Combine positional and flag based arguments

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.