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

Why can't the library provide a `newtype`, one which supports `Applicative` *and* `Monad` interfaces (where the applicative and monadic interfaces match), and an un-newtyped variant that only supports `Applicaative`, as well as conversion functions? I think this would be clearer, as it would force you to switch into "Monad"-mode purposefully, and the documentation on the conversion function can make very clear what is going on. I agree that, now that we have AMP, applicative and monadic interfaces absolutely *should* match, and that it should be considered an error for them not to; semantically different interfaces can be provided with newtypes. -- Andrew On Thu, Apr 30, 2015 at 6:46 PM, Omari Norman wrote:

On Thu, Apr 30, 2015 at 1:32 PM, Nikita Volkov wrote:

...

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`.

One example of a library where the Applicative and Monad instances mismatch is uu-parsinglib.

http://stackoverflow.com/questions/18275123/cannot-compute-minimal-length-of...

There's a good reason the library is this way but the behavior can be rather startling.

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