Instead of fmap, which applies a function to a value-in-a-functor:
而不是fmap，它将函数应用于函数值：
fmap :: Functor f => (a -> b) -> f a -> f b
I needed a function where the functor has a function and the value is plain:
我需要一个函数，其中函子有一个函数，值很简单：
thing :: Functor f => f (a -> b) -> a -> f b
but I can't find one.
但我找不到一个。
What is this pattern called, where I apply a function-in-a-functor (or in an applicative, or in a monad) to a plain value?
这个模式叫做什么，我将函数函数（或者在应用程序中，或者在monad中）应用于普通值？
I've implemented it already, I just don't quite understand what I did and why there wasn't already such a function in the standard libraries.
我已经实现了它，我只是不太明白我做了什么以及为什么标准库中没有这样的功能。
2 个解决方案
#1
19
You don't need Applicative for this; Functor will do just fine:
你不需要申请这个; Functor会做得很好：
apply f x = fmap ($ x) f
-- or, expanded:
apply f x = fmap (\f' -> f' x) f
Interestingly, apply is actually a generalisation of flip; lambdabot replaces flip with this definition as one of its generalisations of standard Haskell, so that's a possible name, although a confusing one.
有趣的是，申请实际上是翻转的概括; lambdabot用这个定义取代了flip作为标准Haskell的一个概括，所以这是一个可能的名称，虽然令人困惑。
By the way, it's often worth trying Hayoo (which searches the entirety of Hackage, unlike Hoogle) to see what names a function is often given, and whether it's in any generic package. Searching for f (a -> b) -> a -> f b, it finds flip (in Data.Functor.Syntax, from the functors package) and ($#) (from the synthesizer package) as possible names. Still, I'd probably just use fmap ($ arg) f at the use site.
顺便说一下，经常值得尝试的是Hayoo（它搜索整个Hackage，不像Hoogle）来查看函数的名称，以及它是否在任何通用包中。搜索f（a - > b） - > a - > f b，它找到flip（在Data.Functor.Syntax中，来自functors包）和（$＃）（来自合成器包）作为可能的名称。不过，我可能只是在使用网站上使用fmap（$ arg）f。
#2
7
As Niklas says, this is application in some applicative functor to a lifted value.
正如尼克拉斯所说，这是应用于一些应用函子，以提升价值。
\f a -> f <*> pure a
:: Applicative f => f (a -> b) -> a -> f b
or more generally (?), using Category (.)
或更一般地（？），使用类别（。）
\f a -> f . pure a
:: (Applicative (cat a), Category cat) => cat b c -> b -> cat a c
#1
19
You don't need Applicative for this; Functor will do just fine:
你不需要申请这个; Functor会做得很好：
apply f x = fmap ($ x) f
-- or, expanded:
apply f x = fmap (\f' -> f' x) f
Interestingly, apply is actually a generalisation of flip; lambdabot replaces flip with this definition as one of its generalisations of standard Haskell, so that's a possible name, although a confusing one.
有趣的是，申请实际上是翻转的概括; lambdabot用这个定义取代了flip作为标准Haskell的一个概括，所以这是一个可能的名称，虽然令人困惑。
By the way, it's often worth trying Hayoo (which searches the entirety of Hackage, unlike Hoogle) to see what names a function is often given, and whether it's in any generic package. Searching for f (a -> b) -> a -> f b, it finds flip (in Data.Functor.Syntax, from the functors package) and ($#) (from the synthesizer package) as possible names. Still, I'd probably just use fmap ($ arg) f at the use site.
顺便说一下，经常值得尝试的是Hayoo（它搜索整个Hackage，不像Hoogle）来查看函数的名称，以及它是否在任何通用包中。搜索f（a - > b） - > a - > f b，它找到flip（在Data.Functor.Syntax中，来自functors包）和（$＃）（来自合成器包）作为可能的名称。不过，我可能只是在使用网站上使用fmap（$ arg）f。
#2
7
As Niklas says, this is application in some applicative functor to a lifted value.
正如尼克拉斯所说，这是应用于一些应用函子，以提升价值。
\f a -> f <*> pure a
:: Applicative f => f (a -> b) -> a -> f b
or more generally (?), using Category (.)
或更一般地（？），使用类别（。）
\f a -> f . pure a
:: (Applicative (cat a), Category cat) => cat b c -> b -> cat a c