Scalacheck, generator for lists between size 5 and 12

Question

I can find many examples of setting maximum sizes for generators, but how do I generate lists between a min and max length?

Answer 1

A neat property about generators is they are composable, so you can simply compose a generator for the length of your list with a listOfN generator.

for {
  numElems <- Gen.choose(5, 12)
  elems <- Gen.listOfN(numElems, elemGenerator)
} yield elems

Answer 2

I am waking up ghosts here, but incase anyone comes along:

As per my comment on the accepted answer, that solution randomly decides the maximum length between 5 and 12, but the actual size of the generated list could still be 0 (zero) or anything below 5, in that case.

I think the following does what the OP describes:

Gen
  .listOfN(12, elemGenerator)
  .suchThat(_.size >= 5)

Would be cool if there was an API to conveniently generate this, or say a list of an exact size, N.

Update (2022/12/22)... as I realise I never actually answered the OP's question (and prompted by a recent comment):

def listOf[E](min: Int, max: Int, factor: Int = 4, retries: Int = 10000)(implicit gen: Gen[E]): Gen[List[E]] = for {
  size <- Gen.choose(min, max)
  list <- Gen.listOfN(factor * size, gen).retryUntil(_.size >= min, retries).flatMap(_.take(size))
} yield list

The factor is some attempt to increase the odds that within 10,000 tries (ScalaCheck's default), a list greater than size min will be generated; perhaps too large a factor will skew away from min-sized lists.

Alternatively, if you like Cats and friends (cats-scalacheck):

def listOf[E](min: Int, max: Int)(implicit gen: Gen[E]): List[E] = for {
  size <- Gen.choose(min, max)
  list <- List.fill(size)(gen).sequence
} yield list

I cannot attest to what happens in cats-scalacheck; maybe they also use suchThat or retryUntil.