Re: [Haskell-cafe] Is it acceptable if Applicative behave not like a Monad

I'm afraid I have to disagree with Adam as well. Recently I've triggered a prolonged discussion on exactly the subject ( https://github.com/ekmett/either/pull/38). Being originally convinced that the instances can behave however it fits, I think I've been over-persuaded in the end. Shortly speaking, while I can't say I like it, the rule seems to be that `<*>` should produce the same side effects as Monad's `ap`. The most convincing argument I've seen so far against the two digressing is that it can cause unexpected behaviour of the "do" notation ( https://github.com/ekmett/either/pull/38#issuecomment-95695814), but you'll find plenty of other arguments in the discussion as well. Best regards, Nikita Volkov 2015-04-30 19:07 GMT+03:00 Kim-Ee Yeoh :

On Thu, Apr 30, 2015 at 10:49 PM, Adam Bergmark wrote:

...

Yes it's okay and sometimes expected for them not not behave the same, but see e.g. haxl and ApplicativeDo.

This is a value of okay I've never seen before.

...

For example, the applicative version may run foo and bar in parallell. But using monad they run sequentially.

What if pure weren't a perfect synonym for return?

What if the methods of the semigroup instance didn't match those of the monoid?

The mind boggles at the confusion that would result.

Such cases typically call for newtypes to sort out the effect classes.

-- Kim-Ee

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