[QuickCheck] [Haskell-cafe] Fwd: an idea for improving 'shrink'

[Petr Pudlák]

Hi Nick,

you're right, this example is indeed problematic. I'll try to think about
it how to get around that, keeping backward compatibility.

  Petr
Dne 11. 9. 2014 21:37 "Nick Smallbone" <nicsma at chalmers.se> napsal(a):

> Hi Petr,
>
> My attitude has always been "just use the shrink instance for tuples".
> I don't think there's all that much difference between:
>   shrink (C x y) = [ C x' y' | (x', y') <- shrink (x, y) ]
> and:
>   shrink (C x y) = C <$> shrink x <*> shrink y
> The second is certainly nicer, but neither way is really very painful.
>
> However, your shrinkOne example shows a case where shrinking tuples
> doesn't work and you really need a shrinking Applicative.
> So this is a good reason to add a shrinking Applicative to QuickCheck
> and I would probably like to do it at some point.
>
> There is a problem with your current patch, though. Namely the default
> implementation of shrinkA:
>   shrinkA = pure
> If someone defines shrink but not shrinkA, then shrinkA will not
> shrink! In fact, one of your examples exhibits this bug:
>   shrinkA (Branch x l r) =
>     Branch <$> shrinkA x <*> shrinkA l <*> shrinkA r
> If the Arbitrary instance for whatever x is defines shrink but not
> shrinkA, then this will do the wrong thing.
>
> So we can't provide a default implementation for shrink in terms of
> shrinkA, or vice versa, because we want the user to be able to define
> either. We can't even define both in terms of the other because then
> the default implementation will loop instead of returning [].
>
> Probably we would just have to move shrinkA outside of the Arbitrary
> class. This does make using the Applicative more cumbersome, e.g.:
>   shrink (C x y) = fromShrink (C <$> shrinkA x <*> shrinkA y)
>   shrinkOne shr = fromShrink . traverse (Shrink . shr)
> but maybe this would still be useful for defining trickier shrink
> functions.
>
> Anyway, this is the only worry I have - how to integrate the
> traditional and applicative-based shrinking without too much syntactic
> overhead. If we can come up with a good solution for that then I'd be
> very eager to add this to QuickCheck. Or maybe calling fromShrink
> isn't so painful and we should add it anyway. I will think it over a
> bit...
>
> Nick
>
> On Saturday 06 September, 2014 at 11:17 am, Petr Pudlák wrote:
> > 2014-09-05 18:22 GMT+02:00 Bryan O’Sullivan <bos at serpentine.com>:
> >
> >
> > > On Thu, Sep 4, 2014 at 1:06 PM, Petr Pudlák <petr.mvd at gmail.com>
> wrote:
> > >
> > >> since I got no response from the QuickCheck mail address, so I'm
> > >> resending it here, if someone wants to comment on the idea.
> > >>
> > >
> > > I don't think it pays its way: it makes shrink a bit tidier, at the
> cost
> > > of breaking backwards compatibility in a widely used library.
> > >
> > That’s a problem. But I believe it the patch addresses it. QuickCheck
> > always uses ‘shrink’, not ‘shrinkA’, and as the default implementation of
> > ‘shrink’ is defined in terms of ‘shrinkA’, then it’s possible to define
> > instances both using ‘shrink’ and using ‘shrinkA’. So both old code and
> new
> > code that wants to take advantage of the Applicative interface work.
> >
> > I believe that in the long term this change would pay off: Compare the
> old
> > code
> >
> >  shrinkOne []     = []
> >  shrinkOne (x:xs) = [ x':xs | x'  <- shr x ]
> >                  ++ [ x:xs' | xs' <- shrinkOne xs ]
> >
> >  shrink (x, y) =
> >       [ (x', y) | x' <- shrink x ]
> >    ++ [ (x, y') | y' <- shrink y ]
> >
> >  shrink (x, y, z) =
> >    [ (x', y', z')
> >    | (x', (y', z')) <- shrink (x, (y, z)) ]
> >
> > against the improved piece
> >
> > traverse shr xs -- replaces the whole shrinkOne
> > shrinkA (x, y) = liftA2 (,) (shrinkA x) (shrinkA y)
> > shrinkA (x, y, z) = liftA3 (,,) (shrinkA x) (shrinkA y) (shrinkA z)
>
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