Question

I m trying my hand at Euler Problem 4 in Haskell. It asks for that largest palindrome formed by multiplying two three-digit numbers. The problem was simple enough, and I thought my Haskell-fu was up to the task, but I m getting a result that looks inconsistent to say the least.

Here s my palindrome detector (which was simplicity itself to code):

isPalindrome :: String -> Bool
isPalindrome [] = True
isPalindrome str = let str2 = reverse str
                   in (str2 == str)

From here it s a simple question of writing a function to detect when a product forms a palindrome (and possibly to subtract one from one of the multiplicands and recurse over a brute-force search if it doesn t). Here s my very simplified version of this, stripped down and returning an IO action for debugging:

findPal :: Integer -> Integer -> IO()
findPal 1 y = putStrLn "reached 1"
findPal x y = let pal = isPalindrome $ show mult 
                  mult = x * y
                  in case pal of
                          true -> putStrLn $ "mult is " ++ (show mult)
                          false -> putStrLn "pal is false"

Here are two separate outputs in GHCi:

*Main> isPalindrome $ show (999*999)
False
*Main> findPal 999 999
mult is 998001

In other words, the call to isPalindrome is always evaluating to true in findPal s case statement, even when it should be false.

What am I not seeing here?

Answer 1

I think you need to capitalize "True" and "False." I don t have a Haskell interpreter handy but you are probably just declaring a new variable "true" to be equal to "pal"

Answer 2

Could it be that in findPal, you should write True and False instead of true and false?

EDIT: Ok, thoroughly beaten by the early bird here...

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