[tikhonj]

This is why we have Data.Dynamic which does safe dynamic types.

It almost never comes up because it turns out to be virtually useless, but that's a story for another time.

[brandonbloom]

> it turns out to be virtually useless

That's debatable, however Data.Dynamic is built on top of Data.Typeable, which provides a lower level runtime type safety facility. I think we can both agree Typable has lots of interesting uses.

[tel]

Typeable is interesting in theory and generic traversals are a godsend, but usually I find that when I'm reaching for that particular hammer I should check twice.

[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.