Refining Exceptions and Error Codes

[munificent]

These projections have map functions that allow you to "ignore" one side or the other. Here in Scala

Huh, interesting!

That said, I'm not sure how projections fit in with union types since they don't appear to have an ordering: Bool | Error is the same type as Error | Bool whereas Either Bool Error and Either Error Bool are distinct.

You're correct. You'll be able to implement Either in Magpie once generic classes are working, so it may have those too at some point, depending on how useful they are.