From: "William Lovas" <wlovas@stwing.upenn.edu>
> More seriously, one might argue -- only slightly hand-wavily -- that with
> =, <, etc., types whose values *cannot* be used as inputs are the exception
> rather than the rule, whereas the reverse is the case for +, -, etc.  For
> example, it may be perfectly clear (or at least possible to specify) how to
> implement comparisons on data types, records, and tuples.  What should the
> arithmetic operators mean on such things?

I don't quite think comparisons are clearer (or "more possible" to
specify) than additions for tuples and records - they can be defined
just element-wise.  Data types indeed seem more problematic as pointed
out in other replies.

> (This argument breaks down in
> the face of code which relies on abstract types to enforce modularity -- in
> such cases, incomparability can become "the rule" rather than the
> exception, putting =, <, etc. on the same footing as +, -, etc.)

Yes, polymorphic comparison already breaks type abstraction.

	Eijiro