collect vs map, filter vs flatMap

gistfile1.scala

// The collect method takes a PartialFunction as argument and maps the 
// values defined for this partial function over it, skipping those
// outside the definition domain. 

// Given a partial function pf, the following code:
//
// val pf: PartialFunction[A, B] = 
// coll.collect(pf)
//
// is roughly equivalent to
//
// coll.filter(pf.isDefinedAt _).map(pf)

// filter and map returns List[Any]
scala> List("hello", 1, true, "world").filter(_.isInstanceOf[String]).map(identity)
res0: List[Any] = List(hello, world)

// flatMap returns List[String]
scala> List("hello", 1, true, "world") flatMap {
     |   case t: String => Some(t)
     |   case _         => None
     | }
res1: List[String] = List(hello, world)

// collect returns List[String] and is concise
scala> List("hello", 1, true, "world") collect { case s: String => s }
res2: List[String] = List(hello, world)

// A instance of Seq, Set or Map is actually a partial function. So,
scala> val pets = List("cat", "dog", "frog")
pets: List[String] = List(cat, dog, frog)

scala> Seq(1, 2, 42) collect pets
res2: Seq[String] = List(dog, frog)