue&lt;NSString&gt;] = ...
  
  layer.iconImageName = MGLConstantStyleValue(interpolationMode: .identity,
                      sourceStops: styledValues,
                      attributeName: "imageId")
  </code></pre>