Crosscut

Some syntax nodes are only valid, if their children are of the correct type and number. Take an fn node for example, which is the parent node for a function literal. It needs exactly two children, parameter and body. The parameter needs to be a pattern, the body an expression. Anything else, and the fn node is not valid.

If you follow a free-form approach, that turns into a confusing mess pretty fast: You edit a node, you check its context (parent, and position among siblings), then you compile it depending on that context. Next, you compile its children, because compiling the original node could have changed their context. And so on, recursively.

At some point, you'll encounter a child that doesn't change as a result of the re-compilation. Then you're done with that part. But of course, for all the children that did change, you need to compile their parents again. Including the original node, but also its parents, up to the root. None of that is impossible to manage, but it's more complicated than I'd like.