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

8 Dec 2011


      On Wed, Dec 7, 2011 at 11:46 PM, Brandon Allbery wrote:
...
On Wed, Dec 7, 2011 at 23:24, Alexej Segeda 
...
wrote:
...
case s of
                   (s == reverse s)    -> putStrLn (s ++ " is a
palindrome")
                   otherwise           -> putStrLn (s ++ " is not a
palindrome")
case does pattern matching, not Boolean expressions.  (s == reverse s) is
not a useful pattern, and in fact is probably a syntax error because ==is not a valid infix constructor.
If you want to do Boolean comparisons in a case, you need to use
something like
...
case () of
  () | s == reverse s -> putStrLn "palindrome"
  _                   -> putStrLn "nope"
This is kind of a hack of case, though. I think what the OP was looking for
is

palindrome :: IO ()
palindrome = do putStrLn "Type in a word"
                s <- getLine
                isPalindrome s

isPalindrome word
  | (word == reverse word) = putStrLn (word ++ " is a palindrome")
  | otherwise              = putStrLn (word ++ " is not a palindrome")


amindfv / Tom

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

Tom Murphy