On Wed, Mar 12, 2008 at 7:48 PM,
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