Haskellのmplusってもしかしてすごい便利じゃない？
束縛(<-)とcaseでインデントが多くネストしてしまいそうな式でも、少ないインデントで済ませられる。
Maybeで使う
import Control.Monad (liftM2)
import Control.Monad (MonadPlus(..))
main :: IO ()
main = do
print $ Nothing `mplus` return 1
print $ return 1 `mplus` (return 2 :: Maybe Int)
print $ Nothing `mplus` (Nothing :: Maybe Int)
let x = return (return 1) :: IO (Maybe Int)
let y = return Nothing :: IO (Maybe Int)
print =<< liftM2 mplus x yJust 1
Just 1
Nothing
Just 1(IO Maybe)で使う
fooDBFile :: (MonadIO m, MonadThrow m) => m (Maybe FilePath)
fooDBFile = do
let maybeDir = liftM2 mplus (getEnv "XDG_DATA_DIR") (getEnv "HOME")
case maybeDir of
dir -> dir ++ "/foo.db"
Nothing -> errorベンリ。
