packages with orphaned instances only

Gábor Lehel illissius at
Sat Jan 8 22:48:10 CET 2011

On Fri, Jan 7, 2011 at 11:22 AM, Christian Maeder
<Christian.Maeder at> wrote:
> Hi,
> referring to previous messages about QuickCheck and Johan's proposal
> about moving instances I wonder how many of us have created obviously
> orphaned "Arbitrary" instances for container data types?
> Maybe indeed separate packages with (orphaned) instances only should be
> created for re-use. I haven't seen such packages on hackage, yet. And
> maybe such packages should be clearly recognizable.
> So is this a bad idea?
> Cheers Christian

I wonder if it might be feasible to add some infrastructural support
for this, e.g. so-called adapter packages, which get installed
automatically when the packages they adapt between are both present?
(Or, thinking further, perhaps even automatic importation of adapter
modules at the language level?) You'd have a combinatorial explosion
in the number of packages/modules, but this is just a consequence of
the combinatorial number of instances... (maybe framing it as
'optional' parts of packages rather than adapter packages would be

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