[Haskell-beginners] Type signature question

[Erik Price]

It's saying that you can't pass parameters to this function unless they are
of the Eq type class, because somewhere in the body of the function is code
that expects parameters to be of this type class.

e


On Tue, Jul 23, 2013 at 10:33 AM, Louis-Guillaume Gagnon <
louis.guillaume.gagnon at gmail.com> wrote:

> Hello beginners,
>
> I'm working on the second set of exercises in ch.03 of Real World Haskell
>
> I wrote a palindromize function (:: [a] -> [a]) which transforms a
> list in a palindrome (eg. [1,2,3] -> [1,2,3,3,2,1]) .
>
> I wrote a isPalindrome function which checks whether a given list is
> such a palindrome.
> it reads:
> isPalindrome xs
>      | odd (length xs)              = False
>      | firstHalf == secondHalf =True
>      | otherwise                       = False
>      where half              = div (length xs) 2
>                 firstHalf       = take half xs
>                 secondHalf = reverse (drop half xs)
>
> I would expect the type signature to be:
> isPalindrome :: [a] -> Bool
>
> but ghci gives me
> is Eq a => [a] -> Bool
>
> and I don't undestand why the "Eq a =>" shows up.
>
> many thanks,
>
> glg
>
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