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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeFamilyDependencies #-}
import Control.Monad.Writer (WriterT, runWriterT, tell)
import Data.Constraint (Dict (Dict))
import Data.Foldable (for_)
import Data.Kind (Type)
data SBool (b :: Bool) where
STrue :: SBool 'True
SFalse :: SBool 'False
data SomeSBool where
SomeSBool :: SBool b -> SomeSBool
fromSBool :: SBool b -> Bool
fromSBool STrue = True
fromSBool SFalse = False
toSBool :: Bool -> SomeSBool
toSBool True = SomeSBool STrue
toSBool False = SomeSBool SFalse
withSomeSBool :: SomeSBool -> (forall (b :: Bool). SBool b -> r) -> r
withSomeSBool (SomeSBool s) f = f s
class Monad (LoggingMonad b) => MonadLogging (b :: Bool) where
type LoggingMonad b = (r :: Type -> Type) | r -> b
logMsg :: String -> LoggingMonad b ()
runLogging :: LoggingMonad b a -> IO a
instance MonadLogging 'False where
type LoggingMonad 'False = IO
logMsg _ = pure ()
runLogging = id
instance MonadLogging 'True where
type LoggingMonad 'True = WriterT [String] IO
logMsg s = tell [s]
runLogging m = do
(a, w) <- runWriterT m
for_ w putStrLn
pure a
program :: MonadLogging b => LoggingMonad b ()
program = do
logMsg "hello world"
pure ()
dict :: (c 'True, c 'False) => SBool b -> Dict (c b)
dict STrue = Dict
dict SFalse = Dict
test :: Bool -> IO ()
test bool =
withSomeSBool (toSBool bool) $ \(sb :: SBool b) ->
case dict @MonadLogging sb of
Dict -> runLogging @b program
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