Haskell-book/26/Embedded/src/OuterInner.hs


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module OuterInner where

import Control.Monad.Trans.Except
import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Reader

-- We only need to use return once
-- because it's one big Monad
embedded :: MaybeT
            (ExceptT String
                    (ReaderT () IO))
            Int
--embedded = return 1
embedded = MaybeT . ExceptT . ReaderT $ return . (const (Right (Just 1)))

-- We can sort of peel away the layers one by one:
maybeUnwrap :: ExceptT String
               (ReaderT () IO) (Maybe Int)
maybeUnwrap = runMaybeT embedded

-- Next
eitherUnwrap :: ReaderT () IO
                (Either String (Maybe Int))
eitherUnwrap = runExceptT maybeUnwrap

-- Lastly
readerUnwrap :: ()
             -> IO (Either String
                           (Maybe Int))
readerUnwrap = runReaderT eitherUnwrap