How to extend a Scala list to enable slicing not by explicit position but by given predicate/condition

Question

For

trait Item
case class TypeA(i: Int) extends Item
case class TypeB(i: Int) extends Item

consider a Scala list of items such as

Answer 1

Aren't you looking for filter, which works (almost) without any tweaks for your examples?

$ sbt console
scala> trait Item
scala> case class TypeA(i: Int) extends Item
scala> case class TypeB(i: Int) extends Item
scala> val myList = List(TypeA(1), TypeB(11), TypeB(12), 
                         TypeA(2), TypeB(21), 
                         TypeA(3), TypeB(31))
myList: List[Product with Serializable with Item] = List(TypeA(1), TypeB(11), TypeB(12), TypeA(2), TypeB(21), TypeA(3), TypeB(31))

your first works unaltered:

scala> myList.filter { x => x.isInstanceOf[TypeA] }
res0: List[Product with Serializable with Item] = List(TypeA(1), TypeA(2), TypeA(3))

your second requires a default case:

scala> myList.filter { case TypeA(x) => x < 10; case _ => false }
res2: List[Product with Serializable with Item] = List(TypeA(1(3))

See also collect, which takes a partial function instead of a boolean predicate:

scala> myList.collect { case z @ TypeA(x) if x < 10 => z }
res3: List[TypeA] = List(TypeA(1), TypeA(2), TypeA(3))

and can transform as well:

scala> myList.collect { case TypeA(x) if x < 10 => x }
res4: List[Int] = List(1, 2, 3)