[Haskell-cafe] Why the exception?

[skynare at gmail.com]

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 <thomas.dubuisson at gmail.com> 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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