[Haskell] How to avoid both duplicate instances and extraneous instance declarations?

[Robin Bate Boerop]

The error given by GHC (with or without the -fallow-overlapping- 
instances) is

question.hs:6:0:
     Duplicate instance declarations:
       question.hs:6:0: instance (B1 x) => A x
       question.hs:7:0: instance (B2 x) => A x

-- 
Robin Bate Boerop



On 22-Apr-06, at 1:01 PM, ihope wrote:

> On 4/22/06, Robin Bate Boerop <robin_bb at acm.org> wrote:
>> This does not work.  The compiler gives the same error with or
>> without "-fallow-overlapping-instances".
>
> Just what is the error?