module Rakka.Utils
    ( parseYesOrNo
    , maybeA
    , defaultTo
    , deleteIfEmpty
    )
    where

import           Control.Arrow
import           Control.Arrow.ArrowList


parseYesOrNo :: ArrowChoice a => a String Bool
parseYesOrNo 
    = proc str -> do case str of
                       "yes" -> returnA -< True
                       "no"  -> returnA -< False
                       _     -> returnA -< error ("Expected yes or no: " ++ str)


maybeA :: (ArrowList a, ArrowChoice a) => a b c -> a b (Maybe c)
maybeA a = listA a
           >>>
           proc xs -> case xs of
                        []    -> returnA -< Nothing
                        (x:_) -> returnA -< Just x


defaultTo :: ArrowChoice a => b -> a (Maybe b) b
defaultTo def
    = proc m -> case m of
                  Nothing -> returnA -< def
                  Just x  -> returnA -< x


deleteIfEmpty :: (ArrowList a, ArrowChoice a) => a String String
deleteIfEmpty
    = proc str -> do case str of
                       "" -> none    -< ()
                       _  -> returnA -< str