[Haskell-beginners] Meaning of (Eq a)

[Lakshmi Narasimhan]

  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