Merge pull request #228 from purescript/ring-law-fix

Fix Ring laws; fixes #212

Author: hdgarrood

src/Data/Ring.purs

@@ -15,10 +15,11 @@ import Type.Data.RowList (RLProxy(..))
 -- | The `Ring` class is for types that support addition, multiplication,
 -- | and subtraction operations.
 -- |
--- | Instances must satisfy the following law in addition to the `Semiring`
+-- | Instances must satisfy the following laws in addition to the `Semiring`
 -- | laws:
 -- |
--- | - Additive inverse: `a - a = (zero - a) + a = zero`
+-- | - Additive inverse: `a - a = zero`
+-- | - Compatibility of `sub` and `negate`: `a - b = a + (zero - b)`
 class Semiring a <= Ring a where
   sub :: a -> a -> a