[Haskell-cafe] Alternative to newtypes or orphan instances

[Miguel Mitrofanov]

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
> ,
>
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