@Andrew, it's not clear how you are posing U(R) as "ring of units." It's a multiplicative group, and is not closed under addition. 2020년 1월 28일 (화) 19:42, Dannyu NDos 님이 작성:

This two-step proposal suggests to alter the official Haskell 2010 standard.

1. Introduction:

The `Num` typeclass effectively represents a ring. Yet it also has `abs` and `signum`. `abs` represents a norm, but its type is wrong:

abs :: a -> a

Mathematically, the return value must be a nonnegative real number. Yet there is no guarantee that the type `a` can have that value. This led us to have too strong restriction to `instance Num Complex`:

instance RealFloat a => Num (Complex a) where ...

I found a way for `abs` (and `signum`) to have much more mathematical place.

2. Step 1:

First, I suggest to add the following variants of `abs` and `signum`:

realAbs :: Real a => a -> a realAbs x = if x >= 0 then x else negate x realSignum :: Real a => a -> a realSignum x = case compare 0 x of LT -> -1 EQ -> 0 _ -> 1

These can replace `abs` and `signum` instances for integral and rational types.

3. Step 2:

Second, I suggest to move `abs` and `signum` from `Num` to `Floating`:

class Floating a where abs :: a -> a signum :: a -> a ...

This exploits the fact that `Floating` represents a field with characteristic 0, regarding exponential and trigonometric functions are defined as Taylor series. The definition of convergence of series is defined using norms.

4. Conclusion:

This enables us to implement rings (Num) and fields (Fractional) without concerning about norms. For example, Gaussian integers.