Improve arbitrary function generation

[rickynils]

This has been discussed for some time. The current way of generating functions are quite stupid, since it will only generate constant functions.
#66 does some changes in this area, but it must be updated to current master. And maybe we should take the Haskell QuickCheck route instead, using some fashion of CoArbitrary.

[tonymorris]

CoArbitrary, as in Haskell, is not possible given the Gen implementation (returning Option).

[pocketberserker]

I tried to implement CoArbitrary pocketberserker/scalacheck@4293d29 and replace arbitrary function implementation pocketberserker/scalacheck@169d1e4 .

[jedesah]

As a side note, it would be really nice if the generated functions could have a nice toString method. When trying to figure out why your property fails, it sort of sucks to see <function2> as the representation of the function that was generated.

I took a stab at a version that uses reflection, and one of the advantages of this approach is that it is possible to generate functions like so:

object Take extends String => String {
  def apply(a: String): String = take(a)
  def toString = "take(_)"
}

But I think reflection is an inferior approach to CoArbitrary. One might use reflection to automate generation of some CoArbitrary or something, but it should not be the main mechanism of generation since that would not allow to take advantage of the type system for customization.

Just something to keep in mind.

Is anyone working on this right now? Is there an estimate?

[non]

Hello, is there any plan to improve function generation for Scalacheck, either along the lines of @pocketberserker's work, or another way?

[xuwei-k]

pocketberserker's codes still does not work correctly due to mutable random number generator(scala.util.Random)

Maybe there are 2 ways

• immutable random number generator + State monad like( I have implemented )
• mutable random number generator + IO monad like (Haskell's QuickCheck ?)

[xuwei-k]

As a side note, it would be really nice if the generated functions could have a nice toString

Is anyone working on this right now? Is there an estimate?

work in progress

• scalaprops/scalaprops@80712db3d92d687a2dd
• https://github.com/nick8325/quickcheck/blob/2.8.1/Test/QuickCheck/Function.hs#L8

[non]

I created a branch to update Scalacheck to use an immutable RNG, and created a basic Cogen type there. I was able to hack together a Gen[A => B] which produces deterministic, but non-constant, functions:

https://github.com/non/scalacheck/blob/topic/immutable-rng/src/main/scala/org/scalacheck/Gen.scala#L614

The warts around using Option are still there -- I may try something a bit more radical.

@rickynils, is this direction something you'd like to see in a PR?