为什么“同时"在Haskell中不是单子?

问题描述：

我正在阅读软件包

I'm reading the doc of package async, and trying to find something similar to JavaScript's Promise, and I find Concurrently, which is the most close concept that implemented Functor, Applicative (Promise.all), Alternative (Promise.race). But it doesn't implement Monad (Promise.then), I'm wondering why.

我认为可能是因为(>>=)是顺序操作，与名称Concurrently冲突，但这是唯一的原因吗?这里还有一些更重要的原因吗?

I think it maybe because (>>=) is a sequential operation, which conflict with the name Concurrently, but is this the only reason? is there some more important reason here?

答

The Monad typeclass states that (<*>) and ap should be equivalent. (<*>) for Concurrently evaluates both the LHS and the RHS at the same time. ap can't evaluate the RHS until the LHS is done, since (>>=) needs to finish evaluating the LHS before it can call the function that gives it the RHS.

为什么“同时"在Haskell中不是单子?

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