Monday, October 18, 2010

Phantom Types In Haskell and Scala

Some literate Haskell and Scala to demonstrate 
1) phantom types
2) I am a masochist
3) ???
4) profit!

The code is probably best viewed with a syntax colorizer
for one language or the other but I've colorized all my

> {-# LANGUAGE EmptyDataDecls #-}
> module RocketModule(test1, test2, createRocket, addFuel, addO2, launch) where

object RocketModule {

None of these data types have constructors, so there are
no values with these types. That's okay because I only 
need the types at compile time. Hence "phantom" - 
ethereal and unreal.

> data NoFuel
> data Fueled
> data NoO2
> data HasO2

   sealed trait NoFuel
   sealed trait Fueled
   sealed trait NoO2
   sealed trait HasO2

The Rocket data type takes two type parameters, fuel and
o2.  But the constructor doesn't touch them.  I don't
export the constructor so only this module can create

> data Rocket fuel o2 = Rocket

   final case class Rocket[Fuel, O2] private[RocketModule]()

createRocket produces a rocket with no fuel and no o2
> createRocket :: Rocket NoFuel NoO2 
> createRocket = Rocket

   def createRocket = Rocket[NoFuel, NoO2]()

addFuel takes a rocket with no fuel and returns one with
fuel.  It doesn't touch the o2
> addFuel :: Rocket NoFuel o2 -> Rocket Fueled o2
> addFuel x = Rocket

   def addFuel[O2](x : Rocket[NoFuel, O2]) = Rocket[Fueled, O2]()

Similarly, addO2 adds o2 without touching the fuel

> addO2 :: Rocket fuel NoO2 -> Rocket fuel HasO2
> addO2 x = Rocket
   def addO2[Fuel](x : Rocket[Fuel, NoO2]) = Rocket[Fuel, HasO2]()

launch will only launch a rocket with both fuel and o2

> launch :: Rocket Fueled HasO2 -> String
> launch x = "blastoff"

   def launch(x : Rocket[Fueled, HasO2]) = "blastoff"

This is just a pretty way of stringing things together,
stolen shamelessly from F#.  Adding infix operations is
a bit verbose in Scala.

> a |> b = b a

   implicit def toPiped[V] (value:V) = new {
      def |>[R] (f : V => R) = f(value)

Create a rocket, fuel it, add o2, then 
launch it

> test1 = createRocket |> addFuel |> addO2 |> launch

   def test1 = createRocket |> addFuel |> addO2 |> launch

This compiles just fine, too.  It doesn't matter which 
order we put in the fuel and o2
> test2 = createRocket |> addO2 |> addFuel |> launch

   def test2 = createRocket |> addO2 |> addFuel |> launch

//This won't compile - there's no fuel
// > test3 = createRocket |> addO2 |> launch

//    def test3 = createRocket |> addO2 |> launch

// This won't compile either - there's no o2

// > test4 = createRocket |> addFuel |> launch

//    def test4 = createRocket |> addFuel |> launch

// This won't compile because you can't add fuel twice

// > test5 = createRocket |> addFuel |> addO2 |> addFuel |> launch

//    def test5 = createRocket |> addFuel |> addO2 |> addFuel |> launch


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Matthias J. Déjà said...

I wonder what bytecode will be produced by the Scala compiler. How much of that stuff will be optimized away?

ssanj said...

Great post! :)

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Jamesbangga said...

can this be more complicated

Ben C said...

I want to do this but suppose you had lots of other parameters that you want to check at various specific points.  Unfortunately you are then carrying around a lot of phantom type parameters.  Would it be possible in Haskell to combine them in a single blob somehow?

This seems to answer the question for Scala, but I'm not using that!