[Haskell-cafe] Some help needed to start Haskell with Yi

[Daniel Fischer]

Am Donnerstag 19 November 2009 01:07:37 schrieb Henning Thielemann:
> 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 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?