[Haskell-cafe] Quickcheck examples and Data.Word32

[Bryn Keller]

I've not had any problems with them, though of course your mileage may 
vary. Have a look at section 7.4.4.3 in 
http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#multi-param-type-classes 
for an explanation. Basically, if you have a cyclic class dependency 
graph, the type checker won't terminate. Well, it will, but only because 
there's a limit on the recursion depth. The trivial example is

instance C a => C a where ...

In practice, I'm not sure that this is really a big issue, and it does 
come in handy. Perhaps someone who's actually been bitten by a problem 
with undecidable instances can comment?

Bryn


Joel Reymont wrote:
> This requires
>
> {-# OPTIONS_GHC -fallow-undecidable-instances #-}
>
> but since I'm using -fglasgow-exts in a lot of places I'm wondering if 
> adding undecidable instances would be a bad habit. I guess not... not 
> until I shoot myself in the foot :-).
>
> Any explanation of undecidable instances, the good and the bad?
>
>     Joel
>
> On Oct 27, 2005, at 6:49 PM, Bryn Keller wrote:
>
>> How about this?
>>
>> class ArbitraryDefault a where {}
>>
>> instance (Integral a, Bounded a, ArbitraryDefault a) => Arbitrary a 
>> where
>>    arbitrary = arbitraryBound
>>    coarbitrary a = error "Not implemented"
>>
>> instance ArbitraryDefault Word16   instance ArbitraryDefault Word32
>> instance ArbitraryDefault Word64
>
> -- 
> http://wagerlabs.com/
>
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