Applicative 类声明为：
class Functor f => Applicative f where
pure :: a -> f a
(<*>) :: f (a -> b) -> f a -> f b
我们可以代表fmapi, i=0,1,2,...按照pure and (<*>):
fmap0 :: a -> f a
fmap0 = pure
fmap1 :: (a -> b) -> f a -> f b
fmap1 g x = pure g <*> x
fmap2 :: (a -> b -> c) -> f a -> f b -> f c
fmap2 g x y = pure g <*> x <*> y
fmap3 :: (a -> b -> c -> d) -> f a -> f b -> f c -> f d
fmap3 g x y z = pure g <*> x <*> y <*> z
在应用上，如何能<*>可以表示为fmap_i, i=0,1,2,...?
Thanks.
也可以看看基于“fmap”的“”实现对于 Maybe applicative 来说是特殊的还是可以推广到其他 applicative？ https://stackoverflow.com/questions/57220345/is-the-implementation-of-based-on-fmap-special-to-maybe-applicative-or-c
你可以写：
(<*>) = fmap2 ($)
或者，如果您发现它不那么晦涩：
f <*> a = fmap2 apply f a
where apply g x = g x
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