Add SmallBounded* type classes and related instances; Fixes #38

src/Data/Enum.purs

@@ -14,14 +14,16 @@ module Data.Enum
   , defaultCardinality
   , defaultToEnum
   , defaultFromEnum
+  , class SmallBounded
+  , class SmallBoundedEnum
   ) where
 
 import Prelude
 
 import Control.MonadPlus (guard)
 import Data.Either (Either(..))
 import Data.Maybe (Maybe(..), maybe, fromJust)
-import Data.Newtype (class Newtype)
+import Data.Newtype (class Newtype, unwrap)
 import Data.Tuple (Tuple(..))
 import Data.Unfoldable (class Unfoldable, singleton, unfoldr)
 import Data.Unfoldable1 (class Unfoldable1, unfoldr1)
@@ -316,3 +318,26 @@ charToEnum _ = Nothing
 
 foreign import toCharCode :: Char -> Int
 foreign import fromCharCode :: Int -> Char
+
+
+-- | A lawful subclass  of `Bounded` to denote
+-- | Cardinality a << Cardinality Int
+class Bounded a <= SmallBounded a
+
+-- | A lawful subclass of `BoundedEnum` to denote
+-- | Cardinality a << Cardinality Int
+class BoundedEnum a <= SmallBoundedEnum a
+
+instance smallBoundedEnum :: (SmallBounded a, BoundedEnum a)
+  => SmallBoundedEnum a
+
+instance boundedEnumMaybe :: (SmallBounded a, BoundedEnum a)
+  => BoundedEnum (Maybe a) where
+    cardinality = Cardinality $ unwrap (cardinality :: Cardinality a) + 1
+    toEnum 0 = Nothing
+    toEnum n = Just <$> toEnum (n - 1)
+    fromEnum Nothing = 0
+    fromEnum (Just e) = fromEnum e + 1
+
+instance smallBoundedBoolean :: SmallBounded Boolean
+