On 9 Apr 2008, at 15:23, Henning Thielemann wrote:
I don't know if it is possible to extend the syntax this way, but
it would be closer to math usage. And one would avoid duplicate
definitions just to indicate different operator names, like:
class AdditiveMonoid a where
o :: a
(+) :: a -> a -> a
as it could be create using
class Monoid (a; o, (+))
I also recognized that problem in the past, but didn't know how to
solve it. In Haskell 98, methods are resolved using the types of
the operands. How would the compiler find out which implementation
of (+) to choose for an expression like x+y using your approach?
Different names result in different operator hierarchies. So a class
like
class Monoid (a; unit, mult) where
unit :: a
mult :: a -> a -> a
must have an instantiation that specifies the names of the operators.
In particular, one will need a
class (Monoid (a; 0; (+)), ...) => Num a ...
if (+) should be used as Monoid.(+) together with Num.(+).
Or give an example you think may cause problems, and I will give it a
try.
Hans
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