15 Apr
2009
15 Apr
'09
7:49 a.m.
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. Jules