This package implements effects, as alternative to monad transformers. Actually, the effects themselves are created without the use of TH, but the binding of nested effects described by mkEff splice. For example.

mkEff "MyReader"    ''Reader    ''Int       ''Lift
mkEff "SomeState"   ''State     ''Bool      ''MyReader
mkEff "OtherRdr"    ''Reader    ''Float     ''SomeState

main:: IO ()
main = do
    r <- runMyReader  100
       $ runSomeState False
       $ runOtherRdr  pi  $ do
            i :: Int   <- ask                    -- MyReader
            f :: Float <- ask                    -- OtherRdr
            b <- get                             -- SomeState
            put $ not b                          -- SomeState
            lift $ putStrLn "print from effect!" -- Lift
            return $ show $ fromIntegral i * f
    print r

For more information about extensible effects , see the original paper at http://okmij.org/ftp/Haskell/extensible/exteff.pdf. But, this package is significantly different from the original. It uses a chains of ordinary GADTs created by TH. No Typeable, no unsafe... , no ExistentialQuantification ...

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