On Tue, 20 Mar 2007, Duncan Coutts
H98 requires that Eq and Ord be equality classes and total orders
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). The report states the following: "The Eq class provides equality (==) and inequality (/=) methods." "The Ord class is used for totally ordered datatypes." I interpret this as being mere usage guidelines, and nothing more. What would it mean to state that Ord is a total order, by the way? Should (<=) always return True or False, never ⊥? In that case there is only one implementation of (<=): const (const True).
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.
If we cannot rely on Ord then the current Data.List.sort implementation is wrong because it gives different answers from the H98 spec sort when the Ord instance is not a total order (this is easiest to see with sortBy).
Then I would say it is wrong (according to H98), yes. -- /NAD