On Fri, Jan 7, 2011 at 11:22 AM, Christian Maeder
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?
It's often not possible to define instances separately, as you often needs to access data constructors internal to the module that defines them. This is the case for the containers package. It's largely accidental that you can define e.g. an NFData instance for Data.Map outside the module and the instances are usually less efficient if defined outside the module [1]. 1. You often end up converting the data type to another data type (e.g. lists) that expose their data constructors and can thus be 'seq'ed. Johan