[grammarians] Error recovery / parsing partial source

[Jason Morrison]

Hey there folks,

I was wondering if anyone could point me in the right direction.  I'm
looking into parsing partial/invalid Ruby code, specifically for the intent
of type inference for code completion.  (See http://soc.jayunit.net... I've
disappeared under school work for a while, but am trying to reach the
surface under the guise of a project for my Language Processors/Compiler
Construction course ;)  The biggest roadblock I'd hit upon, and would like
to revisit, is the fact that,inside an IDE code completion (and therefore
type inference) is often requested at a point at which the code is not
syntactically valid.  I.e.:

def foo
  if ( cond )
    puts myvar. # invoke completion

class MyKlass
  class << self; def method
#... and so on

So we're in the middle of a method-send on the myvar expr, a puts call, an
if statement, and a method definition node, none of which are closed before
the MyKlass class declaration node is opened (or, equally often, EOF is hit
when we are ten nodes deep).  My knowledge of error hooks inside parsers is
spotty at best (and limited to error symbols in yacc-based Jay), so I appeal
to you all: is there research on heuristics in this area I can read up on?
Is there such error recovery functionality in C-Ruby's parse.y (I spy some
nice yyerror() calls in there, do they bubble up into JRuby's Jay parser?)
or Rubyfront's ruby.g (didn't see any, but I don't know ANTLR)?

If not, any thoughts on the idea?  Is this something that can be usefully
resolved via error symbols?  (Perhaps I am simply ignorant of an
error-recovery enabling flag in my trusty JRuby parsing calls, and would be
better served directing inquiry to the JRuby list?)  I'd rather not just
blast an arbitrary number of close-parens and "end"s at the cursor position
until a valid parse surfaces, the quickest-and-dirtiest solution that
originally came to mind ; )

Thanks very much for any input!
Jason

-- 
Jason Morrison
jason.p.morrison at gmail.com
http://jayunit.net
(585) 216-5657
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