On 09/05/2010 02:28 PM, Rohit Garg wrote:
Hi,
RWH: chapter 3 - in question 5, you have to write a function which determines if a list is a palindrome. Here is my solution
isPalindrome :: (Eq a) => [a] -> Bool isPalindrome [] = False isPalindrome x = compareLists x (reverse x) where compareLists [x] [y] = x == y compareLists (x:xs) (y:ys) = if x == y then compareLists xs ys else False
Although it works, my question is why ghci refuses to run it without the "(Eq a) => " being added to the type signature of the function. Presumably, it is to let ghc know that you can perform equlity tests on a. If so, then why does the sumList function below work without any type signature of any kind? I haven't told ghc that the input list elements can be added together.
sumList [] = 0 sumList (x:xs) = x + sumList xs
Thanks, Hi Rohit, You are correct about the assumption. Giving (Eq a) is constraining the set of types that can be used with to the palindrome function. The elements of the type a can be tested for equality.
The compiler can perform type inference. It will find out the type signature if you do not provide one. Assume that you did not provide the type signature for the isPalindrome function, here is what I get when I loaded this code into ghci. *Main> :t isPalindrome isPalindrome :: (Eq t) => [t] -> Bool For the sumList function, since you did not provide the type information, the compiler will infer the type for the argument and result of the function. However, as you point out correctly, not all types can be added. Hence constraint (Num t) will be added to the type signature. This means that the elements of the type "t" can be added. If not, then that type cannot be used and a compiler error will result. sumList :: (Num t) => [t] -> t Hope that helps -Lakshmi Narasimhan