Created
October 4, 2011 07:28
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Unwrap monad stacks within the enumeator package
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{-# LANGUAGE RankNTypes #-} | |
module Data.Enumerator.Unwrap | |
( unwrapEnumeratee | |
, unwrapStep | |
, unwrapIter | |
, liftStep | |
) where | |
import Control.Monad.Trans.Class (MonadTrans) | |
import Control.Exception (Exception, toException) | |
import Data.Enumerator (Enumeratee, Step (..), Iteratee (..), liftTrans) | |
import Control.Monad (liftM) | |
unwrapEnumeratee :: (Exception e, MonadTrans t, Monad m, Monad (t m)) | |
=> (forall x. t m x -> m (Either e x)) | |
-> Enumeratee ao ai (t m) b | |
-> Enumeratee ao ai m b | |
unwrapEnumeratee unwrap orig step = | |
unwrapIter unwrap $ liftM (unwrapStep unwrap) $ orig $ liftStep step | |
liftStep :: (MonadTrans t, Monad m, Monad (t m)) | |
=> Step a m b | |
-> Step a (t m) b | |
liftStep (Yield b s) = Yield b s | |
liftStep (Error e) = Error e | |
liftStep (Continue k) = Continue $ liftTrans . k | |
unwrapStep :: (Monad m, Exception e) | |
=> (forall x. t m x -> m (Either e x)) | |
-> Step a (t m) b | |
-> Step a m b | |
unwrapStep _ (Yield b s) = Yield b s | |
unwrapStep _ (Error e) = Error e | |
unwrapStep unwrap (Continue k) = Continue $ unwrapIter unwrap . k | |
unwrapIter :: (Monad m, Exception e) | |
=> (forall x. t m x -> m (Either e x)) | |
-> Iteratee a (t m) b | |
-> Iteratee a m b | |
unwrapIter unwrap (Iteratee mstep) = Iteratee $ do | |
estep <- unwrap mstep | |
case estep of | |
Left e -> return $ Error $ toException e | |
Right x -> return $ unwrapStep unwrap x |
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