On Saturday, 7 July 2012 at 21:52:09 UTC, David Piepgrass wrote:
it seems easier to tell what the programmer "meant" with three phases, in the face of errors. I mean, phase 2 can tell when braces and parenthesis are not matched up properly and then it can make reasonable guesses about where those missing braces/parenthesis were meant to be, based on things like indentation. That would be especially helpful when the parser is used in an IDE, since if the IDE guesses the intention correctly, it can still understand broken code and provide code completion for it. And since phase 2 is a standard tool, anybody's parser can use it.
There could be multiple errors that compensate each other and make your phase 2 succeed and prevent phase 3 from doing proper error handling. Even knowing that there is an error, in many cases you would not be able to create a meaningful error message. And any error would make your phase-2 tree incorrect, so it would be difficult to recover from it by inserting an additional token or ignoring tokens until parser is able to continue its work properly. All this would suffer for the same reason: you loose information.
