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
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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