I think you're underestimating the effort cost of D sumtypes at scale. Every individual instance of StrOrInt(10) is cheap, but once you're actually constructing

I think you're underestimating the effort cost of D sumtypes at scale. Every individual instance of StrOrInt(10) is cheap, but once you're actually constructing structs in the same line, you end up writing a lot of terrible code like publish(TrackingEvent(UpdateTrackerEvent(trackerId, position))); that just adds visual noise. And the fact that you cannot return out of sumtype apply expressions just makes so many neat idioms impossible. Something like Object obj = nullableObj.case(null: return false); is just fundamentally impossible in D. So it's not one thing, it's a lot of things coming together. :) Mostly I just realized one day that D was never going to be the perfect language for me, because it wasn't even interested in being that language. > The effort to create a language and a stdlib and a package manager etc. is ridiculously high compared to improving existing languages. Have you seen the DMD source code? Genuinely, writing my own compiler was easier than improving DMD.