Perfect Storm

I will document my effort in learning Scala. The main objective is to help in building up as much content as possible on the Web about Scala. Caveat though: I am learning, therefore what I document here might be rubbish.

The problem: I need an arithmetic expression parser to be able to deal with drug isobolograms like these 2 taken from Wang et al.:

Very roughly speaking, the concentration “curve” that you see is when the drug combination becomes active against the parasite. In this case the compounds are Sulfadoxine (SDX) and Pyrimethamine (PYR) used to combat P. falciparum malaria.

I want to be able to build expressions for those curves. These expressions run in contexts, that is, if one “curve” (here approximated by a line) is 133*SDX + 2000 - PYR then there is a need for having a variable SDX and another PYR.

[People with a functional programming background might immediately recognize the typical newbie exercise of doing an expression evaluator… In fact you can find it in various Scala documentation… and Caml]

So I need, what is called an environment, a store of mappings from symbols to values. Something like

{case "PYR" => 1800.0 case "SDX" => 5.0}.

First problem: Doubles and Ints. There seems to be no way to specify that the return is a Number. I would like, sometime, to use Numbers irrespective of the specific subclass. I would like to do something like:

type Exp : String => Number

The problem is that one cannot have type at all, unless inside a class. But, but, but the type is not visible outside the class as a type (i.e. just for the type info, without the need to instantiate to access). Maybe putting it into an object? Did not try it, but it would be clumsy.

I ended up with this solution for now:

trait Env {
  def apply(name : String) : Double
}

Can live with it. The Double instead of Number hurts more.

Again, I am beginning. Please don’t be too harsh on me

By the way: It would be nice to have Scala support on GeSHi so that Wordpress blogs could render Scala code beautifully. No, I am suggesting only, I don’t have the time to actually do it myself.

Please share your thoughts