ListT examples

test/Example/List.purs

@@ -0,0 +1,38 @@
+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.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
+  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 = L.iterate (add one) zero -- lazy infinite list
+    squares = L.toUnfoldable <<< L.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"]
+  runListT $ 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