[SOLVED] fs2 dropWhile with X => IO[Boolean]

fs2 dropWhile with X => IO[Boolean]

I can write something like the following and it works just as it should (it yields 10):

import cats.effect.IO
import cats.effect.unsafe.implicits.global
val zz: fs2.Stream[IO, Int] = fs2.Stream.iterate[IO, Int](1)(_+1).map(_*2)
val qq: IO[Int] = zz.dropWhile(_ < 10).take(1).compile.toList.map(_.head)
qq.unsafeRunSync()

However, supposing that, instead of a predicate like _ < 10 which returns Boolean, I have a predicate that returns an IO[Boolean]. Is there an alternative form of dropWhile which could handle this situation? I don't want to have to do something like unsafeRunSync() inside the predicate just so that it can yield a Boolean.

Clearly, this is a toy case, not the actual problem that I'm working on.

Solution

You can easily define that combinator yourself:

def dropWhileEval[A](sa: Stream[IO, A])(p: A = IO[Boolean]): Stream[IO, A] =
  sa.evalMap(a => p(a).map(b => (b, a))).dropWhile(_._1).map(_._2)