Alex Ferguson <[EMAIL PROTECTED]>
wrote (8 Dec 1997) on Partially annoying orders:
> I recently found myself needing a partial order class, and found
> that:
> The most likely-looking operator symbols are all "taken";
> It'd be desirable to have such a class be a superclass of Ord, but
> this isn't possible without hacking the Prelude;
> It's not possible to have an instance declaration making all total
> orders also partial orders.
...
> ... looks like a PartOrd class in the Prelude, and a Jones, P-J and
> Meijer-style generalisation of instance declarations would do the
> trick.
In my program class (Eq a,Show a,Read a) => OrdereSet a
where
compare_m :: a -> a -> Maybe Order
...
compare_m expresses the partial ordering. Further, for many instances
of Ord, i define compare_m as agreed with compare:
\x y-> Just (compare x y)
Indeed, it is better to have a possibility to make Ord a subclass of,
say, PartOrd.
---------------------
Sergey Mechveliani
[EMAIL PROTECTED]
