How to make `switch` parse successfully only when the flag is passed?

[Dario Oddenino]

Hello,
not sure if the title is correct, but I'll explain my use case:

I want to be able to pass a --version to show the current version, so I had this (not working) implementaiton:

parser =
           SomeCommand1 <$ T.subcommand ...
  T.<|> SomeCommand2 <$ T.subcommand...
  T.<|> Version <$ T.switch "version" 'v' "print the version"

The problem is that Version always parse successfully, so, for example, when I run my script without any command instead of the help message I get the version.

What's the correct way to handle this?

Thanks a lot!

[Gabriella Gonzalez]

@dariooddenino: One solution is to restructure the options like this:

data Options = Options { command :: Command, version :: Bool }

parser = Options <$> commands <*> switch "version" 'v' "print the version"

commands = SomeCommand1 <$ T.subcommand ... <|> ...

Another option is to use the lower-level optparse-applicative library and use the flag' utility, which fails to parse if you don't supply the flag:

http://hackage.haskell.org/package/optparse-applicative-0.14.3.0/docs/Options-Applicative-Builder.html#v:flag-39-

[Dario Oddenino]

Thanks! I went with the second approach :)

[Gabriella Gonzalez]

You're welcome! 🙂