[Haskell-cafe] Alternative to newtypes or orphan instances

htebalaka goodingm at gmail.com
Mon Aug 24 14:52:09 UTC 2015

Another issues is there's lots of code polymorphic over an instance, which
you lose out on if you simply make a new class rather than newtype to
support an alternative instance. For instance, lots of datatypes have a
second non-trivial Applicative ([] : ZipList, IO : Concurrently , Either :
Validation), and there's lots of code that's polymorphic over Applicative
(traverse, Data.Functor.Compose, Data.Functor.Product). Using a new class
means you still need some newtype and "instance Applicative2 f =>
Applicative (WrappedA2 f)" to avoid duplicating all this work, so you don't
really escape from needing newtypes in non-trivial examples anyway.

Sean Leather-2 wrote
> On Mon, Aug 24, 2015 at 1:37 PM, Clinton Mead wrote:
>> I see the problem now Sean. So in your example, is this correct:
>> "To use the machinery of 'Show', I have to add and instance Show T. But
>> that would be an orphan instance. So there is no way for T to use show
>> machinery without adding an orphan instance (aside from modifying a
>> module
>> not under your control)?"
> Correct.
> Even newtype doesn't seem to help in your example. Show has all this
>> machinery, but it seems completely unusable without adding an orphan
>> instance or re-writing other people's modules. Is there no other way?
>> This
>> situation seems incredibly against re-usability, which I thought was
>> Haskell's strength. Am I missing something?
> Orphan instances are one of unfortunate pain points of Haskell, though
> they
> are sometimes a necessary “evil.” Erik's first response describes a common
> technique for publishing and sharing orphan instances. See, for example,
> https://hackage.haskell.org/packages/search?terms=instances .
> Regards,
> Sean
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