{-# LANGUAGE TypeSynonymInstances #-}
import Test.QuickCheck
import Test.QuickCheck.Variant
import Test.Hspec
import Test.Hspec.Variant

type Natural = Integer

instance Variant Natural where
  invalid = do
    n <- arbitrary
    if (n<0) then return n else return ((-1)*(n+1))
  valid = do
    n <- arbitrary
    if (n>=0) then return n else return ((-1)*n)

main::IO ()
main = hspec $
  describe "Naturals" $
    propValid "succ" $
      \x -> succ (x::Natural) > 0