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

[Tony Morris]

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On 20/11/15 20:15, Joachim Breitner wrote:
> Hi,
> 
> Am Freitag, den 20.11.2015, 12:02 +0200 schrieb Oleg Grenrus:
>> - `Errors` [4], which could be specialised to `Either (NonEmpty
>> err) a` and `ap` would gather all errors!
> 
> this is actually a pretty nice and convincing example: „A
> effectful computation (e.g. a stateless parser) that may fail in
> various spots with errors, where the type system can guarantee that
> _all_ errors will be reported (and not just the first found).“
> 

It's a pretty nice example of further splitting of the type-class
hierarchy, since if given,

instance Monoid a => Applicative (Either a) where
  -- we gather all the errors

then we could not have Either (NonEmpty err), since (NonEmpty err)
does not have a Monoid, only a binary, associative operation (Semigroup)
.

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