Anyone help a newbie at parsing/syntax error reporting?

Hi all, having "lurked" for a while in this group, I thought I might
post a message. I'm posting normally through MS Outlook Express, so if
that's the wrong thing to do, then I apologise.


Anyway, I'm currently writing a Sinclair BASIC interpreter. I've
written interpreters before, but as I was the only person expected to
code in them, the error and syntax checking was a little down the list
of priorities. Now however, I am trying to parse a line of basic
code. I have tried a few methods - reducing using rules (# + # becomes
#) but the best one by far that I have tried is BNF - using a parse
tree. It's only simple, but I've come up against a small problem. I
can tell when and why the parser drops out with a syntax error, but I
want to be able to report more than "Expected X, but found Y" errors,
such as "The keyword TO cannot be used to subscript a numeric
variable"


However, due to the recursive nature of my BNF checker, I only find
which rule failed, not why. Does anybody have experience of this sort
of thing, and can point me in the right direction?


Thanks
        Paul.

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