使用谓词提升到Maybe [英] Lift to Maybe using a predicate

问题描述

我正在寻找类似于

  liftPredMaybe ::（a  - > Bool） - > a  - >也许是
 liftPredMaybe p a 
 | p a =只是
 |否则= Nothing 
  

Haskell中是否有这样的函数？
<不是现成的解决方案，而是使用 guard （来自 Control。 Monad ）和（< $）（来自 Data.Functor ）， ：

 确保::可选f => （a  - > Bool） - > a  - > fa 
确保pa = a <$ guard（pa）
  

（感谢Daniel Wagner ）

可疑的可疑拼写是 \ p - > （< $）*>守卫。 p 。

I'm searching for something like

liftPredMaybe :: (a -> Bool) -> a -> Maybe a
liftPredMaybe p a
  | p a = Just a
  | otherwise = Nothing

Is there such a function in Haskell already?

解决方案

Not quite a ready-made solution, but with guard (from Control.Monad) and (<$) (from Data.Functor) we can write:

ensure :: Alternative f => (a -> Bool) -> a -> f a
ensure p a = a <$ guard (p a)

(Thanks to Daniel Wagner for suggesting a nice name for this function.)

A more pointfree spelling of dubious taste is \p -> (<$) <*> guard . p.

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