Why ZipList is not the default Applicative Instance for List

喜夏-厌秋 提交于 2019-12-05 12:30:38

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.

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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