[Haskell-cafe] Re: (flawed?) benchmark : sort

David Menendez dave at zednenem.com
Wed Mar 12 22:28:26 EDT 2008

On Wed, Mar 12, 2008 at 7:48 PM,  <ajb at spamcop.net> 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 <dave at zednenem.com>

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