On 06/15/2018 07:31 AM, Petr Pudlák wrote:
That's interesting! Could you please elaborate how this is implemented?
Sure. Whereas QuickCheck declares "shrink" as a function, in Hedgehog generating and shrinking are performed simultaneously. Instead of generating a single value, we can generate a rose tree of them, where the main output is at the root, and the other nodes carry shrunk versions of it, with the smallest ones at the leaves. It works quite nicely and efficiently in Haskell thanks to laziness, so that the shrinking part only happens as needed. The main drawback might be that there is some duplicated work during shrinking ((>>=) applies its second argument to different arguments), but it is quite minor since shrinking is only needed in exceptional cases, and it usually terminates in few steps. Li-yao