Expected behavior of "deriving Ord"?

Christian Maeder Christian.Maeder at dfki.de
Thu Mar 20 05:00:30 EDT 2008

Conal Elliott wrote:
> I have an algebraic data type (not newtype) that derives Ord:
>     data AddBounds a = MinBound | NoBound a | MaxBound
>         deriving (Eq, Ord, Read, Show)

The class Ord is not suited for partial orders. If you write your own
Ord instances anyway, I'd suggest to introduce a proper new class (say
Lattice), too!

I hope that the computation of "uncomparable" does terminate in your
case. Maybe the lattice operation "join and "meet" are even more
appropriate than "min" and "max".

However, if your type parameter "a" has a total order, the above derived
instance looks correct.

HTH Christian

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