On Wed, Nov 18, 2009 at 8:15 PM, Daniel Fischer
Kapil Hari Paranjape schrieb:
Hello,
On Sat, 14 Nov 2009, Jaco van Iterson wrote:
Only installation with 'cabal install yi' in a Cygwin shell under MS Windows XP ended in: Yi\Prelude.hs:182:9: Duplicate instance declarations: instance Category Accessor.T -- Defined at Yi\Prelude.hs:182:9-38 instance Category Accessor.T -- Defined in data-accessor-0.2.1:Data.Accessor.Private cabal.exe: Error: some packages failed to install: yi-0.6.1 failed during the building phase. The exception was: exit: ExitFailure 1
Seems easy to fix but I can't even find where on my drive I can find
Am Donnerstag 19 November 2009 01:07:37 schrieb Henning Thielemann: the
source code.
Where is the source?
Seems to be that the author defined an orphan instance - something one should never do!
So what do you do if you need an instance ClassX TypeY but the author of the package that defines TypeY hasn't provided one?
You can define an orphan instance or duplicate packageY but with the instance. Both are bad. Providing an orphan instance until packageY has one seems the lesser evil to me. Are there any good options?
Whether it qualifies as 'good' or not, I'm not sure, but I think the standard recommendation is to newtype-wrap the type you want to make an orphan instance of, and make the instance on that (new) type.