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
