Haskell 中关于 <$> 和 <*> 的优先级混淆

Question

两个例子都来自http://learnyouahaskell.com/functors-applicative-functors-and-monoids#applicative-functors，

1）。 (+) <$> (+3) <*> (*100) $ 5

 (+) <$> (+3) <*> (*100) $ 5, the 5 first got applied to (+3) and
 (*100), resulting in 8 and 500. Then, + gets called with 8 and 500,
 resulting in 508.

从第一个示例来看，它的<*>优先级似乎高于<$>.

2）。 (++) <$> Just "johntra" <*> Just "volta"

 (++) <$> Just "johntra" <*> Just "volta",   resulting in a value
 that's the same as Just ("johntra"++),and now Just ("johntra"++) <*>
 Just "volta" happens, resulting in Just "johntravolta".

从第二个例子来看，它的<$>优先级似乎比<*>.

那么它们的优先级相同吗？有人可以给我一些解释/参考吗？

Answer 1

实际上，它们都具有相同的优先级（infixl 4:(<*>)和(<$>)），您可以从左到右阅读 -

(+) <$> (+3) <*> (*100) $ 5
= ((+) <$> (+3)) <*> (*100) $ 5
= (\ a b -> (a+3) + b) <*> (\ a -> a*100) $ 5
= (\ a -> (a+3) + (a*100)) $ 5
= 8 + 500 = 508

记住在这种情况下我们有f <*> g = \x -> f x (g x)