Hi,
For quite some time I have working on this home project trying to parse a very
complex grammar. I had a brain wave yesterday on how to skip the difficult
bits, at least for the moment. If I were to handle the difficult bits now, I
would end up producing a parser for a complete programming language, almost.
Below is an example of the sort of thing I am trying to skip - i.e. the 'where'
statement. Now, because the grammar is such that the 'where' statement, or
statements, immediately precede the 'end_type' keyword, I thought I'd gobble to
'end_type'.
In the example below, at the moment, 'typeid' and 'underlyingtype' eventually
come down to a simple string identifier of 'a..z'('a-z'|'_')*
type dayinmonth = integer;
where
validrange : {1 <= self <= 31};
end_type;
The modified grammar for this is...
typedecl
: 'type' typeid '=' underlyingtype ';' (options {greedy=false;} : .* )
'end_type' ';'
;
I can't just skip to the next ';' because there may be several statements i.e.
Where
Label1 : stuff1;
Label2: stuff2;
What I find is that the '{' and '}' within the gobble process become
significant. In other similar cases I find a '|', or even a carriage return
'\r', is significant. Using the Eclipse add-in, testing just this sub-graph
produces different (although successful in both cases) results depending on
whether the '{' or '}' is surrounded by whitespace or not. Somehow, if it is
surrounded by whitespace, the '{' token disappears from the parse tree. But
when trying to parse the text properly in context, it throws up an error. I
also found that changing the '{' to '(' removed the error.
In the end, I managed to parse an 12000 line file with only this type of error.
This was a long introduction for just a couple of short questions. Are there
significant characters that can affect the gobble process? Do I need other
options to be able to skip everything to 'end_type'?
Thanks.
Dugald Wilson
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