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Add evalListT to get results from ListT as a list #184

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Add evalListT to get results from ListT as a list #184

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6b6a9b5
Add evalListT to get results from ListT as a list
Oct 20, 2016
bb96432
Make evalListT actually return the list and not an empty list
Oct 20, 2016
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11 changes: 11 additions & 0 deletions src/Pipes.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -46,6 +46,7 @@ module Pipes (
-- * ListT
, ListT(..)
, runListT
, evalListT
, Enumerable(..)

-- * Utilities
Expand Down Expand Up @@ -78,7 +79,9 @@ import Control.Monad.Writer (MonadWriter(..))
import Control.Monad.Zip (MonadZip(..))
import Pipes.Core
import Pipes.Internal (Proxy(..))
import Pipes.Lift (writerP, execWriterP)
import qualified Data.Foldable as F
import qualified Control.Monad as M

#if MIN_VERSION_base(4,8,0)
import Control.Applicative (Alternative(..))
Expand Down Expand Up @@ -566,6 +569,14 @@ runListT :: Monad m => ListT m a -> m ()
runListT l = runEffect (enumerate (l >> mzero))
{-# INLINABLE runListT #-}

-- | Run a self-contained `ListT` computation and accumulate the produced values as a list in the underlying monad
evalListT :: Monad m => ListT m a -> m [a]
evalListT listT =
runEffect
$ execWriterP
$ (writerP $ (\() -> ((), [])) <$> enumerate listT)
>-> (M.forever $ pass ((\element -> ((), (element :))) <$> await))

{-| 'Enumerable' generalizes 'Data.Foldable.Foldable', converting effectful
containers to 'ListT's.

Expand Down