Probably you need to implement coarbitrary, too. My ghci session for your code: $ ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :l t.hs [1 of 1] Compiling Main ( t.hs, interpreted ) t.hs:4:0: Warning: No explicit method nor default method for `coarbitrary' In the instance declaration for `Arbitrary L.ByteString' Ok, modules loaded: Main. *Main> quickCheck prop_LPS Loading package array-0.1.0.0 ... linking ... done. Loading package bytestring-0.9.0.1 ... linking ... done. Loading package old-locale-1.0.0.0 ... linking ... done. Loading package old-time-1.0.0.0 ... linking ... done. Loading package random-1.0.0.0 ... linking ... done. Loading package QuickCheck-1.1.0.0 ... linking ... done. OK, passed 100 tests. On 6/24/08, Thomas M. DuBuisson wrote:

Cafe I'm a bit lost on this exception and curious about what's going on. Is there a valid reason for this exception that I am missing? Note the hard-coded [0..100] could be any Word8 list you want (generated via arbitrary, [], or other) and it gives the same result.

Load the module and perform: :break prop_LPS quickCheck prop_LPS :step :force ps *** Exception: Prelude.head: empty list

...

import qualified Data.ByteString as L import Test.QuickCheck

instance Arbitrary L.ByteString where arbitrary = do return $ L.pack [0..100]

prop_LPS :: L.ByteString -> Bool prop_LPS ps = ps `seq` True

P.S. Same result with GHCi 6.8.2 and 6.8.3.

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