[Haskell-cafe] instance Monad m => Functor m
Henning Thielemann
lemming at henning-thielemann.de
Wed Apr 9 10:26:32 EDT 2008
On Wed, 9 Apr 2008, Hans Aberg wrote:
> 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.
I think a classical example are number sequences which can be considered
as rings in two ways:
1. elementwise multiplication
2. convolution
and you have some function which invokes the ring multiplication
f :: Ring a => a -> a
and a concrete sequence
x :: Sequence Integer
what multiplication (elementwise or convolution) shall be used for
computing (f x) ?
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