Raph Levien defines a reactive UI as a pipeline of incremental tree transformations. I'm working on my compiler right now, which is also a pipeline of tree transformations. When you build a projectional editor, which supports live coding, then you have a tree representation of your program, which needs to be incrementally rendered in the UI as changes are made, and which also needs to be incrementally compiled as changes are made. The same tree structure is at the head of two incremental transformation pipelines (reactive UI, and compiler).