On Wed, Mar 12, 2008 at 7:48 PM, <[EMAIL PROTECTED]> wrote: > Adrian Hey wrote: > > >> This might be a reasonable thing to say about *sortBy*, but not sort > >> as the ordering of equal elements should not be observable (for any > >> correct instance of Ord). It should be impossible to implement a > >> function which can discriminate between [a,a],[a,b],[b,a],[b,b] if > >> compare a b = EQ. > > Nonsense. Consider a Schwartzian transform wrapper: > > data OrdWrap k v = OrdWrap k v > > instance (Ord k) => Ord (OrdWrap k v) where > compare (OrdWrap k1 v1) (OrdWrap k2 v2) = OrdWrap k1 k2 > > It would be incorrect (and not sane) for sort [a,b] to return [a,a] in > this case, though a case could be made that either [a,b] or [b,a] make > sense.
Adrian is arguing that compare a b == EQ should imply compare (f a) (f b) == EQ for all functions f (excluding odd stuff). Thus, the problem with your example would be in the Ord instance, not the sort function. -- Dave Menendez <[EMAIL PROTECTED]> <http://www.eyrie.org/~zednenem/> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
