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Optparse-applyative: how to handle a situation without arguments in Arrow syntax

Example: https://github.com/pcapriotti/optparse-applicative/blob/master/tests/Examples/Cabal.hs#L46-L62

parser :: Parser Args
parser = runA $ proc () -> do
  opts <- asA commonOpts -< ()
  cmds <- (asA . hsubparser)
            ( command "install"
              (info installParser
                    (progDesc "Installs a list of packages"))
           <> command "update"
              (info updateParser
                    (progDesc "Updates list of known packages"))
           <> command "configure"
              (info configureParser
                    (progDesc "Prepare to build the package"))
           <> command "build"
              (info buildParser
                    (progDesc "Make this package ready for installation")) ) -< ()
  A version >>> A helper -< Args opts cmds

...

pinfo :: ParserInfo Args
pinfo = info parser
  ( progDesc "An example modelled on cabal" )

main :: IO ()
main = do
  r <- execParser pinfo
  print r

So, and by default, when I do not use arguments, it shows usage information. I want to use a case with no arguments, or with one [User argument] (with user I get the following error: Invalid argument 'regreg')

How can I handle empty and custom arguments here?

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1 answer

A https://hackage.haskell.org/package/optparse-applicative-0.11.0.2/docs/Options-Applicative-Arrows.html ArrowPlus. , (, f Alternative), Alternative:

subparser (command "foo" ... ...) <|> pure DefaultCommand
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