Simon Peyton-Jones wrote: > Yes, Haskell says that in any program there should be only one > instance for any particular type (here Monoid Int). GHC > doesn't check that, but it should really do so. It's not > necessary for soundness (ie no runtime crash) but it is > necessary for coherence (ie when you run the program the answer > you get doesn't depend on which dictionary the typechecker > arbitrarily chose).
Unless of course, your program implicitly depends on the coherence of dictionary choice for its own soundness, for example, a program using Data.Typeable to implement Dynamic or similar.
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