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

2014-09-05 18:22 GMT+02:00 Bryan O’Sullivan :

On Thu, Sep 4, 2014 at 1:06 PM, Petr Pudlák 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) ​