[Haskell-cafe] Different advice for type classes for end-user app and libraries?

Hécate hecate at glitchbra.in
Thu Aug 12 16:29:47 UTC 2021

You're using classes in the context of MTL in an effect system-ish 
fashion, which is not *quite* what was the original intent and wisdom of 
Typeclasses in Haskell.
Here Typeclasses are being used to implement a query interface, which is 
not the most "natural" usage of ad-hoc polymorphism.
As such, I wouldn't expect such typeclasses to fully comply to the 
customs of Typeclasses.

Don't worry too much about it. You're already playing outside of the 
customary bounds for Typeclasses. :)

Le 12/08/2021 à 17:41, Ruben Astudillo a écrit :
> Hello list!
> Currently I am working a end user application. Through the years I had
> incorporated the advice from r/haskell that classes without laws were not
> desirable. Their lack meant that we didn't have a contract boundary that
> should be respected by the instances. They secretly could `throw exc` on
> their implementation and you wouldn't notice.
> But on the current app I am working on, I follow the advice from this
> blogpost [1] that I should use classes for domain modelling. This manifest
> on classes like
>      class UserQueryCapability f where
>          getUser :: Id -> f User
> for interacting with a DB. Functions can be written against the interface
> and I can mock up with QuickCheck some pretty broad cases. The classes
> usually don't have a superclass constrain on `Monad` as in `MonadReader`
> because I *cannot* establish a law relating the monadic behavior and
> `getUser`. The name has Capability in it and not Monad because I won't lie
> that way.
> Making sure that these effect classes are orthogonal is something I have to
> check by hand. But in the back of my head there is the concern about the
> lack of laws could bite me even on a end user-app. Is my intuition well
> founded that this case is OK?
> Thanks for your time!
> [1]: https://www.parsonsmatt.org/2018/03/22/three_layer_haskell_cake.html
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