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

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 9/5/10 04:58 , Rohit Garg wrote:

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.

Haskell won't do type inference if you provide a function with a type signature, so if you say anything at all you need to say everything about it. If you say *nothing* about it, as with the second function, Haskell will infer everything including required constraints. - -- brandon s. allbery [linux,solaris,freebsd,perl] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.10 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkyDrYwACgkQIn7hlCsL25U60ACguFkSUsfyPdk77Q7AmA5McVcu 0S0An3lbVUtVyzGCd6SSer2bksufokpd =NAqj -----END PGP SIGNATURE-----