Monad for Const? [英] Monad for Const?

问题描述

为什么没有Control.Applicative.Const的monad实例?遵循以下定义正确无误，还是违反了monad法律?

Why is there no monad instance for Control.Applicative.Const? Is following definition correct, or violates it the monad laws?

instance Monoid a => Monad (Const a) where
  return _ = Const mempty
  (Const x) >>= _ = Const x

您能想到任何有用的应用程序吗?

And can you think of any useful application?

推荐答案

它违反了左身份法:必须与f x相同，但请考虑f x = Const (x + 1).

It violates the left identity law: return x >>= f must be the same as f x, but consider f x = Const (x + 1).

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