[antlr-interest] Re: look ahead side effects on interactive parsers
clay_douglass_at_work
clay.douglass at conformative.com
Wed Jun 23 09:25:44 PDT 2004
--- In antlr-interest at yahoogroups.com, "clay_douglass_at_work"
<clay.douglass at c...> wrote:
> I am implementing an expression parser for an interactive
debugger.
> Unfortunately the generated parser causes underactivity issues.
e.g.
> It doesn't return the command till I hit return twice. I guess
this
> is an obvious side effect of the look-ahead k=2. I had though
since
> each command pattern ended in NEWLINE that I would get command
> returned as soon as the parser saw the NEWLINE. Any helpful
> suggestion or is my plan to use ANTLR to parse the interactive
> commands just a bad idea?
>
> Thank,
>
> Clay
Well I figured out that
NEWLINE :('\r' '\n' | '\n' | '\r'){newline();};
was the issue not look ahead in general. Since my input source is
generating only '\r' when I hit enter the rule needed the next
character to correctly determine the match. To fix this I removed
the '\r''\n' part
NEWLINE :('\n' | '\r'){newline();};
Now it work right except it counts lines wrong on some
systems/files. Is there an easy mechanism to look at the previous
character to decide if I should call newline() ?
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