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Comment by catpolice | Hacker News Reader

Fast re-parsing is tricky. I'm building a JS IDE for a simple language, and it needs to only re-parse the part of the syntax tree that an edit affects (and only update the minimal number of affected DOM nodes). Writing the (recursive descent) lexer/parser by hand was easy. Being able to stream text diffs to the lexer/parser and have it rebuild only the appropriate part of the AST took... a bit of work. Writing unit tests might kill me.

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You will probably be interested in this explanation of how xi-editor does it: https://google.github.io/xi-editor/docs/rope_science_11.html

Thanks! The entire site is very interesting.

I cannot find the link now, but I remember reading a recent article saying that what VSCode does is basically storing some context information for each line. When input changes, the line is reparsed, and then the context provides enough information on how to modify the parse tree.

Yeah, that's pretty similar to what I ended up doing - it comes pretty natural with recursive descent to just store enough of the parser state on relevant nodes that you can restart it at natural breaks.

For my application, I end up having to make expensive async queries about the expressions the user writes. In order to minimize server load/make things responsive/avoid invalidating perfectly good caches, this means that I have to know exactly what to keep and what to throw out with each edit, which means that I never want to replace any node in the AST with an identical one that was needlessly re-parsed if I can help it.

So my lexer does a lot of work trying not to throw out parts of lines that the user hasn't touched, and the parser can figure out what changes are relevant to invalidate my caches.

Oddly the restartable lexer has turned out to be the most frustrating part - selectively diffing parts of two doubly linked lists and snipping them together in the way I ended up having to is one of those things that isn't conceptually very difficult but has the potential for unusually subtle bugs.

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