Thinking about this a bit more (and a couple months later…), maybe there is a simpler alternative.
Currently, a reason why writer evolution is hard (the union issue described below is a special case of this) is that aliases are only used on the reader side. Why not also allow readers to use the writer’s aliases? Resolution would first be done on names, then fall back to reader aliases, and finally fall back to writer aliases. In the example below, it would be enough to add an alias to the base record inside any new records to have evolution work. -Matthieu > On Apr 22, 2016, at 8:42 AM, Matthieu Monsch <mon...@alum.mit.edu> wrote: > > The second solution sounds like a great alternative. > > Branch aliases are more straightforward than an implicit order-sensitive > policy. They also have the additional benefit of giving users a bit more > flexibility: since defaults are specified on the branches’ types, it is > possible to have different branches have different defaults inside the same > union. There are probably a few edge cases (e.g. allowing multiple such > aliases would be useful) but they should be simple to address. > > What would be a good attribute name for this? `baseTypes`? > > -Matthieu > > > >> On Apr 21, 2016, at 10:52 AM, Doug Cutting <cutt...@gmail.com> wrote: >> >> On Wed, Apr 20, 2016 at 9:09 PM, Ryan Blue <rb...@netflix.com.invalid> wrote: >>> Making the default a property of an >>> inner schema makes me think that we will have to deal with multiple schemas >>> with such a label at some point. >> >> On Thu, Apr 21, 2016 at 6:54 AM, Matthieu Monsch <mon...@alum.mit.edu> wrote: >>> Delegating default selection to the branches themselves is a great idea but >>> it >>> will be tricky to handle reference branches smoothly. More minor but it also >>> doesn’t feel intuitive to not have the union “own” its default attribute. >> >> If I understand your concerns correctly, I attempted to address this above: >> >> "Note however that, when using a record as the default branch, one >> could not then >> use that same record as a non-default branch in another union. To >> ameliorate that, we might permit multiple default branches in a union >> to be specified as default with the convention that the first such is >> used." >> >> Does that make sense? >> >> This isn't ideal syntax, but it's not terrible, and it doesn't change >> schema syntax incompatibly, which seems important, especially when its >> unlikely that all implementations would implement such a syntax change >> in a synchronized manner. >> >> Alternately, one might annotate each derived record with the name of >> its base record, then one wouldn't need to alter union definitions. >> This would work like an alias. If a record doesn't exist in the >> reader's schema, then an alias to the missing record would be added in >> the reader's schema to the base record it names in the writer's >> schema. Aliases work by rewriting the writer's schema at read-time, >> updating names, including those in unions. Might that work? It seems >> like perhaps a more elegant approach. It has compatible syntax and >> only alters behavior of a case that fails today. >> >> Doug >
