Consider more expressive types for declaring command-line parsing

[istathar]

The Config type as set up by calling simpleConfig or complexConfig and passing to configure is pretty good. It meets most of what we were going for in a declarative, obvious, and simple setup to configure the command-line parsing of a program.

Having used this API a bit now, it's starting to bother me that the usage side of this is a bit weak. The combination of pattern matching on the Maybe that comes back from lookupKeyValue and then pattern matching on the ParameterValue if the option is there is a bit awkward. It's more awkward for the case of flags (where no value is expected [or perhaps, allowed] and very awkward for arguments, where the value must be present at this point of the program because we require arguments to be identified and found in configure.

So, perhaps a better type for or within Config would allow the current lookupOptionFlag, lookupOptionValue and lookupArgument to be combined or significantly simplified.

That would be a major API bump breaking change. Ok.

AfC

[istathar]

If not superseded this is papered over by the new functions queryOptionFlag, queryArgument, and so on. It's all pretty good at the moment, but it we can consider a better design here now that it's seeing wider usage.