Expose internal Int in IDs and make the type an instance of Random (generating only positive IDs)

src/Hanafuda/ID.hs

@@ -1,10 +1,17 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module Hanafuda.ID (
-      ID(..)
-    , getID
+    ID(..)
   ) where
 
-newtype ID a = ID Int deriving (Eq, Ord, Enum, Read, Show)
+import System.Random (Random(..))
 
-getID :: ID a -> String
-getID (ID n) = show n
+newtype ID a = ID {
+    getID :: Int
+  } deriving (Eq, Ord, Enum, Read, Show)
+
+rIntToRID :: (Int, g) -> (ID a, g)
+rIntToRID (someID, g) = (ID someID, g)
+
+instance Random (ID a) where
+  random = rIntToRID . randomR (0, maxBound)
+  randomR (ID idA, ID idB) = rIntToRID . randomR (idA, idB)