Haskell：是否有这样的运算符：（<$$>）:: Functor f => fa->（a-> b）-> fb

Question

Finding out that map and flat map in Haskell could be performed the this way,

xs >>= f          ≃ flatMap f xs
xs >>= return . f ≃ map f xs

I thought it would be convenient if we have such an operator

(<$$>) :: Functor f => f a -> (a -> b) -> f b
f <$$> fn = fn <$> f

so that map could be expressed more tidily:

xs >>=  f ≃ flatMap f xs
xs <$$> f ≃ map f xs

Does such an operator exist in Haskell's standard library?

Answer 1

It's not anywhere in the base package. The most commonly used library that exports this function is lens , as <&> .

Answer 2

Note also that, as often to flip arguments, operator sections may be "enough" and may perhaps preserve readability by limiting the introduction of new operators:

> :{
| (<$> Just 0) $ \x ->
|   x + 1 + 1 + 1
|     + 1 + 1 + 1
|     + 1 + 1 + 1
|     + 1 + 1 + 1
| :}
Just 12