BalancedDelimeters.hs

{-
http://programmingpraxis.com/2014/06/10/balanced-delimiters-2/
Examples of strings that pass: {}, [], (), a(b)c, abc[d], a(b)c{d[e]}
Examples of strings that don’t pass: {], (], a(b]c, abc[d}, a(b)c{d[e}]
-}
import Control.Monad(foldM,mapM_)

isHead _ [] = False
isHead x xs = x == head xs

balanced = maybe False null . foldM read []
	where
		read xs s
			| s `elem` ['{','(','['] = Just $ s:xs
			| s `elem` ['}',')',']'] = if (inv s) `isHead` xs then Just $ tail xs else Nothing
			| otherwise = Just xs
		inv '}' = '{' 
		inv ')' = '('
		inv ']' = '['

pass = [ "{}", "[]", "()", "a(b)c", "abc[d]", "a(b)c{d[e]}" ]
fails = [ "{]", "(]", "a(b]c", "abc[d}", "a(b)c{d[e}]" ]	
expectPass = zip pass (repeat True)	
expectFails = zip fails (repeat False)
		
main = mapM_ test $ expectPass  ++ expectFails
	where 
		test (str,expect) = let r = balanced str 
				    in putStrLn $ "balanced " ++ str ++ " = " ++ (show r) ++ ", expected " ++ (show expect)
{- outputs:
balanced {} = True, expected True
balanced [] = True, expected True
balanced () = True, expected True
balanced a(b)c = True, expected True
balanced abc[d] = True, expected True
balanced a(b)c{d[e]} = True, expected True
balanced {] = False, expected False
balanced (] = False, expected False
balanced a(b]c = False, expected False
balanced abc[d} = False, expected False
balanced a(b)c{d[e}] = False, expected False
-}