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Just curious, how do they handle broken code? (Like when you start writing a line in the middle of the file, not yet done with it, but already need all the good

Just curious, how do they handle broken code? (Like when you start writing a line in the middle of the file, not yet done with it, but already need all the goodness like highlighting and code completion to work.) A common approach with libraries I've encountered is that parser just stops with error - but that's almost unacceptable for use in a proper code editor, which should really try its best to recover and continue processing, even if some chunk in the middle is failing.

mwsherman 12y ago
That part of Visual Studio impresses me a lot, though it’s not obvious to a user. I haven’t (say) closed the brace yet, so the code is invalid and therefore a correct AST can’t be parsed. It has to be heuristic? Or a given (say) line falls back to last known good state?
- lobster_johnson 12y ago
  I suspect the latter, helped by an ability to recover by inserting missing characters such as braces and quotes. (Xcode actually offers to fix trivial errors such as those, I'm sure VS does the same.) It's an interesting problem. I suppose that as it knows the point of breakage, it can annotate the AST to indicate breakage, but preserve the subsequent node; breakage itself becomes a kind of AST node. It's possible that in such a situation, any subsequent AST nodes probably have to point to their pre-breakage nodes as parents in order to stay sane. Thus the AST tree becomes a kind of Git-like revision history that stays fragmented until the next time the AST fully parses. It could easily be something even simpler, however.
- edwinyzh 12y ago
  In my case, the later, I use a very tolerated css and html tokenizer/parser to provide basic html code-completion in LIVEditor - the real-time css and html tweaker/editor (http://liveditor.com http://liveditor.com)
kazagistar 12y ago
Most IDEs use pretty smart heuristics for this sort of thing out of necessity. You notice when it is missing: for example, MonoDevelop starts compulsively indenting wrong if you mismatch your brackets anywhere in the file, but other IDEs more gracefully match the local indentation. In other words, JetBrains has lots of experience with correctly handling partially valid syntax trees in a friendly way.

mwsherman 12y ago

That part of Visual Studio impresses me a lot, though it’s not obvious to a user. I haven’t (say) closed the brace yet, so the code is invalid and therefore a correct AST can’t be parsed. It has to be heuristic? Or a given (say) line falls back to last known good state?

lobster_johnson 12y ago
I suspect the latter, helped by an ability to recover by inserting missing characters such as braces and quotes. (Xcode actually offers to fix trivial errors such as those, I'm sure VS does the same.) It's an interesting problem. I suppose that as it knows the point of breakage, it can annotate the AST to indicate breakage, but preserve the subsequent node; breakage itself becomes a kind of AST node. It's possible that in such a situation, any subsequent AST nodes probably have to point to their pre-breakage nodes as parents in order to stay sane. Thus the AST tree becomes a kind of Git-like revision history that stays fragmented until the next time the AST fully parses. It could easily be something even simpler, however.
edwinyzh 12y ago
In my case, the later, I use a very tolerated css and html tokenizer/parser to provide basic html code-completion in LIVEditor - the real-time css and html tweaker/editor (http://liveditor.com http://liveditor.com)

kazagistar 12y ago

Most IDEs use pretty smart heuristics for this sort of thing out of necessity. You notice when it is missing: for example, MonoDevelop starts compulsively indenting wrong if you mismatch your brackets anywhere in the file, but other IDEs more gracefully match the local indentation. In other words, JetBrains has lots of experience with correctly handling partially valid syntax trees in a friendly way.