withFilter instead of filter

Question

Is it always more performant to use withFilter instead of filter, when afterwards applying functions like map, flatmap etc.?

Why are only map, flatmap and fore

Answer 1

In addition of the excellent answer of Shadowlands, I would like to bring an intuitive example of the difference between filter and withFilter.

Let's consider the following code

val list = List(1, 2, 3)
var go = true
val result = for(i <- list; if(go)) yield {
   go = false
   i
}

Most people expect result to be equal to List(1). This is the case since Scala 2.8, because the for-comprehension is translated into

val result = list withFilter {
  case i => go
} map {
  case i => {
    go = false
    i
  }
}

As you can see the translation converts the condition into a call to withFilter. Prior Scala 2.8, for-comprehension were translated into something like the following:

val r2 = list filter {
  case i => go
} map {
  case i => {
    go = false
    i
  }
}

Using filter, the value of result would be fairly different: List(1, 2, 3). The fact that we're making the go flag false has no effect on the filter, because the filter is already done. Again, in Scala 2.8, this issue is solved using withFilter. When withFilter is used, the condition is evaluated every time an element is accessed inside a map method.

Reference: - p.120 ,Scala in action (covers Scala 2.10), Manning Publications, Milanjan Raychaudhuri - Odersky's thoughts about for-comprehension translation