Avoiding the hazards of orphan instances without dependency problems

RodLogic dev at rodlogic.net
Tue Oct 21 18:45:39 UTC 2014

One other benefit of multiple files to use a single module name is that it
would be easy to separate testing code from real code even when testing
internal/non-exported functions.

On Tue, Oct 21, 2014 at 1:22 PM, John Lato <jwlato at gmail.com> wrote:

> Perhaps you misunderstood my proposal if you think it would prevent anyone
> else from defining instances of those classes?  Part of the proposal was
> also adding support to the compiler to allow for a multiple files to use a
> single module name.  That may be a larger technical challenge, but I think
> it's achievable.
> I think one key difference is that my proposal puts the onus on class
> implementors, and David's puts the onus on datatype implementors, so they
> certainly are complementary and could co-exist.
> On Tue, Oct 21, 2014 at 9:11 AM, David Feuer <david.feuer at gmail.com>
> wrote:
>> As I said before, it still doesn't solve the problem I'm trying to solve.
>> Look at a package like criterion, for example. criterion depends on aeson.
>> Why? Because statistics depends on it. Why? Because statistics wants a
>> couple types it defines to be instances of classes defined in aeson. John
>> Lato's proposal would require the pragma to appear in the relevant aeson
>> module, and would prevent *anyone* else from defining instances of those
>> classes. With my proposal, statistics would be able to declare
>> {-# InstanceIn Statistics.AesonInstances AesonModule.AesonClass
>> StatisticsType #-}
>> Then it would split the Statistics.AesonInstances module off into a
>> statistics-aeson package and accomplish its objective without stepping on
>> anyone else. We'd get a lot more (mostly tiny) packages, but in exchange
>> the dependencies would get much thinner.
>> On Oct 21, 2014 11:52 AM, "Stephen Paul Weber" <singpolyma at singpolyma.net>
>> wrote:
>>> Somebody claiming to be John Lato wrote:
>>>> Thinking about this, I came to a slightly different scheme.  What if we
>>>> instead add a pragma:
>>>> {-# OrphanModule ClassName ModuleName #-}
>>> I really like this.  It solve all the real orphan instance cases I've
>>> had in my libraries.
>>> --
>>> Stephen Paul Weber, @singpolyma
>>> See <http://singpolyma.net> for how I prefer to be contacted
>>> edition right joseph
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