I just found out that my carefully crafted parser fails to parse any string I throw at it:
roi :: Parser (Maybe ROI)
roi = optional $ option (ROI <$> auto <*> auto <*> auto <*> auto)
$ long "roi" <> metavar "ROI" <> help "Only process selected region of interest"
where ROI = ROI Int Int Int Int
If that is important, it is nested in a higher parser
options :: Parser Opts
options = Opts <$> input <*> output <*> roi <*> startT <*> endT
where Opts is an appropriate ADT.
Now I assumed that the roi parser will parse expressions such as --roi 1 2 3 4 but it fails with Invalid argument '128' and giving me usage message.
--roi 1 instead parses but returns Just (ROI 1 1 1 1)
Is there a way to make this work?
I don't think options are supposed to consume multiple arguments. At least I'm not sure how you'd go about implementing that. I'd suggest simply going away from that idea and putting your ROI options into a single argument, using syntax like --roi 1,2,3,4.
You'd simply have to implement a custom reader for that, here's an example of how you could do that:
module Main where
import Options.Applicative
data ROI = ROI Int Int Int Int
deriving Show
-- didn't remember what this function was called, don't use this
splitOn :: Eq a => a -> [a] -> [[a]]
splitOn sep (x:xs) | sep==x = [] : splitOn sep xs
| otherwise = let (xs':xss) = splitOn sep xs in (x:xs'):xss
splitOn _ [] = [[]]
roiReader :: ReadM ROI
roiReader = do
o <- str
-- no error checking, don't actually do this
let [a,b,c,d] = map read $ splitOn ',' o
return $ ROI a b c d
roiParser :: Parser ROI
roiParser = option roiReader (long "roi")
main :: IO ()
main = execParser opts >>= print where
opts = info (helper <*> roiParser) fullDesc