From 6b6a9b530a304020b58e64891c0e8741ca124b98 Mon Sep 17 00:00:00 2001
From: Edward Amsden <edwardamsden@gmail.com>
Date: Thu, 20 Oct 2016 10:25:37 -0400
Subject: [PATCH 1/2] Add evalListT to get results from ListT as a list

---
 src/Pipes.hs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/Pipes.hs b/src/Pipes.hs
index 91071c0..63e4419 100644
--- a/src/Pipes.hs
+++ b/src/Pipes.hs
@@ -46,6 +46,7 @@ module Pipes (
     -- * ListT
     , ListT(..)
     , runListT
+    , evalListT
     , Enumerable(..)
 
     -- * Utilities
@@ -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(..))
@@ -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 >> mzero))
+  >-> (M.forever $ pass ((\element -> ((), (element :))) <$> await))
+
 {-| 'Enumerable' generalizes 'Data.Foldable.Foldable', converting effectful
     containers to 'ListT's.
 

From bb96432ec23f5e03f08a111a9bc73f0d8cb1c3ee Mon Sep 17 00:00:00 2001
From: Edward Amsden <edwardamsden@gmail.com>
Date: Thu, 20 Oct 2016 10:44:01 -0400
Subject: [PATCH 2/2] Make evalListT actually return the list and not an empty
 list

---
 src/Pipes.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/Pipes.hs b/src/Pipes.hs
index 63e4419..9d43247 100644
--- a/src/Pipes.hs
+++ b/src/Pipes.hs
@@ -574,7 +574,7 @@ evalListT :: Monad m => ListT m a -> m [a]
 evalListT listT =
       runEffect
   $   execWriterP
-  $   (writerP $ (\() -> ((), [])) <$> (enumerate $ listT >> mzero))
+  $   (writerP $ (\() -> ((), [])) <$> enumerate listT)
   >-> (M.forever $ pass ((\element -> ((), (element :))) <$> await))
 
 {-| 'Enumerable' generalizes 'Data.Foldable.Foldable', converting effectful