[Haskell-beginners] applicative instance

[sasa bogicevic]

Yep that worked, thanks.
Is there a way to do it without defining a separate function like plusPlus ?





> On Jan 28, 2017, at 10:43, Francesco Ariis <fa-ml at ariis.it> wrote:
> 
> On Sat, Jan 28, 2017 at 10:09:10AM +0100, sasa bogicevic wrote:
>> Ok so how would the implementation look to get the correct result ?
>> I can't seem to write something that will compile except ZipList version.
> 
> One way is by implementing your own (++):
> 
>    data List a = Nil | Cons a (List a) deriving (Eq, Show)
> 
>    plusPlus :: List a -> List a -> List a
>    plusPlus Nil         bs = bs
>    plusPlus (Cons a as) bs = Cons a (as `plusPlus` bs)
> 
>    instance Functor List where
>        fmap f Nil = Nil
>        fmap f (Cons a b) = Cons (f a) (fmap f b)
> 
>    instance Applicative List where
>        pure x = Cons x Nil
>        Nil <*> _ = Nil
>        _ <*> Nil = Nil
>        (Cons x xy) <*> ys = (fmap x ys) `plusPlus` (xy <*> ys)
> 
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