Tree, Type and TypeTree

[liufengyun]

This is one of the divergence points between scala.reflect and scala.meta. The former has the Tree/TypeTree/Type separation, whereas the latter merges all three into one concept.

Some observations from my experience with scala.reflect:

• Almost all newcomers need to be explained why the separation exists.
• Trees and types are different ADT families, which means that stuff that works for trees doesn't work for types and vice versa.
• Types throw away some syntactic information, which means that they can't be reliably converted back to trees.
• Most importantly, types can't be manipulated using quasiquotes, so people have to learn additional APIs.

[liufengyun]

This is a very good question about user experience. I'd like to come back to it later, as it doesn't influence feasibility of the current approach, but more of a design choice.