Advanced Pattern Matching

For Case

Using for case

Let's write a simple array function which only returns the non-nil elements

func nonnil<T>(array: [T?]) -> [T] {
   var result: [T] = []
   for case let x? in array {
      result.append(x)
   }
   return result
}

print(nonnil(["a", nil, "b", "c", nil]))

The case keyword can be used in for loops just like in switch cases. Here's another example. Remember the game we talked about earlier? Well, after the first refactoring, our entity system now looks like this:

enum Entity {
    enum EntityType {
        case soldier
        case player
    }
    case Entry(type: EntityType, x: Int, y: Int, hp: Int)
}

Fancy, this allows us to draw all items with even less code:

for case let Entity.Entry(t, x, y, _) in gameEntities()
where x > 0 && y > 0 {
    drawEntity(t, x, y)
}

Our one line unwraps all the necessary properties, makes sure we're not drawing beyond 0, and finally calls the render call (drawEntity).

In order to see if the player won the game, we want to know if there is at least one Soldier with health > 0

func gameOver() -> Bool {
    for case Entity.Entry(.soldier, _, _, let hp) in gameEntities() 
    where hp > 0 {return false}
    return true
}
print(gameOver())

What's nice is that the .soldier match is part of the for query. This feels a bit like SQL and less like imperative loop programming. Also, this makes our intent clearer to the compiler, opening up the possibilities for dispatch enhancements down the road. Another nice touch is that we don't have to spell out Entity.EntityType.soldier. Swift understands our intent even if we only write .soldier as above.