On http://hackage.haskell.org/trac/haskell-prime/wiki/
DerivedInstances it says:
"- There is no way to derive an instance of a class for a data type
that is defined elsewhere (in another module)."
Though there is no proposal to fix this. Would such a proposal be
appropriate for Haskell'?
If so, I propose to add a top-level declaration on the form:
'deriving' qtycls 'for' qtycon
which produces the same instance as a deriving clause in the
declaration of the datatype or newtype would.
I have recently (thanks to the GHC Hackathon) implemented this in GHC.
/Björn
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