同类对的monad实例的已知/已确定用例 [英] Known/estabilished usecases for the monad instance of an homogeneous pair

问题描述

一旦我问了这个问题，已正确标记为

Once I asked this question, which was correctly marked as duplicate of this other one.

现在我有好奇心，一对同类类型的monad实例是否有任何已知用例?

Now I have a curiosity, is there any known usecase for monad instance of a pair of homogeneous types?

这是它的实例:

data Pair a = Pair a a deriving Show

instance Functor Pair where
  fmap f (Pair a b) = Pair (f a) (f b)

instance Applicative Pair where
  pure a = Pair a a
  Pair f g <*> Pair x y =  Pair (f x) (g y)

instance Monad Pair where
    m >>= f = joinPair (f <$> m)

joinPair :: Pair (Pair a) -> Pair a
joinPair (Pair (Pair x _) (Pair _ y)) = Pair x y

推荐答案

您的 Pair a 与 Reader Bool a / Bool->同构；一个:

to (Pair f t) = \b -> if b then t else f

from f = Pair (f False) (f True)

因此，读者monad 也是您monad的潜在用例.这些类型的数据类型的通用术语是可表示的函子.

As such, any use-case for the Reader monad is also a potential use-case for your monad. The general term for these sorts of data types is representable functors.

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