Easy optimization in the pattern match

[jvican]

When compiling the code used in #3, we get this (phase simplify):

def getFoodFor(a: Animal): Food & Product0 = {
  case val selector15: Animal = a
  if selector15.isInstanceOf[Cat] then {
    val c: Cat = selector15.asInstanceOf[Cat(c)]
    Meat
  } else 
    if selector15.isInstanceOf[Cow] then {
      val cw: Cow = selector15.asInstanceOf[Cow(cw)]
      Grass
    } else throw new MatchError(selector15)
}

In this case, the values c and cw could be removed because they are not used at all. Not sure if this is a new optimization target or something that should be fixed in dotty.

[jvican]

Also, one question: is the type in asInstanceOf correct? It doesn't look like it is. Maybe a prettyprinting error?

[DarkDimius]

@jvican, in order to get you unstuch I've disabled the optimization that should have eliminated those vals. It still needs a proper fix.

[jvican]

Great, makes sense!

[DarkDimius]

I've just pushed a better fix to unsuch branch that generates such code:

  def getFoodFor(a: Animal): Food & Product0 =
    if a.isInstanceOf[Cat] then {
      a.asInstanceOf[Cat(c)]
      Meat
    } else
      if a.isInstanceOf[Cow] then {
        a.asInstanceOf[Cow(cw)]
        Grass
      } else throw new MatchError(a)

Note that casts are still there as we lack a phase that would know that those casts always succeed.

[jvican]

Summary of the discussion: mark a.asInstanceOf[Cow(cw)] as pure and then a concrete optimization will remove any unused pure expression.