Empty Expression

[lglaz]

I'm trying to write a parser for a filter string which can be translated into SQL WHERE clause or some client-side table filter. How to write a parser where an empty expression (meaning no filtering) is a valid part of the grammar?

[nblumhardt]

Hi Łukasz,

If you mean that the whole input being empty implies true, then the top level parser might look something like:

static readonly TokenListParser<XToken, XExpression> Filter =
    Expression.OptionalOrDefault((XExpression)new XConstantExpression(true)).AtEnd();

(Assuming your language is called X :-))

Where I've encountered this, though, I tend to just check the input first with string.IsNullOrWhiteSpace() and skip parsing altogether.

HTH!

[lglaz]

Thanks for the answer and the additional tip. I'll probably use the latter :)