'Convertible' class?

[Dylan Thurston]

In thinking about various issues with the numeric classes, I came up
with the following question:  Is there a problem with having a class
'Convertible' as follows?

class Convertible a b where
    convert :: a -> b

So, e.g., fromInteger and fromRational could be replaced with
convert.  (But if you did the same thing with toInteger and toRational
as well, you run into problems of overlapping instances.  I think
there are good semantic reasons for this: fromInteger and fromRational
can be defined in terms of the basic +,*,zero,one, while toInteger and
toRational rely on details of the representation.)

What do people think of this idea in general?  Perhaps a better name
would be 'Subtype'?

Note that "convert . convert" would be up there with "show . read" as
an ambiguous term.

Best,
	Dylan Thurston