instance visibility (was: Re: The base library and GHC 6.10)

Ian Lynagh igloo at
Thu Sep 25 14:34:44 EDT 2008

Hi Claus,

On Wed, Sep 24, 2008 at 07:21:19PM +0100, Claus Reinke wrote:
> >I've put an example showing how multiple instances can cause Data.Set to
> >go wrong here:
> >
> >
> >Do you think that that behaviour is OK?

I'm not sure from your reply what your answer to this is.

Is it "I don't know whether I think that that behaviour is OK or not"?

> Note that I submitted that ticket;-) I think that the behaviour, the 
> Haskell'98 vs Ghc discrepancy, the comments, and this thread,
> show that the rules need to be specified carefully and clearly,

While that would be helpful, I think that the behaviour you are
interested in only works because it would be expensive to fix it (at
least, that's how I understand Simon's
    "So I'm not sure what to do; and the only thing that comes to mind
    isn't cheap."
comment), not because of any unclarity.

I'd be interested to see a design that allows for local instances, in a
way that doesn't cause problems like in the URL above, but as far as I
know none exist yet.


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