class Providers(private val project: Project) {

  fun <T : Any> toProvider(list: Iterable<Provider<T>>): Provider<List<T>> {
    val seed = project.providers.provider { emptyList<T>() }
    return list
      .filter { it.isPresent }
      .fold(seed) { acc: Provider<List<T>>, provider ->
        acc.zip(provider) { list, item -> list + item }
      }
  }

  fun <T : Any> toProvider2(list: Iterable<Provider<T>>): Provider<List<T>> {
    return project.provider { 
      list
        .filter { it.isPresent }
        .map { it.get() } 
    }
  }
}

internal class ProvidersTest {

  private val project = ProjectBuilder.builder().build()
  private val providerFactory = project.providers
  private val providers = Providers(project)

  @Test fun `can convert list of providers to provider of lists`() {
    // Given
    val a = providerFactory.provider { "a" }
    val b = providerFactory.provider { "b" }
    val notPresent = providerFactory.gradleProperty("notpresent")
    val listOfProviders = listOf(a, b, notPresent)

    // Expect
    assertThat(providers.toProvider(listOfProviders).get()).containsExactly("a", "b")
    assertThat(providers.toProvider2(listOfProviders).get()).containsExactly("a", "b")
  }
}