[Haskell-cafe] Alternative to newtypes or orphan instances

[Clinton Mead]

After implementing the steps in (c), T is an instance of both C and C1, and
both those instances define foo the same way. The only way I see a problem
if is a module imports both C1 and C unqualified (they'll clash) but I see
no reason to do that.

On Monday, 24 August 2015, Clinton Mead <clintonmead at gmail.com> wrote:

> Why does 'foo' fail? The module that 'foo'is defined in still can see C. C
> still exists. So as long as I import foo it should still work yes?
>
> On Monday, 24 August 2015, Miguel Mitrofanov <miguelimo38 at yandex.ru
> <javascript:_e(%7B%7D,'cvml','miguelimo38 at yandex.ru');>> wrote:
>
>> OK. There is no point in having a class C, if there is no functions to
>> work with it. So, this "other package" likely contains some functions that
>> take types (of that class) as input. Like this:
>>
>> foo :: C a => a -> Int
>>
>> You don't need class C if you aren't using any functions from the
>> library. So, with your (c), if you try to do
>>
>> a :: T
>> a = ...
>>
>> b :: Int
>> b = foo a
>>
>> you'll get a compile error. THAT is the problem.
>>
>> 24.08.2015, 13:28, "Clinton Mead" <clintonmead at gmail.com>:
>> > There's no new data. Class C already exists in another package. Data T
>> already exists in another different package.
>> >
>> > The options are:
>> >
>> > a) Create a newtype MyT and an instance MyT of C.
>> > b) Just create an orphan instance T of C.
>> > c) Create a new class C1, forward its implementation to C, and add an
>> instance T of C1.
>> >
>> > I'm suggesting (c) is best, but I haven't seen this elsewhere, the
>> debate is usually between (a) and (b).
>> >
>> > I don't really understand the problems you're proposing with (c), but
>> I'm not sure if that's because I'm misunderstanding you or I'm not
>> explaining myself well.
>> >
>> > On Monday, 24 August 2015, Miguel Mitrofanov <miguelimo38 at yandex.ru>
>> wrote:
>> >> If, for your data, you create an instance of the new class — but not
>> the original one, then you can't use all the machinery that expects input
>> being of the old class. Which is the point.
>> >>
>> >> 24.08.2015, 12:55, "Clinton Mead" <clintonmead at gmail.com>:
>> >>> The original class still exists, I can't see how making a new class
>> based on the old on affects that. Won't existing functions in modules which
>> import the old class instead of the new class continue to work?
>> >>>
>> >>> On Monday, 24 August 2015, Erik Hesselink <hesselink at gmail.com>
>> wrote:
>> >>>> On 24 August 2015 at 09:18, Clinton Mead <clintonmead at gmail.com>
>> wrote:
>> >>>>> A second approach is an orphan instance. The recommendation here is
>> to put
>> >>>>> the orphan instances in their own module, so the user can choose to
>> import
>> >>>>> them.
>> >>>>>
>> >>>>> This may works ok if your user is writing an executable. But what
>> if your
>> >>>>> user is writing a library themselves. But once, you, or your user,
>> directly
>> >>>>> uses one of the instances, they need to import it, and they pollute
>> the
>> >>>>> global instance namespace for anyone that uses their package.
>> >>>>
>> >>>> For this reason, I think the recommended course of action is to make
>> a
>> >>>> canonical place for the instance, so that everyone can use it. For
>> >>>> example, if you have a library 'foo' providing T, and a library 'bar'
>> >>>> providing C, put the instance in a new package 'foo-bar' (or
>> >>>> 'bar-foo'). Then everyone can use that one instance, since Haskell is
>> >>>> built on the assumption that every type has one unique instance per
>> >>>> class.
>> >>>>
>> >>>>> I want to suggest a third option:
>> >>>>>
>> >>>>> (3) Copying the class.
>> >>>>
>> >>>> This would make a new distinct class, which means you can't call any
>> >>>> methods which have the original class as the context (f :: C a => a
>> ->
>> >>>> a) since that class won't exist for type T (you are trying to avoid
>> >>>> defining that orphan instance). So I don't think this is usable in
>> >>>> most cases, unless I'm missing something.
>> >>>>
>> >>>> Erik
>> >>> ,
>> >>>
>> >>> _______________________________________________
>> >>> Haskell-Cafe mailing list
>> >>> Haskell-Cafe at haskell.org
>> >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>
>
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