Revamping the numeric classes
Wed, 7 Feb 2001 20:25:03 -0500
On Tue, Feb 06, 2001 at 10:29:36PM +0100, Andreas Gruenbacher wrote:
> Also not all instances of Num can be shown. I have a monad that is an
> instance of Num, for example. I cannot possibly show the monad.
I've been thinking about this a little. It's quite an interesting problem
in general to write classes that can be defined for monads. This
can be done for any class in which each member returns the type variable:
class C a where
foo :: ... -> a
is good, but anything else seems to cause problems. So '+', '-',
'max', etc., are good, but '<' and 'show' cause problems. 'quotRem'
and 'divMod' are interesting cases: they return a pair (a,a), which is
OK for some monads but not for others.
I wonder if there is a way to set things up so that all classes could
be written for monadic types.