The docs[1] do say that the last of a particular type wins, though as I say in [2] it's not clear to me whether that's actually desirable, or if it's essentiall

The docs[1] do say that the last of a particular type wins, though as I say in [2] it's not clear to me whether that's actually desirable, or if it's essentially an implementation detail (process the arguments in order and mutate the object) that got blessed because it's easier that way. I'd love to hear arguments for last-write-wins over immutable assignment. It seems you can't implement custom interfaces on the core data types[3], so a do-nothing interface isn't an option to emulate sum types and enforce type-safety (E switches on string to set the error message). At least the code[4] gives you a run-time error instead of silently ignoring unsupported types. [1] https://godoc.org/upspin.io/errors#E https://godoc.org/upspin.io/errors#E [2] https://news.ycombinator.com/item?id=15870334 https://news.ycombinator.com/item?id=15870334 [3] http://www.jerf.org/iri/post/2917 http://www.jerf.org/iri/post/2917 [4] https://upspin.googlesource.com/upspin/+/master/errors/errors.go#154 https://upspin.googlesource.com/upspin/+/master/errors/error...