为什么``加入``Monad`类的一部分 [英] Why isn't `join` part of the `Monad` class
问题描述
众所周知，（>> =）可以使用 fmap 和 join ，而 join 可以使用>> = 。是否有任何理由我们没有定义包含 join 的 Monad 类并使用下列默认定义？
It is a well known fact that (>>=) can be implemented using fmap and join while join can be implemented using >>=. Is there any reason we don't define the Monad class with join included and using the following default definitions?
join x = x >>= id
x >>= f = join $ f <$> x
这将允许最小定义包含（>> ; =）或 join ，而不是强制（>> =）。考虑到类别理论倾向于支持 join 。
This would allow a minimal definition to include either just (>>=) or join, instead of forcing (>>=). Might be a bit helpful considering category theory tends to favour join.
反对修改类的常见论据是我们打破向后兼容性。但是，在这种情况下，这种情况不会发生 - 我们只添加使用 join 来定义 Monad 的可能性。
The usual argument against modifying classes is that we break backwards compatibility. However, in this case, that wouldn't happen - we only add the possibility of defining Monad using join.
推荐答案
这意味着要发生 Applicative-Monad提案（已将其提交给GHC 7.10）。但是，技术问题涉及在GHC中键入角色，它已无限期地推迟了您的建议。
That was meant to happen with the Applicative-Monad proposal (which has made it to GHC 7.10). However, there is a technical issue involving type roles in GHC which has postponed indefinitely the implementation of what you suggest.
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