Gen.nonEmptyListOf...map generates empty list

[AntonIvanov87]

import org.scalacheck.Gen
import org.scalacheck.Prop.forAll

object Main extends App {

  val charGen = Gen.oneOf('A', 'B')
  val charsGen = Gen.nonEmptyListOf(charGen)
  val stringGen = charsGen.map(_.mkString)

  forAll(stringGen) { (string: String) =>
    if (string.isEmpty) {
      throw new IllegalArgumentException("empty string!")
    }
    assert(false, "should see this message")
    true
  }.check

}

charsGen should generate non empty list of chars, that is why IllegalArgumenException("empty string!") should be never thown.
Unfortunately if an iteration fails, you see:

! Exception raised on property evaluation.
> ARG_0: ""
> ARG_0_ORIGINAL: "A"
> Exception: java.lang.IllegalArgumentException: empty string!

Empty string is very confusing, because it hides the real cause and contradicts with generator definition (nonEmptyListOf).

If you substitute stringGen with charsGen the output is correct:

! Exception raised on property evaluation.
> ARG_0: List(" ")
> ARG_0_ORIGINAL: List("B")
> Exception: java.lang.AssertionError: assertion failed: should see this me
  ssage

It looks like any map operation on a generator of lists causes this problem.

[ashawley]

You'd need to use forAllNoShrink.

That constraints in generators aren't respected in shrinking is known issue, see #129.