[Haskell-cafe] deriving instances in GHC 7.*

Alberto G. Corona agocorona at gmail.com
Mon Apr 18 11:53:46 CEST 2011

By my little experience with GHC 7.*  it seems that this version is
more "deterministic" when deriving instances. For example
if there is a general instance declaration such is:

instance C1 x=> C2 x where ....

then   the absence of the C1 instance becomes an error. C1 instance
becomes a prerequisite for any C2 instance, where in previous
versions, that was just a overlapping instance in collision with  any
other less general instance of the C2 class, so it could be overcomed
with the "OverlappingInstances" flag.

Given that is not possible to hide instances,  this is annoying. I´m
not saying that this is wrong or even If this is so, because this
could be a misinterpretation on my side based on the tests I have
done. Where can I read about the rationale of this change?. General
Instance derivations rules, as any rule system seem to me naturally
flexible and with different degrees of generalization rather tan

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