Nice! This looks right. Let's get the other piece of this merged into syntax tree first and then we can merge this.

@kddnewton This is what I've got so far.

I added the onLanguage event like you mentioned, then I thrashed around a lot and added the documentSelector as well. That seems to get me to the point where VSCode is using Syntax Tree to trigger.

This may or may not be doing something in the config:

"[haml]": {
    "editor.defaultFormatter": "ruby-syntax-tree.vscode-syntax-tree"
  }

At this point I was going to switch over to Syntax Tree and try to figure out how to get Haml formatting working there, does that sound right?

Edit: You beat me! 👍 🎉

ruby-syntax-tree/syntax_tree#127 is merged :D