Add default values to help output

[asarkar]

It'd be nice to have a column in the help output listing the default values

[xenomachina]

Right now we only know how to parse from a string into value, but not go from an arbitrary value back into a string. We could use toString(), but this would often produce output inconsistent with the transform function.

Straw man idea: default() could simply have an optional String parameter for how to represent the default value. If unset we use toString().

[asarkar]

default() could simply have an optional String parameter for how to represent the default value. If unset we use toString()

Sounds good to me

[tgockel]

I implemented this fairly naively...and added a bogus flag to HelpTest:

class Args(parser: ArgParser) {
    val dryRun by parser.flagging("-n", "--dry-run",
            help = "don't do anything")
    val includes by parser.adding("-I", "--include",
            help = "search in this directory for header files")
    val outDir by parser.storing("-o", "--output",
            help = "directory in which all output should be generated")
    val language by parser.storing("-l", "--language",
            help = "language to output help message in")
            .default("Latin")
    val verbosity by parser.counting("-v", "--verbose",
            help = "increase verbosity")
    val sources by parser.positionalList("SOURCE",
            help = "source file")
    val destination by parser.positional("DEST",
            help = "destination file")
}

The output looks like this:

usage: program_name [-h] [-n] [-I INCLUDE]... -o OUTPUT
                    [-l LANGUAGE] [-v]... SOURCE... DEST


This is the prologue. Lorem ipsum dolor sit amet,
consectetur adipiscing elit. Aliquam malesuada maximus eros.
Fusce luctus risus eget quam consectetur, eu auctor est
ullamcorper. Maecenas eget suscipit dui, sed sodales erat.
Phasellus.

This is the second paragraph of the prologue. I don't have
anything else to say, but I'd like there to be enough text
that it wraps to the next line.


required arguments:
  -o OUTPUT,            directory in which all output
  --output OUTPUT       should be generated


optional arguments:
  -h, --help            show this help message and exit
                        (default: kotlin.Unit)

  -n, --dry-run         don't do anything (default: false)

  -I INCLUDE,           search in this directory for
  --include INCLUDE     header files (default: [])

  -l LANGUAGE,          language to output help message in
  --language LANGUAGE   (default: Latin)

  -v, --verbose         increase verbosity (default: 0)


positional arguments:
  SOURCE                source file

  DEST                  destination file


This is the epilogue. Lorem ipsum dolor sit amet,
consectetur adipiscing elit. Donec vel tortor nunc. Sed eu
massa sed turpis auctor faucibus. Donec vel pellentesque
tortor. Ut ultrices tempus lectus fermentum vestibulum.
Phasellus.

I'm not a fan of all the semi-confusing default values ((default: Kotlin.Unit), etc). So I added an opt-in boolean:

class Args(parser: ArgParser) {
    val dryRun by parser.flagging("-n", "--dry-run",
            help = "don't do anything")
    val includes by parser.adding("-I", "--include",
            help = "search in this directory for header files")
    val outDir by parser.storing("-o", "--output",
            help = "directory in which all output should be generated")
    val language by parser.storing("-l", "--language",
            help = "language to output help message in")
            .default("Latin", show = true)
    val verbosity by parser.counting("-v", "--verbose",
            help = "increase verbosity")
    val sources by parser.positionalList("SOURCE",
            help = "source file")
    val destination by parser.positional("DEST",
            help = "destination file")
}

Which makes the output look like this:

usage: program_name [-h] [-n] [-I INCLUDE]... -o OUTPUT
                    [-l LANGUAGE] [-v]... SOURCE... DEST


This is the prologue. Lorem ipsum dolor sit amet,
consectetur adipiscing elit. Aliquam malesuada maximus eros.
Fusce luctus risus eget quam consectetur, eu auctor est
ullamcorper. Maecenas eget suscipit dui, sed sodales erat.
Phasellus.

This is the second paragraph of the prologue. I don't have
anything else to say, but I'd like there to be enough text
that it wraps to the next line.


required arguments:
  -o OUTPUT,            directory in which all output
  --output OUTPUT       should be generated


optional arguments:
  -h, --help            show this help message and exit

  -n, --dry-run         don't do anything

  -I INCLUDE,           search in this directory for
  --include INCLUDE     header files

  -l LANGUAGE,          language to output help message in
  --language LANGUAGE   (default: Latin)

  -v, --verbose         increase verbosity


positional arguments:
  SOURCE                source file

  DEST                  destination file


This is the epilogue. Lorem ipsum dolor sit amet,
consectetur adipiscing elit. Donec vel tortor nunc. Sed eu
massa sed turpis auctor faucibus. Donec vel pellentesque
tortor. Ut ultrices tempus lectus fermentum vestibulum.
Phasellus.

This is all ultimately backed by fun <T> ArgParser.Delegate<T>.default(defaultValue: () -> T, showDefault: ((T) -> String)?): ArgParser.Delegate<T>, which allows a user to use a non-toString output if they're so inclined.

Let me know if this approach is makes sense or if there's some other ideas for formatting you had in mind.