Add a standard validator for range

[KathleenDollard]

I'd like to be able to add a validator for a range (upper/lower bound for the value). I made a stab at this, but ran into a couple of issues:

• At the point of validation, we have a token (string) not a value. It seems inappropriate to convert an extra time just to have the value for a range validator.
• I'm working from metadata, so I have a Type, not an actual known type. I can compare to IComparable, but the only way I know to do this is some gnarly reflection. Maybe someone sees an easier way.

I wonder if as a strongly typed parser, we need to have a set of validators that run before and after conversion. In addition to the range challenge, I can imagine cases where there are equivalent values that appear as different strings (1.0 vs 1.00).

If we don't want range as a standard validator, it would be nice to at least have guidance about how to write one.

[amis92]

I'd just like to add that I've hit it as well - a need of validation of an already parsed/converted value. Of course, it's a simple workaround to just parse manually for validation, but as said above, it might be a common enough scenario.

Specifically, I had an argument of type Uri, but it's required to be UriKind.Absolute, so I had to parse and throw away result. If I could validate an Uri value, I could just check Uri.IsAbsoluteUri.

[jonsequitur]

If we think the new ParseArgument<T> delegate has the right shape and capabilities, it might be that we can just change the existing validators to run after conversion and receive T rather than the tokens.

[dtv42]

I have implemented a set of Option extensions for validation and ease of use. They allow to write statements such as:

AddOption(new Option<int>  ("-d", "default value").Name("Number").Default(1));

or

AddOption(new Option<int>  ("-r", "range value [0..10]").Name("Number").Range(0, 10));

I also use this for validation for IPAddress, IPEndpoint, Uri or regular expressions. It is fairly simple but helps.