単相化された存在型のランク2での謎
ランク2多相をランク1多相として使おうとしたときの問題？
これはできるのですが
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE Rank2Types #-}
import Control.Monad (void)
import Control.Monad.IO.Class (MonadIO, liftIO)
import Data.String (IsString, fromString)
import System.Environment (getEnv)
type FilePath' = forall s. IsString s => s
inHomeDir :: MonadIO m => (FilePath' -> m a) -> m a
inHomeDir f = do
homeDir <- liftIO $ getEnv "HOME"
f $ fromString homeDir
printFooAppDir :: IO ()
printFooAppDir = inHomeDir body
where -- 型を明示するために関数分割してます
body :: FilePath -> IO ()
body homeDir = putStrLn $ homeDir ++ "/.config/foo"
main :: IO ()
main = printFooAppDirこれは
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE Rank2Types #-}
import Control.Monad (void)
import Control.Monad.IO.Class (MonadIO, liftIO)
import Data.Monoid ((<>))
import Data.String (IsString, fromString)
import Data.Text (Text)
import Data.Typeable (Typeable)
import Data.Typeable (cast)
import System.Environment (getEnv)
import qualified Data.Text.IO as TIO
type FilePath' = forall s. IsString s => s
newtype BakaFilePath = BakaFilePath { unBakaFilePath :: Text }
deriving (IsString, Monoid, Typeable)
putBakaLn :: BakaFilePath -> IO ()
putBakaLn = TIO.putStrLn . unBakaFilePath
inHomeDir :: MonadIO m => (FilePath' -> m a) -> m a
inHomeDir f = do
homeDir <- liftIO $ getEnv "HOME"
f $ fromString homeDir
printFooAppDir :: IO ()
printFooAppDir = id' . inHomeDir $ body . cast'
where
-- MonadIO mからMonadIO nへの単相的な変換。 IO ()を期待する
id' :: IO () -> IO ()
id' = id
cast' :: FilePath -> Maybe BakaFilePath
cast' = cast
body :: Maybe BakaFilePath -> IO ()
body Nothing = error "baka"
body (Just bakaHomeDir) = putBakaLn $ bakaHomeDir <> "/.config/foo"
main :: IO ()
main = printFooAppDirこうなり
Test.hs|34 col 24| error:
• Couldn't match type ‘FilePath’ with ‘FilePath'’
Expected type: (FilePath -> IO ()) -> IO ()
Actual type: (FilePath' -> IO ()) -> IO ()
• In the second argument of ‘(.)’, namely ‘inHomeDir’
In the expression: id' . inHomeDir
In the expression: id' . inHomeDir $ body . cast'コンパイルに失敗します。
(bakaHomeDir :: BakaFilePath) <> ("/.config/foo" :: BakaFilePath)としてもだめ。
回答
inHomeDirを単相化すればいいみたい。
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE Rank2Types #-}
import Control.Monad (void)
import Control.Monad.IO.Class (MonadIO, liftIO)
import Data.Monoid ((<>))
import Data.String (IsString, fromString)
import Data.Text (Text)
import Data.Typeable (Typeable)
import Data.Typeable (cast)
import System.Environment (getEnv)
import qualified Data.Text.IO as TIO
type FilePath' = forall s. IsString s => s
newtype BakaFilePath = BakaFilePath { unBakaFilePath :: Text }
deriving (IsString, Monoid, Typeable)
putBakaLn :: BakaFilePath -> IO ()
putBakaLn = TIO.putStrLn . unBakaFilePath
inHomeDir :: MonadIO m => (FilePath' -> m a) -> m a
inHomeDir f = do
homeDir <- liftIO $ getEnv "HOME"
f $ fromString homeDir
printFooAppDir :: IO ()
printFooAppDir = id' . (inHomeDir :: (FilePath -> IO ()) -> IO ()) $ body . cast'
where
id' :: IO () -> IO ()
id' = id
cast' :: FilePath -> Maybe BakaFilePath
cast' = cast
body :: Maybe BakaFilePath -> IO ()
body Nothing = error "baka"
body (Just bakaHomeDir) = putBakaLn $ bakaHomeDir <> "/.config/foo"
main :: IO ()
main = printFooAppDirこれcastに失敗してNothingしてerror呼ぶけど、問題ないね！
コンパイル通れば、あとはどうとでもできる。
これは、xmonadとshellyを使う際に、 xmonadの関数がStringを期待するのに対して、 shellyがsystem-filepathのFilePathという独自のデータ型を扱っている故に遭遇したもの…… の簡略系となっています。
皆様どうか、楽しい多相ライフを！
