Type safe method chaining that doesn't allow repeats of operations

Question

I want to implement method chaining like in those questions:

Best practice to implement Scala trait which supports method chaining ;

Scala DSL: method chaini

Answer 1

See Phantom Types In Haskell and Scala by James Iry.

You could also use type-safe builder pattern:

trait TTrue
trait TFalse

@annotation.implicitNotFound(msg = "Cannot call same method twice.")
sealed abstract class =^=[From, To]
object =^= {
  private val singleton_=^= = new =^=[Any, Any]{}
  implicit def tpEquals[A]: A =^= A = singleton_=^=.asInstanceOf[A =^= A]
}

class Myclass[TFoo, TBar, TBuz] private(){
  def foo(implicit e: TFoo =^= TFalse) = new Myclass[TTrue, TBar, TBuz]
  def bar(implicit e: TBar =^= TFalse) = new Myclass[TFoo, TTrue, TBuz]
  def buz(implicit e: TBuz =^= TFalse) = new Myclass[TFoo, TBar, TTrue]
}

object Myclass{
  def apply() = new Myclass[TFalse, TFalse, TFalse]
}

to be used like this

scala> Myclass().foo.bar.buz
res0: Myclass[TTrue,TTrue,TTrue] = Myclass@12ac706a

scala> Myclass().bar.buz.foo
res1: Myclass[TTrue,TTrue,TTrue] = Myclass@1e69dff6

scala> Myclass().foo.buz.foo
<console>:12: error: Cannot call same method twice.
              Myclass().foo.buz.foo
                                ^

Answer 2

here you can ifnd an improved solution :

http://jim-mcbeath.blogspot.fr/2009/09/type-safe-builder-in-scala-part-3.html

(which actually relies on the phantom types)