Scala3 inline if reuse result of function-CodePudding

val num = 3
val x = if (func(num) > 3) func(num) else 4

How can i avoid calling func(num) twice here? My project is fully functional so calling func(num) twice is guaranteed to yield the same result. Does the compiler optimize this automatically? I am aware of this:

val num = 3
val x = List(num).map(f(_)).map(a => if (a > 3) a else 4).head

EDIT: I'm looking for a way to improve this

val temp = tl.indexWhere(<bool>)
val insertIndex = if(temp == -1) tl.size else temp

CodePudding user response：

The Scala compiler will generally not know if a function is pure, so it cannot perform that optimization automatically.

val temp = tl.indexWhere(???)
val insertIndex = if(temp == -1) tl.size else temp

Is very likely the clearest and (by explicitly preventing double evaluation) most performant expression of this logic. Assuming that temp is a local variable, the Scala compiler can see that it's not used later in your function/method and reuse the local variable slot.

CodePudding user response：

Just in case you want to make it abundantly clear that the local variable is only used for this particular block of code, you could utilize … a block of code:

val insertIndex = { val temp = tl.indexWhere(<bool>); if (temp == -1) tl.size else temp }

val insertIndex = {
  val temp = tl.indexWhere(<bool>)
  if (temp == -1) tl.size else temp
}

As mentioned in another answer, Scala cannot, generally, know that tl.indexWhere is pure, since Purity Analysis is equivalent to solving the Halting Problem.

CodePudding user response：

Not sure if this is an improvement but you can use pattern matching with guard:

val num = 3
val x = num match {
  case v if v > 3 => v
  case _ => 4
}

Or in second case simply:

val insertIndex = tl.indexWhere(...) match {
  case -1 => tl.size
  case v => v
}