Marc Van Dongen
writes about the need of constant :: a -> b ->
and explains that it is needed, for example, to convert constant
to polynomial.
I consider this as kind of support for the
Domain conversion proposal in basAlgPropos, section 'dcon'.
It suggests class Convertible
which I would like to reformulate now as
class Cast a b where cast :: CastMode -> a -> b -> a
Example: cast _ f 2
makes a polynomial from 2 :: Integer,
if instance (Pol ..) Integer is defined.
f serves as the sample that denotes the particular domain of
polynomials. This may be, for example - in [x] or in [x,y] -
different domains.
Of course, the eager usage of Cast would often cause the ambiguity
- compile-time report.
Still the thing is useful in practice. And mathematically, it is a
highly cultural approach - to cast between the domains.
It is on the user how wide and how wise to set the Cast instances
- the user knows that it is often hard for Haskell to solve such
instances.
I tried this in practice with GHC, Hugs - a very useful thing.
------------------
Sergey Mechveliani
[EMAIL PROTECTED]
