How do you provide a specific error message with optparse-applicative when multiple mutually exclusive options are provided?

Question

It's easy to specify mutually exclusive options with optparse-applicative:

data Exclusive = E1 | E2

exclusiveParser :: Parser ExclusiveOption
exclusiveParser = 
        (flag' E1 (short 'e1')
    <|> (flag' E2 (short 'e2')

The above parser will parse either -e1 or -e2, but not both. The default optparse-applicative action when both -e1 and -e2 are provided is to print the application's usage message. I would like to provide the user with a specific error message informing them that they cannot provide both -e1 and -e2, but I don't see an obvious way to do that.

Any suggestions (or solutions) would be appreciated?

Answer 1

I'm not familiar with optparse-applicative, so I'm not sure what error-printing facilities it provides. (Sometimes parser combinator libraries offer a primitive that changes the error that's printed, but I didn't see anything for that on a quick skim of the optparse-applicative docs. Entirely possible that I missed it.)

But in case nothing is available from the library itself, you could always print your own message by accepting both flags; e.g.

data Exclusive = E1 | E2 | Both
exclusiveParser
     =  (flag' E1 (short 'e'))
    <|> (flag' E2 (short 'f'))
    <|> (flag' Both (short 'e') <* flag' Both (short 'f'))

Then in your top-level handler (i.e. once the options are all parsed), if you see Both, you can issue an error message of your own crafting at that moment.