Question

While working with macros, I have reached the point (I have been trying hard to avoid it) where I need to update those nodes in the AST which hold certain condition. For instance, let’s say I would like to update each node:

Literal(Constant(1))

with the value:

Literal(Constant(2))

Those AST nodes could be anywhere in the expression tree, so I cannot use an ad-hoc pattern matcher. Obviously, the last thing I would like to do is to code a full pattern matcher which is able to cover all the compiler primitives. I have been searching in the API but I have the impression that methods such as collect and the traversable family are not good enough to fulfill my needs, since they treat the tree as a linear thing, and I want the whole updated tree as a result. So, is it possible to update an immutable expression tree in a smart way? Why doesn’t exist such ‘update’ operation in the standard API?

Answer 1

Here’s an example of using an AST transformer in a Scala macro:

https://github.com/retronym/macrocosm/blob/171be7e/src/main/scala/com/github/retronym/macrocosm/Macrocosm.scala#L129