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

[Joachim Breitner]

Hi,

Am Freitag, den 20.11.2015, 22:20 +1000 schrieb Tony Morris:
> 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)

I believe that there are plans to move Semigroup into Base. Eventually,
there will be a transition to Monoid having Semigroup as a superclass¹,
and I guess then the instance can be changed to 
    instance Monoid a => Applicative (Either a) where
and that worry is gone.

Greetings,
Joachim

¹ https://ghc.haskell.org/trac/ghc/ticket/10365 and
  https://ghc.haskell.org/trac/ghc/wiki/Proposal/SemigroupMonoid


-- 
Joachim “nomeata” Breitner
  mail at joachim-breitner.de • http://www.joachim-breitner.de/
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  Debian Developer: nomeata at debian.org

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