[Haskell-beginners] Meaning of (Eq a)

[Hein Hundal]

> Date: Sun, 5 Sep 2010 14:28:09 +0530
> From: Rohit Garg <rpg.314 at gmail.com>
>
> 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 equality 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

When I compile isPalindrome with ghc 6.12.1, I do not get any errors even if I drop the type signature.  If I drop the type signature and use ghci to get the type signature (":t isPalindrome"), then I get 


*Main> :t isPalindrome
isPalindrome :: (Eq t) => [t] -> Bool


So, it seems that ghc 6.12.1 correctly infers the types.

Cheers,
Hein

PS:  Haskell does equality of lists, so you can also use the definition 

isPalindrome v = v == reverse v