为什么ZipList不是List的默认应用实例 [英] Why ZipList is not the default Applicative Instance for List

问题描述

我目前正在Haskell学习Applicatives.如果我没看错，则List有两个不同的Applicative实例(List和ZipList-第二个被定义为包装List值的新类型). ZipList适用实例对我来说似乎更直观.

I am currently learning Applicatives in Haskell. If I am not wrong, there are two different Applicative instances for Lists, (List and ZipList - the second being defined as a newtype wrapping a List value). The ZipList applicative instances seems more intuitive for me.

这可能是一个愚蠢的问题，但是是否有特定原因导致ZipList不是列表的默认应用实例.

It might be a dumb question, but is there a specific reason ZipList is not the default Applicative instance for Lists.

pure (+) <*> [1,2,3] <*> [4,5,6]
-- [5,6,7,6,7,8,7,8,9]

pure (+) <*> ZipList [1,2,3] <*> ZipList [4,5,6]
-- ZipList [5,7,9]

是因为分发版的应用列表"也恰好有一个Monad实例吗?

Is it because the distributive version of Applicative List also happens to have a Monad instance?

推荐答案

我怀疑诚实的回答是历史性事故". monad抽象在应用抽象之前就很流行.由于存在一个自然列表monad，因此定义了该实例.为了向后兼容，必须使函子和应用实例保持一致并向前发展.

I suspect the honest answer here is "historical accident". The monad abstraction was popular before the applicative one was; since there is a natural list monad, that instance was defined. For backwards compatibility, it makes sense to keep the functor and applicative instances consistent moving forward.

也就是说，如果我们今天必须再次做出选择，我想无论如何我都会支持当前的情况:不确定性单子作为语法上便宜的回溯搜索是有用的，我已经采取了经常这样做的优势； (以我的个人经验)随意压缩拉链的频率要低得多.

That said, if we had to make the choice again today, I think I would support the current situation anyway: the nondeterminism monad is useful as a syntactically cheap backtracking search, and I've taken advantage of that frequently; arbitrary-arity zipping is (in my personal experience) a much less frequent need.

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