within.scala

import scalaz.{PLens, @?>}

// Ah, breaks the prism laws!

// > val two = within(_ == 2, PLens.plensId)
// > two.set(2, 3).flatMap(two.get)
// None

def within[A, B](f: A => Boolean, p: A @?> B): A @?> B = PLens.plens { (a: A) =>
  if (f(a))
    p(a)
  else
    None
}