Combine positional and flag based arguments

[xldenis]

I'm trying to combine both positional and flag based arguments, I've tried embedding a record in a normal constructor like so:

data Config = Command [String] FlagArguments
data FlagArguments = Args {a :: Bool }

While the parser is correct, the help message is fairly useless:

Usage: proj-exe build [STRING] FlagArguments

What I'd like to get is:

Usage: proj-exe build [STRING] --a

I don't understand haskell generic programming well enough to provide the correct instances though.

[Gabriella439]

I think the fix here is to change the instances for tuples from this:

instance (ParseFields a, ParseFields b) => ParseRecord (a, b)

... to this:

instance (ParseRecord a, ParseRecord b) => ParseRecord (a, b)

... and likewise for other tuple instances.

Then the types a and b would be Config and FlagArguments in your case and it would do the right thing.

How does that sound?

[xldenis]

Hmm, I don't think that would really solve my issue, I just realized that my example might not have been the best.

A more accurate example could look something like this:

data Config = Build [String] FlagArguments
            | Other [OtherArgs]
data FlagArguments = ...

though I suppose I could restructure it so that Build holds a tuple. 😕

[Gabriella439]

Yeah, in general what you want to do is probably not going to work. For example, consider the case where FlagArguments was a labeled field of Build or if it were a sum type

[Gabriella439]

For cases like that you will probably have to hand-write the top-level parser to do what you want

[xldenis]

Addressing both examples you've given:

1. If it is a labeled field then we can't generate a meaningful parser since in this case the important distinction is that FlagArguments is itself a record.
2. Nested sum types pose a different, equally interesting problem: If FlagArguments is a sumtype, then I would expect app build --help to display style of usage message you would get for app --help

Currently, it seems like any nested type gets displayed as TYPENAME in help messages with no further explanation, making it totally useless.

Both nested subcommands and positional + flag based arguments are important for cli tools, I'd be happy to take a stab at implementing these if you feel it's worth supporting.

[Gabriella439]

If you think there is a clean way to implement this then I'll be happy to review it