Adds helper to create Shrink using LazyList

[froth]

This allows to use Shrink from scala 2.13 without getting deprecation
warnings on scala.collection.immutable.Stream

fixes #626

[mpilquist]

Awesome! I'd personally prefer this as a method directly on Shrink companion (via the normal pattern of a ShrinkPlatform trait that's mixed in to the companion and defined as an empty trait in 2.12 and prior and a non-empty trait on 2.13.

[ashawley]

Yes, that would be the preference. Though, I don't think it's possible to have both definitions of apply on 2.13 with different override arguments for the function type (T => Stream[T] vs. T => LazyList[T]).

[froth]

Though, I don't think it's possible to have both definitions of apply on 2.13 with different override arguments for the function type (T => Stream[T] vs. T => LazyList[T]).

You are right. Although to be honest, I don't get why the compiler can not differentiate between Function1[T, Stream[T]] and Function1[T, LazyList[T]] here.

I followed your suggestions (and am sorry for not looking at the codebase thoroughly before). And I renamed the function from apply to fromLazyList. Again naming is hard, not exactly accurate. Maybe fromLazyListFunction but that seems clunky. Ideas welcome.

[ashawley]

I believe it's because Function1 is implemented with type parameters, as well. There may be a way to overload methods having type parameters with a dummy implicit, but that seems unsavory.

[ashawley]

I suggested it, but maybe fromLazyList is a poor name. Perhaps, withLazyList?

[ashawley]

If we have a name for the Shrink factory method, instead of using apply, the interface to Shrink has diverged. Should we deprecate apply and add withStream for symmetry sake?

[mpilquist]

Indeed, DummyImplicit can de-conflict the erased overloads if we want it to be called apply:

def apply[T](s: T => LazyList[T])(implicit d: DummyImplicit): Shrink[T]

[froth]

Actually DummyImplicit does not do the trick on its own, if I add the apply method with the DummyImplicit I still get a lot of errors like the following:

[error] /home/froth/projects/scalacheck/src/main/scala/org/scalacheck/Shrink.scala:24:49: missing parameter type                                                                                      
[error]   implicit def shrinkAny[T]: Shrink[T] = Shrink(_ => Stream.empty)  

I therefore opted for renaming the factory method to withLazyList.

If we have a name for the Shrink factory method, instead of using apply, the interface to Shrink has diverged. Should we deprecate apply and add withStream for symmetry sake?

I don't have an opinion on that, as I do not know what the further plans for LazyList and scalacheck 2.0 are. I am happy to do whatever you decide ;)

[ashawley]

Thanks for bearing with us and being amenable to our whims. I predict the change is pretty benign, since it's just a factory method for Shrink. It shouldn't paint us in to a corner, and more likely it should be something we can use going forward.

[froth]

No worries :) I learned something in the process, which is even dearer to me, then getting rid of those annoying warnings.

[ashawley]

I wish there was a way to easily dogfood this new factory in the library. Alternatively, we could add tests, but there aren't many existing example tests for Shrink nor any version-specific tests.

[ashawley]

Thanks again for working on this. We'll wait and see if there are other comments.

[LukaJCB]

This looks good to me :)

[SethTisue]

thanks @froth!