instance Num Vector2の実装を簡単に

va.hs

import Control.Applicative

data Vector2 a = Vector2 a a deriving (Show, Eq)

instance Applicative Vector2 where
  pure x = Vector2 x x
  Vector2 fx fy <*> Vector2 x y = Vector2 (fx x) (fy y)

instance Functor Vector2 where
  fmap = liftA

instance Num a => Num (Vector2 a) where
   (+) = liftA2 (+)
   (-) = liftA2 (-)
   (*) = liftA2 (*)
   negate = fmap negate
   abs = fmap abs
   signum = fmap signum
   fromInteger = pure . fromInteger

instance (Fractional a) => Fractional (Vector2 a) where
   (/) = liftA2 (/)
   recip = fmap recip
   fromRational = pure . fromRational