[Haskell-beginners] Type signature question

[Brandon Allbery]

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

> 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.
>

It shows up because of `firstHalf == secondHalf`; you used (==), this
requires an Eq constraint so that the type-appropriate implementation of
(==) is available.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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