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diff --git a/Utility/QuickCheck.hs b/Utility/QuickCheck.hs
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+{- QuickCheck instances
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+{-# LANGUAGE TypeSynonymInstances #-}
+
+module Utility.QuickCheck where
+
+import Test.QuickCheck
+import Data.Time.Clock.POSIX
+import System.Posix.Types
+
+{- Times before the epoch are excluded. -}
+instance Arbitrary POSIXTime where
+	arbitrary = nonNegative arbitrarySizedIntegral
+
+instance Arbitrary EpochTime where
+	arbitrary = nonNegative arbitrarySizedIntegral
+
+{- Pids are never negative, or 0. -}
+instance Arbitrary ProcessID where
+	arbitrary = arbitrarySizedBoundedIntegral `suchThat` (> 0)
+
+{- Inodes are never negative. -}
+instance Arbitrary FileID where
+	arbitrary = nonNegative arbitrarySizedIntegral
+
+{- File sizes are never negative. -}
+instance Arbitrary FileOffset where
+	arbitrary = nonNegative arbitrarySizedIntegral
+
+nonNegative :: (Num a, Ord a) => Gen a -> Gen a
+nonNegative g = g `suchThat` (>= 0)