[TYPES] Interaction between type classes and modules

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I am trying to track down a paper I once read about type classes and
modules. The key result I recall is that if an instance of a type class is
declared in a module then, to preserve coherence, signatures exported by
the module should not mention the type class. Can someone please point me
to the paper? Cheers, -- P

.   \ Philip Wadler, Professor of Theoretical Computer Science,
.   /\ School of Informatics, University of Edinburgh
.  /  \ and Senior Research Fellow, IOHK
. http://homepages.inf.ed.ac.uk/wadler/

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