Hi
Does it? I was under the impression that a compiler can never assume that any laws hold of any user-defined instances (to do optimisations, for instance).
That was my impression.
So one could say that we can't really rely on any laws that should go along with type class instances, but that's clearly not right since then we cannot sort!
Libraries can of course depend on various laws (hopefully this dependence is explicit in the documentation). I guess you can argue whether rewrite rules are part of the compiler or the library. To me a rewrite rule is fishy unless the two sides are observationally equivalent, though.
Taking an example, if your Ord instance is not transitive then Data.Map may loop infinitely. Tracking this down was not particularly fun... I'd guess that if there is an Ord and an Eq instance, they should be in agreement, but until the compiler can prove it, I'd be wary of exploiting it. Thanks Neil