[brandonbloom]

I'll agree it's best to avoid fully open unions when you can, but some (super useful) things truly don't work that way. Check out http://okmij.org/ftp/Computation/monads.html#ExtensibleDS for a cool example.

[tel]

I always think of Control.Exception as the poster child for open unions.

[brandonbloom]

Perfect, since exceptions are a subset of effects! Check out http://math.andrej.com/eff/ and its literature.