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

[ihope]

On 4/22/06, Robin Bate Boerop <robin_bb at acm.org> wrote:
> I am having trouble making a type system for my EDSL in Haskell.  Can
> someone help?  A simplified description of my problem follows.
>
> I have three classes, A, B1, and B2.  I want all instances of B1 to
> be instances of A.  I want all instances of B2 to be instances of A.
> None of the classes have methods.
>
> The following does NOT work, because of a duplicate instance
> declaration for A:

I think with GHC, you can use the flag -fallow-overlapping-instances,
and for GHCi, there's :set -fallow-overlapping-instances and then
:reload.