[Haskell] Abusing quickcheck to check existential properties

[Ben Moseley]

 > there are similar
 > scenarios on which I've already given up in despair, such as  
writing a
 > generator for creating well-typed terms in a nontrivial language.

This is something I've also struggled with. I'm coming to the  
conclusion that it'd really be useful for the "Gen" monad to be  
parameterizable with  some user state, or maybe even be made into a  
transformer.

The only alternative really seems to be passing around the extra  
information manually between generator-creation functions, and this  
quickly gets to be a pain.

For me this is probably the most serious short-coming of QuickCheck.

--Ben

On 14 Oct 2008, at 23:36, Norman Ramsey wrote:

>>> But how do I use QuickCheck to check an existential?
>>
>> The "standard" method in QuickCheck is to be constructive, and
>> actually implement the function that constructs for the value. So,
>> instead of
>>
>>  forAll x . exists y . P(x,y)
>>
>> you write
>>
>>  forAll x . P(x, find_y(x))
>
>
> Interesting.  For A-E properties I can see where this approach would
> be helpful, especially if it's not too hard to think of a good skolem
> function.  In my case x is empty and so I'm left with
>
>  'find a y such that P(y)'
>
> or a bit more exactly
>
>  'find an x in the four-dimensional unit cube such that 0.9 < f(x)'
>
> I've already written f and I'd really rather not write f-inverse;
> I want the computer to do some of the work for me.  So perhaps
> SmallCheck would be a better way to go.
>
> It does seem a pity, as almost all the QuickCheck machinery would be
> useful in such a search.  On the other hand there are similar
> scenarios on which I've already given up in despair, such as writing a
> generator for creating well-typed terms in a nontrivial language.
>
> Anyway, thanks for suggesting a skolem function---I'm sure I'll find
> use for one sooner or later.
>
>
> Norman
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