# Semantics of signum

William Lee Irwin III wli@holomorphy.com
Sat, 10 Feb 2001 13:22:32 -0800

On Sat, Feb 10, 2001 at 11:25:46AM -0500, Dylan Thurston wrote:
> Can you elaborate?  What do you mean by signum for functions?  The
> pointwise signum?  Then abs would be the pointwise abs as well, right?
> That might work, but I'm nervous because I don't know the semantics
> for signum/abs in such generality.  What identities should they
> of these operations, in the same way it says nothing about the meaning
> of (+), (-), and (*).  Compare this to the situation for the Monad class,
> where the fundamental identities are given.  Oddly, there are identities
> listed for 'quot', 'rem', 'div', and 'mod'.  For +, -, and * I can guess
> what identities they should satisfy, but not for signum and abs.)

Pointwise signum and abs are common in analysis. The identity is:

signum f * abs f = f

I've already done the pointwise case. As I've pointed out before,
abs has the wrong type for doing anything with vector spaces, though,
perhaps, abs is a distinct notion from norm.

On Sat, Feb 10, 2001 at 11:25:46AM -0500, Dylan Thurston wrote:
> (Note that pointwise abs of functions yields a positive function, which
> are not ordered but do have a sensible notion of max and min.)

The ordering you're looking for needs a norm. If you really want a
notion of size on functions, you'll have to do it with something like
one of the L^p norms for continua and the \ell^p norms for discrete
spaces which are instances of Enum. There is a slightly problematic
aspect with this in that the domain of the function does not entirely
determine the norm, and furthermore adequately dealing with the
different notions of measure on these spaces with the type system is
probably also intractable. The sorts of issues raised by trying to
define norms on functions probably rather quickly relegate it to
something the user should explicitly define, as opposed to something
that should appear in a Prelude standard or otherwise. That said,
one could do something like

instance Enum a => Enum (MyTree a) where
... -- it's tricky, but possible, you figure it out

instance (Enum a, RealFloat b) => NormedSpace (MyTree a -> b) where
norm f = approxsum $zipWith (*) (map f . enumFrom$ toEnum 0) weights
where
weights = map (\x -> 1/factorial x) [0..]
approxsum [] = 0
approxsum (x:xs)| x < 1.0e-6 = 0
| otherwise = x + approxsum xs

and then do the usual junk where

instance NormedSpace a => Ord a where
f < g = norm f < norm g
...

Cheers,
Bill