add runListTRec and use in examples

Author: matthewleon

src/Control/Monad/List/Trans.purs

@@ -21,6 +21,7 @@ module Control.Monad.List.Trans
   , prepend'
   , repeat
   , runListT
+  , runListTRec
   , scanl
   , singleton
   , tail
@@ -75,6 +76,11 @@ data Step a s
 runListT :: forall f a. Monad f => ListT f a -> f Unit
 runListT = foldl' (\_ _ -> pure unit) unit
 
+-- | Drain a `ListT`, running it to completion and discarding all values.
+-- | Stack safe: Uses tail call optimization.
+runListTRec :: forall f a. MR.MonadRec f => ListT f a -> f Unit
+runListTRec = foldlRec' (\_ _ -> pure unit) unit
+
 -- | The empty list.
 nil :: forall f a. Applicative f => ListT f a
 nil = ListT $ pure Done

test/Example/List.purs

@@ -2,18 +2,17 @@ module Example.List where
 
 import Prelude
 import Data.Array as A
-import Data.List.Lazy as L
 import Control.Monad.Eff (Eff)
 import Control.Monad.Eff.Class (liftEff)
 import Control.Monad.Eff.Console (CONSOLE, log)
-import Control.Monad.List.Trans (ListT, runListT)
+import Control.Monad.List.Trans (ListT, runListTRec, iterate, takeWhile)
 import Control.MonadZero (guard)
 
 -- based on http://hackage.haskell.org/package/list-transformer
 logList :: forall eff.
    ListT (Eff (console :: CONSOLE | eff)) String
 -> Eff (console :: CONSOLE | eff) Unit
-logList l = runListT do
+logList l = runListTRec do
   liftEff $ log "logging listT"
   str <- l
   liftEff $ log str
@@ -24,8 +23,8 @@ sumSqrs :: forall eff.
 -> ListT (Eff (console :: CONSOLE | eff)) Unit
 sumSqrs n = do
   let
-    nats = L.iterate (add one) zero -- lazy infinite list
-    squares = L.toUnfoldable <<< L.takeWhile (_ <= n) $ map (\x -> x * x) nats
+    nats = iterate (add one) zero
+    squares = takeWhile (_ <= n) $ map (\x -> x * x) nats
   x <- squares
   y <- squares
   liftEff $ log ("<" <> show x <> "," <> show y <> ">")
@@ -35,4 +34,4 @@ sumSqrs n = do
 main :: forall eff. Eff (console :: CONSOLE | eff) Unit
 main = do
   logList $ A.toUnfoldable ["one", "two", "three"]
-  runListT $ sumSqrs 10
+  runListTRec $ sumSqrs 10