[Haskell-cafe] AMP - how do you motivate this in teaching?

[Oleg Grenrus]

> On 20 Nov 2015, at 11:31, Erik Hesselink <hesselink at gmail.com> wrote:
> 
> On 20 November 2015 at 02:30, Manuel Gómez <targen at gmail.com> wrote:
>> I haven’t found a helpful example of an Applicative that is not a Monad
>> that is practical for a lesson.
> 
> There's ZipList [0], which depending on the type of audience might be
> doable. There's also Const [1], but that needs a Monoid constraint,
> and since you said you considered ((,) e) as not having an
> Applicative, which it does have with a Monoid constraint, perhaps
> Const isn't suitable to you for that reason. There are more
> interesting examples here [2].
> 
> Erik
> 
> [0] https://hackage.haskell.org/package/base-4.8.1.0/docs/Control-Applicative.html#t:ZipList
> [1] http://hackage.haskell.org/package/base-4.8.1.0/docs/Control-Applicative.html#v:Const
> [2] http://stackoverflow.com/questions/7220436/good-examples-of-not-a-functor-functor-applicative-monad
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There are also (which I find actually useful):

-  `Concurrently = Concurrently (IO a)` which is Monad only IO effects are commutative. [3]
    - one can argue that Applicative and Monad instances aren’t compatible
- `Errors` [4], which could be specialised to `Either (NonEmpty err) a` and `ap` would gather all errors!
    - and of course `Lift` using which `Errors` is defined.

[3] https://hackage.haskell.org/package/async-2.0.2/docs/Control-Concurrent-Async.html#t:Concurrently <https://hackage.haskell.org/package/async-2.0.2/docs/Control-Concurrent-Async.html#t:Concurrently>
[4] https://hackage.haskell.org/package/transformers-0.4.3.0/docs/Control-Applicative-Lift.html#t:Errors <https://hackage.haskell.org/package/transformers-0.4.3.0/docs/Control-Applicative-Lift.html#t:Errors>

- Oleg
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