June 2018 Update

Task/Loops-While/Haskell/loops-while-1.hs

@@ -1,4 +1,5 @@
 import Control.Monad (when)
+
 main = loop 1024
   where loop n = when (n > 0)
                       (do print n

Task/Loops-While/Haskell/loops-while-2.hs

@@ -1,5 +1,10 @@
-import Control.Monad (when)
+import Data.IORef
+import Control.Monad.Loops
 
-whileM :: (Monad m) => m Bool -> m a -> m ()
-whileM cond body = do c <- cond
-                      when c (body >> whileM cond body)
+main :: IO ()
+main = do r <- newIORef 1024
+          whileM_ (do n <- readIORef r
+                     return (n > 0))
+                  (do n <- readIORef r
+                     print n
+                     modifyIORef r (`div` 2))

Task/Loops-While/Haskell/loops-while-3.hs

@@ -1,9 +1,11 @@
+{-# LANGUAGE MonadComprehensions #-}
 import Data.IORef
+import Control.Monad.Loops
 
 main :: IO ()
-main = do r <- newIORef 1024
-          whileM (do n <- readIORef r
-                     return (n > 0))
-                 (do n <- readIORef r
-                     print n
-                     modifyIORef r (`div` 2))
+main = do
+   r <- newIORef 1024
+   whileM_ [n > 0 | n <- readIORef r] $ do
+        n <- readIORef r
+        print n
+        modifyIORef r (`div` 2)