Help output when no arguments are provided. about optparse-applicative HOT 7 CLOSED

It is certainly possible. Use customExecParser (or any of the other functions in Extra that take a ParserPrefs) to run your option parser, and give it a ParserPrefs parameter containing showHelpOnError. For example:

import Options.Applicative

opts :: Parser Int
opts = option (  short 'n'
              <> metavar "THREADS"
              <> help "Set the number of threads" )

main :: IO ()
main = do
  r <- customExecParser
         (prefs showHelpOnError)
         (info (opts <**> helper) idm)
  print r

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Hi! Thank you for your response. I'm afraid this is not exactly what I want. I do not want to print help on error. I want print it only when there are no arguments provided. When error occurs I want to display only a message (like it is done by default).

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I'm afraid the only way to get this behaviour is to create your own version of customExecParser. You can find it in Options.Applicative.Extra. I'll leave this ticket open as a remainder to make customExecParser a bit more modular, so that you wouldn't need to copy-paste as much to get a custom behaviour.

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+1 I was also looking for a way to print help when a command is called with no arguments or flags.

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Hi! I'm currently considering one other use case, that I believe may be related (if not let me know, I'll post this as a new issue): would it be possible to define a default command? Lets say my program supports two commands, load and save, each receives specific options. But if the user did not specify either command, I would like to assume it is a load command, as it is almost always the case.

The same principle could be considered for help, I guess.

I've tried adding an empty command <> command "" (info parseLoadOpts ..., but then I must explicitly do:
program "" filetoload

Thanks

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@jcristovao I think you can just duplicate the options of your default command outside subparser, then run the parser with noBacktrack, see #31. Please do open another issue if that doesn't work for you or if I misunderstood your problem.

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#100 was marked as a duplicate of this, but I don't understand how to make a version of customExecParser that prints the help output.

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