fix runListT (#89)

* stack-safe foldl

* name TCOd foldl fodlRec

* add stack-safe foldl'

* Adapt runListT from Haskell's List.Transformer

* ListT examples

* simplify runListT

Unnecessary use of `void action`.

* add runListTRec and use in examples

Author: matthewleon

src/Control/Monad/List/Trans.purs

@@ -20,6 +20,8 @@ module Control.Monad.List.Trans
   , prepend
   , prepend'
   , repeat
+  , runListT
+  , runListTRec
   , scanl
   , singleton
   , tail
@@ -70,9 +72,14 @@ data Step a s
   | Skip (Lazy s)
   | Done
 
--- | Run a computation in the `ListT` monad.
-runListT :: forall f a. ListT f a -> f (Step a (ListT f a))
-runListT (ListT fa) = fa
+-- | Drain a `ListT`, running it to completion and discarding all values.
+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

test/Example/List.purs

@@ -0,0 +1,37 @@
+module Example.List where
+
+import Prelude
+import Data.Array as A
+import Control.Monad.Eff (Eff)
+import Control.Monad.Eff.Class (liftEff)
+import Control.Monad.Eff.Console (CONSOLE, log)
+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 = runListTRec do
+  liftEff $ log "logging listT"
+  str <- l
+  liftEff $ log str
+
+-- based on https://wiki.haskell.org/ListT_done_right#Sum_of_squares
+sumSqrs :: forall eff.
+   Int
+-> ListT (Eff (console :: CONSOLE | eff)) Unit
+sumSqrs n = do
+  let
+    nats = iterate (add one) zero
+    squares = takeWhile (_ <= n) $ map (\x -> x * x) nats
+  x <- squares
+  y <- squares
+  liftEff $ log ("<" <> show x <> "," <> show y <> ">")
+  guard $ x + y == n
+  liftEff $ log "Sum of squares."
+
+main :: forall eff. Eff (console :: CONSOLE | eff) Unit
+main = do
+  logList $ A.toUnfoldable ["one", "two", "three"]
+  runListTRec $ sumSqrs 10

test/Test/Main.purs

@@ -11,6 +11,7 @@ import Example.State as State
 import Example.StateEff as StateEff
 import Example.Writer as Writer
 import Example.RWS as RWS
+import Example.List as List
 
 main :: Eff (console :: CONSOLE) Unit
 main = do
@@ -20,3 +21,4 @@ main = do
   StateEff.main
   Writer.main
   RWS.main
+  List.main