haskelloptparse-applicative

optparse-applicative non empty string argument

How can I implement a non empty string argument?

parserStart :: Parser String
parser = strArgument (metavar "EXAMPLE")

How can prevent it from successfully parsing an empty string - ""?

Having previous experience with Parsec/Attoparsec, I assume I could write my own parser or possibly use something applicative to inspect the value and fail if it's an empty string (I think this won't be possible as I need a Monad for this)?

Solution 
  • import Data.String
import Data.Text
import Options.Applicative.Types (ReadM, readerAsk)

nonEmptystr :: IsString s => ReadM s
nonEmptystr = do
  readerAsk >>= \case
    "" -> fail "Invalid argument: Empty string"
    x -> pure $ fromString x

f :: Parser Text
f = argument nonEmptystr (metavar "task")