[Haskell-cafe] Why doesn't this work? (palindrome :: IO)

[Alexej Segeda]

Hi!

A couple of months ago, I wrote an exam in an introductory Haskell course and failed, all because of an assignment that I was convinced would work, but for some reason, it didn't. The assignment was to write a function that would take a line, then determine whether it's a palindrome or not. My code follows:

palindrome :: IO ()
palindrome = do putStr "Type in a word"
                s <- getLine
                case s of
                   (s == reverse s)    -> putStrLn (s ++ " is a palindrome")
                   otherwise           -> putStrLn (s ++ " is not a palindrome")

The interesting thing is, that if I change the "case ... of" statement to an "if ... then ... else" statement, this magically starts to work. Since I no longer am enrolled (I have to take the course next year), I can't ask a teacher, but my curiosity still bugs me. Why doesn't this work? And why does it work with a "if ... then ...else" statement?
 		 	   		  
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