Question

in Scala, if I want to implement a DSL, is there a way to do the following:

I have an Object called “Draw” which contains the function def draw(d:Drawable)

how can I make it so that I can import the Object and call it outside the object like:

draw ball

if ball extends the Drawable trait? The problem is that I want to use draw in a kind of infix notation, but I dont want to qualify the function draw by denoting it’s implementing class/object.

Answer 1

I quickly tried it out, but could quite make it work using an object. There I had to use draw(ball) instead of draw ball, as you wanted:

Welcome to Scala version 2.8.0.RC2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_20).

scala> trait Drawable{def doSomething}
defined trait Drawable

scala> object Draw {
def draw(d:Drawable) = d.doSomething
}
defined module Draw

scala> val ball = new Drawable{def doSomething = println("doing ball")}
ball: java.lang.Object with Drawable = $anon$1@3a4ba4d6

scala> import Draw._
import Draw._

scala> draw ball
:11: error: missing arguments for method draw in object Draw;
follow this method with `_' if you want to treat it as a partially applied function
draw ball
^

scala> draw(ball)
doing ball

However by defining Draw as a class, it did work:

scala> trait Drawable{def doSomething: Unit}
defined trait Drawable

scala> class Draw {
def draw(d:Drawable) = d.doSomething
}
defined class Draw

scala>

scala> val ball = new Drawable{def doSomething = println("doing ball")}
ball: java.lang.Object with Drawable = $anon$1@36a06816

scala> val d = new Draw
d: Draw = Draw@7194f467

scala> d draw ball
doing ball

I'm not completely sure why this doesn't work the same way with an object, might be a bug or perhaps that's specified behaviour. However I didn't have the time to look it up at the moment.