使用谓词提升到Maybe

本文介绍了使用谓词提升到Maybe的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我正在寻找类似于

  
 
 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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