Initial data commit

Task/Balanced-brackets/Haskell/balanced-brackets-3.hs

@@ -0,0 +1,31 @@
+import Control.Monad
+import System.Random
+import Text.Printf
+import VShuffle
+
+-- Return whether a string contains balanced brackets.  Nothing indicates a
+-- balanced string, while (Just i) means an imbalance was found at, or just
+-- after, the i'th bracket.  We assume the string contains only brackets.
+isBalanced :: String -> Maybe Int
+isBalanced = bal (-1) 0
+  where
+    bal :: Int -> Int -> String -> Maybe Int
+    bal _ 0 [] = Nothing
+    bal i _ [] = Just i
+    bal i (-1) _ = Just i
+    bal i n ('[':bs) = bal (i + 1) (n + 1) bs
+    bal i n (']':bs) = bal (i + 1) (n - 1) bs
+
+-- Print a string, indicating whether it contains balanced brackets.  If not,
+-- indicate the bracket at which the imbalance was found.
+check :: String -> IO ()
+check s = maybe (good s) (bad s) (isBalanced s)
+  where
+    good = printf "Good \"%s\"\n"
+    bad s n = printf "Bad  \"%s\"\n%*s^\n" s (n + 6) " "
+
+main :: IO ()
+main = do
+  let bs = cycle "[]"
+  rs <- replicateM 10 newStdGen
+  zipWithM_ (\n r -> check $ shuffle (take n bs) r) [0,2 ..] rs