数据2 ab = 2 ab的应用程序纯函数是什么

Question

Given this type:

data Two a b = Two a b deriving (Eq, Show)

What would be the Applicative definition. I'm not able to get the pure right without adding a constraint on a like for example Num a

instance (Num a, Monoid a) => Applicative (Two a) where
  pure b = Two 1 b
  Two f f' <*> Two a b = Two a (f' b)

Answer 1

You can't have it without a constraint. You need to do something about a and to be able to you need to have operations for that type, which you can get two ways: either use a specific type (eg, Int , or an abstract one, requiring a typeclass instance).

The most general constraint that comes to mind in such a case is Monoid , which renders your instance the same as for the 2-element tuple:

instance Monoid a => Applicative ((,) a) where
    pure x = (mempty, x)
    (u, f) <*> (v, x) = (u `mappend` v, f x)