[Haskell-beginners] Type signature question

Louis-Guillaume Gagnon louis.guillaume.gagnon at gmail.com
Tue Jul 23 17:21:16 CEST 2013

2013/7/23 Gesh hseG <gesh at gesh.uni.cx>:

> The inference works somewhat like this:
> GHC sees the definitions of firstHalf, secondHalf and infers they have
> the same type as xs,
> and that all three of them must be some lists for them to be passed to
> and be returned by take and drop.
> GHC sees firstHalf == secondHalf. Since (==) has type Eq a => a -> a
> -> Bool, firstHalf (and secondHalf and xs)
> must have types which are instances of the typeclass Eq.
> Looking at the instance declarations for Eq, we find that for all
> types t which are instances of Eq, the type [t]
> (lists of values of type t) is also an instance of Eq.
> In other words, instance Eq a => Eq [a] where ...
> Thus, we have that firstHalf, secondHalf, xs :: Eq t => [t] and cannot
> infer a more specific type for these three names,
> leaving us with the signature Eq t => [t] -> Bool for isPalindrome.
> Intuitively, since you're checking whether the first half of the list
> is equal to the second half, you need to be able to
> check that the first element is equal to the last, second to the
> before-last, etc.

great post, thanks!

> Also, notice that you could just have written
> isPalindrome xs = xs == reverse xs

Why didn't I think of that!?


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