Add a Random instance over Ratios.

semantic-diff.cabal

@@ -60,6 +60,7 @@ library
                      , filepath
                      , hashable
                      , mtl
+                     , MonadRandom
                      , pointed
                      , QuickCheck >= 2.8.1
                      , quickcheck-text

src/Data/Gram.hs

@@ -1,6 +1,7 @@
 module Data.Gram where
 
 import Data.DList
+import Control.Monad.Random
 import Data.Hashable
 import Prologue
 
@@ -15,3 +16,6 @@ type Bag = DList
 instance Hashable label => Hashable (Gram label) where
   hashWithSalt _ = hash
   hash = hash . serialize
+
+instance (Random a, Integral a) => Random (Ratio a) where
+  random = first (% 1) . random