The best example I have is a closed source one and I can't be more specific on what it is than to say that it's probably installed on at least one of your Apple devices (assuming you have any).

Implementation-wise, the syntax tree nodes have additional attributes that hold pre- and/or post-element text. What's on disk is the serialized tree. You edit a text version, and it's parsed on every edit so it doesn't have to be parsed again at evaluation time, and what's stored is the parse tree with enough whitespace and comment hints to reconstruct the text for editing.

This is a case where looking at the textual code is rare, but hundreds of results must get updated in realtime on every change. This might be enough of a hint as to what program it is.