[Haskell-cafe] programming style...and type classes...

Miguel migmit at gmail.com
Fri Nov 4 13:46:28 UTC 2016

More that one Monad instance is actually pretty easy: type String ->
(String, a) has different monad instances if you treat it as a State monad,
or as a combination of Reader and Writer. Thanks to Denis Moskvin for

On Fri, Nov 4, 2016 at 2:29 PM, Tobias Dammers <tdammers at gmail.com> wrote:

> My rule of thumb is that if there is one obvious instance (or none) for
> every given type, then your abstraction is a good candidate for a
> typeclass; if multiple instances make sense, then it's a judgment call;
> and if many instances could arbitrarily make sense or not depending on
> the usage / circumstances, then it's probably not a good candidate for a
> typeclass.
> For example, Functor is a very good typeclass - every type can have
> exactly zero or one lawful Functor instance; if you think about it a
> little, this means that with Functor, you can never run into overlapping
> or undecidable instances, and the open universe is a fairly benign
> problem - even if you derive or implement an orphan instance, it will at
> least be functionally and logically equivalent to other possible
> instances.
> Most of the typeclasses in base are tougher calls, but overall, I would
> say they still make a lot of sense. I'd be hard pressed to come up with
> a type that has more than one sensible Monad instance, for example.
> Something like Aeson's ToJSON / FromJSON is kind of a judgment call -
> On the one hand, they shouldn't be typeclasses, because ideally you want to
> separate JSON format specifications from data types (i.e., the user code
> of a given type should decide on the serialization format, not the type
> itself), but on the other hand, more often than not one designs types
> specifically for the purpose of representing a certain JSON schema, and
> an extra translation step sits between these types and the actual domain
> types (at least that's how I often do it).
> On a side note; it's not like the open universe principle is a bad idea.
> There are lots of benefits to be had here.
> On Fri, Nov 04, 2016 at 09:57:31AM +0000, Nicholls, Mark wrote:
> > I’m nervous of the statement “used properly”…it’s a bit tautological,
> and the inference is often used the wrong way around…i.e. if it doesn’t
> work you didn’t use it properly….and if it did…then you did!
> >
> > i.e. the logic goes.
> > Creating an open data abstraction is not a “proper” usage?
> > But not knowing future extensions requirements is a reality?
> > So having a language where data abstraction are open by default (unless
> you WANT to close them) is an attractive objective?
> > Typeclasses provide this?
> > Eureka!
> > Becomes DISASTER?
> >
> > So is the objection is really empirical ?…i.e. typeclasses will cause
> you problems IF you work like this (and you don’t know what you’re doing)?
> >
> > Once you know where the problems are…best steer clear….once you know the
> pitfalls, you can use them “properly” (successfully)?
> >
> > (ironically the process of finding the problems IS not using them
> properly!...but that’s life I think).
> >
> >
> >
> >
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> Tobias Dammers - tdammers at gmail.com
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