Generic random generators

Say goodbye to Constructor <$> arbitrary <*> arbitrary <*> arbitrary-boilerplate.

    {-# LANGUAGE DeriveGeneric #-}

    import GHC.Generics (Generic)
    import Test.QuickCheck
    import Generic.Random

    data Tree a = Leaf | Node (Tree a) a (Tree a)
      deriving (Show, Generic)

    instance Arbitrary a => Arbitrary (Tree a) where
      arbitrary = genericArbitraryRec uniform `withBaseCase` return Leaf

    -- Equivalent to
    -- > arbitrary =
    -- >   sized $ \n ->
    -- >     if n == 0 then
    -- >       return Leaf
    -- >     else
    -- >       oneof
    -- >         [ return Leaf
    -- >         , resize (n `div` 3) $
    -- >             Node <$> arbitrary <*> arbitrary <*> arbitrary
    -- >         ]

    main = sample (arbitrary :: Gen (Tree ()))

• User-specified distribution of constructors, with a compile-time check that weights have been specified for all constructors.
• A simple (optional) strategy to ensure termination: Test.QuickCheck.Gen's size parameter decreases at every recursive genericArbitrary' call; when it reaches zero, sample directly from a trivially terminating generator.