[Haskell-beginners] applicative instance

[Benjamin Edwards]

You are zipping rather than taking the cross product.

Ben

On Fri, 27 Jan 2017, 22:09 sasa bogicevic, <brutallesale at gmail.com> wrote:

> What is wrong with my applicative instance for custom List type ?
>
> http://lpaste.net/351723
>
> data List a = Nil | Cons a (List a) deriving (Eq, Show)
>
>
>
> instance Applicative List where
>     pure x = Cons x Nil
>     Nil <*> _ = Nil
>     _ <*> Nil = Nil
>     (Cons x xy) <*> (Cons z dy)   =  Cons (x z)  (xy <*> dy)
>
> Prelude> let functions = Cons (+1) (Cons (*2) Nil)
> Prelude> let values = Cons 1 (Cons 2 Nil)
> Prelude> functions <*> values
> Cons 2 (Cons 3 (Cons 2 (Cons 4 Nil)))  -- I get Cons 2 (Cons 4 Nil) what
> is wrong with my Applicative instance ?
>
>
> {
>         name: Bogicevic Sasa
>         phone: +381606006200
> }
>
>
>
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