Use special object instead of null

[Mingun]

Why not use some singleton internal object as result of parsing instead of null for indicate match failures? Then in actions you free return any value (including null).

[otac0n]

Or a wrapper type. In Pegasus, I used a type called ParseResult. It has a property, value which can be null, but you can also return a null parse result to indicate parse failure. Then, labeled expressions and repetitions simply unwrap the value.

[dmajda]

Resolved by fixing #198.