Splitting SYB from the base package in GHC 6.10
bart at cs.pdx.edu
Tue Sep 2 11:59:49 EDT 2008
Ian Lynagh <igloo <at> earth.li> writes:
> * You want to use a different instance for the same type.
> Instances are global in Haskell, so if one library needs Ratio instance 1,
> and another library needs Ratio instance 2, then those libraries cannot be
> used in the same program.
For me it's not so much about the libraries. I've run into cases (not in SYB)
where I want my program to use a different definition of an instance than the
one provided by a library, while still using the other stuff provided by that
The pervasive globality of instances does seem to me to be a Haskell bug; it
seems to me like they ought to be controlled using the same module export
control rules as anything else. But I imagine there's some reason I don't
understand that makes this hard or impossible...
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