Single hyphen/dash argument for indicating stdin

[moll]

Hey

Thanks for you continued maintenance of Optparse Applicative!

I'm having trouble finding a way to specify a single - as an expected value, a pattern used to refer to reading from standard input. I'd like the help text to show (-|<path>) and the parsing to result in either, let's say, Nothing given - on the command line and Just path :: Maybe FilePath if a path other than - was given. Is that possible at the level of Optparse?

In other words, I'm looking for something to replace … with:

(Opt.flag' Nothing (…) <|>
(Just <$> Opt.argument Opt.str (Opt.metavar "<path>")))

This should then permit invoking an app with ./foo - to read from stdin and ./foo file to read from file, with the --help text nicely differentiating the two use-cases.

Thanks in advance!

[moll]

I'm thinking, if there were a mechanism for specifying expected constants, one could use that to both expect a single dash - argument and even support other advanced syntax in the style of ./foo rename <from> to <target>.

I noticed eitherReader. Maybe that combined with argument and metavar could be used to fake what I'm going for...

[moll]

For now, I ended up going with:

data PathArg path = StdinArg | PathArg path

Opt.argument
  (Opt.maybeReader parsePathArg)
  (mconcat [Opt.metavar "(-|<csv-path>)"])

parsePathArg "-" = Just StdinArg
parsePathArg path = Just (PathArg path)

[HuwCampbell]

That's more or less what I would have suggested.

[HuwCampbell]

There's a solution to this which you've figured out for yourself, but obviously this could be better documented.

I'll add a page to the wiki for it when I get some time.

[moll]

Thanks! And a built-in constant parser may be useful in the future, too, as my solution is rather verbose.