Aha, here's the syntax in case you're curious (using an example lifted from the playground)

  [type Shape
    [Circle f64]
    [Rect f64 f64]
    Point
  ]

  [sig test_sig : Shape -> Float]
  [fn test_sig [shape]
    [match shape
      [Circle r] [* 3.14159 [* r r]]
      [Rect w h] [* w h]
      Point 0.0
    ]
  ]

Unfortunately it seems like this doesn't currently work as expected when I use it in the playground, so I'm going to go file an issue