[Haskell] A problem with overlapping instances and super-classes

Ian Lynagh igloo at earth.li
Tue Jun 10 06:58:04 EDT 2008

On Mon, Jun 09, 2008 at 09:21:02AM +0100, Simon Peyton-Jones wrote:
> This isn't great, but it's not really different than is the case for
> non-overlapping instances.  Suppose module B1 declares 'instance C T',
> and uses that instance; and module B2 declares a *different* 'instance
> C T', and uses that instance; and Main imports B1 and B2, but does not
> use either instance directly.  Then GHC will compile the program
> without complaint, although it is incoherent.

Isn't this a divergence from H98? Shouldn't we therefore list it on


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