Filtering keeps a value only when a predicate accepts it. On a List, it removes elements that fail the predicate; on an Option, it preserves Some when the predicate is true and produces None otherwise. section 3.3.3 section 4.3.1
Filtering an optional department
A lookup can remain inside Option while filtering its result: lookupByName("Joe").map(_.department).filter(_ != "Accounting").getOrElse("Default Dept"). The computation proceeds only when the employee exists and the department is not Accounting; otherwise it selects the default. section 4.3.1
The Option implementation makes the behavior explicit: flatMap applies a function that returns Some or None, so filter returns Some(a) when f(a) succeeds and None when it fails. This mirrors the List version and lets filtering compose with map, flatMap, and getOrElse without explicit pattern matching. section 4.6
Listing 4.2 The Option data type
enum Option[+A]: case Some(get: A) case None def map[B](f: A => B): Option[B] #1 def flatMap[B](f: A => Option[B]): Option[B] #2 def getOrElse[B >: A](default: => B): B #3 def orElse[B >: A](ob: => Option[B]): Option[B] #4 def filter(f: A => Boolean): Option[A] #5
Use the Option interface to compare filtering with transformation, flattening, default selection, and fallback. section 4.3.1
