Scala部分功能中案例评估的顺序

Question

Can I assume an order on the evaluation of cases of a partial function in Scala?

So for instance, given

protected val eval: PartialFunction[(TestPrimitive, Seq[Literal]), Boolean] = {
    case (L3IntLt, Seq(IntLit(x), IntLit(y))) => x < y
    case (L3IntLe, Seq(IntLit(x), IntLit(y))) => x <= y
    case (L3IntGe, Seq(IntLit(x), IntLit(y))) => x >= y
    case (L3IntGt, Seq(IntLit(x), IntLit(y))) => x > y
    case (L3Eq, Seq(x, y)) => x == y
    case (L3Ne, Seq(x, y)) => x != y
  }

If I could assume that cases are evaluated in order I can factor the code as:

protected val eval: PartialFunction[(TestPrimitive, Seq[Literal]), Boolean] = {
    case (L3Eq, Seq(x, y)) => x == y
    case (L3Ne, Seq(x, y)) => x != y
    case (arithp, Seq(IntLit(x), IntLit(y))) => arithp match{
      case L3IntLt => x < y
      case L3IntLe => x <= y
      case L3IntGe => x >= y
      case L3IntGt => x > y  
    }
  }

Is it a good programming practice to assume there is an order in the evaluation of the cases?

Answer 1

In Programming in Scala (First Edition), chapter 15 , you'll find:

A match expression is evaluated by trying each of the patterns in the order they are written. The first pattern that matches is selected . . .

Answer 2

Yes, cases are evaluated top to bottom, first one to match wins. It is a fine practice as it is generally understood by scala programmers, and it's a very common pattern used by a lot of scala code.

For example, for non-exhaustive matches, it is common to specify a catch all:

x match {
  case "value1" => ...
  case "value2" => ...
  case other => ...
}

Clearly this is dependent on order, since if the catch-all case was at the beginning, it would catch everything.

Answer 3

假定评估顺序是我们无论如何都需要做的事情之一。此外，我将像您一样重构我的代码！