What is a Trees#Tree type?

Question
1

Hi,

In the scala synthax tree the Trees#Tree type appears quite often for a block of code. What is it exactly and how is it different from the Tree type? Is there a way to convert between the two?

2

Why did you think it’s different? In what specific case do you need to convert something?

If you are inside something extending Trees you can refer to Trees#Tree as just Tree.

In macros there is context.universe.Tree, in runtime reflection there is scala.reflect.runtime.universe.Tree.

Generally maybe the question is about the difference between type projection A#B and path-dependent type a.B.

1 Like
3

I need to pass in a Tree to a function and it won’t take Trees#Tree.
This is inside a traverser, where I want to call super.traverse on the body of a function.

But actually I think it was just the IDE complaining and it does just work.

4

I found a place where this is relevent and why I was confused. There are two libraries which have Trees and there is a type mismatch between those.

error: type mismatch; found : scala.reflect.api.Trees#Tree required: MyComponent.this.global.Tree

5

There’s only one library here. As Dmytro hinted, many types in the compiler are “path-dependent”; the compiler uses the “cake pattern” heavily internally. Trees#Tree is the general type (that’s a “type projection”), MyComponent.this.global.Tree is the path-dependent type specific to the particular instance of MyComponent. The latter is a subtype of the former.

It’s good that you quoted the exact error message — that helps us give relevant advice.