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Re: Re: [scala-user] Phantom types

by Henry Ware-2 :: Rate this Message:

On Wed, Jul 9, 2008 at 6:26 PM, Rafael de F. Ferreira
<rafael@...> wrote:

> Also, I was wondering if there is any way to remove the boilerplate
> from the code. It becomes apparent when looking at the version with
> abstract members (http://snippets.dzone.com/posts/show/5741).
>
> Leaving aside the phantom types, it is easy to define a BuilderPart
> class to encapsulate each field (a thin wrapper over Option[] that
> returns the builder for method chaining); the price is a little
> mutable state. But when the types are added to the picture, I can't
> think of a way to avoid threading them all in each builder method.
> Moreover, when threading the types, I couldn't find a way to avoid
> threading the values as well.

The boilerplate for the values in the anonymous ScotchBuilders can be
abstracted into a common parent. I also don't see anyway to avoid repeating
the types.

class DefaultBuilder[a,b,c](default:ScotchBuilder) extends ScotchBuilder {
protected[BuilderPattern] val theBrand:Option[String] = default.theBrand;
protected[BuilderPattern] val theMode:Option[Preparation] = default.theMode;
protected[BuilderPattern] val theDoubleStatus:Option[Boolean] =
default.theDoubleStatus;
protected[BuilderPattern] val theGlass:Option[Glass] = default.theGlass;
type HAS_BRAND = a;
type HAS_MODE = b;
type HAS_DOUBLE_STATUS = c;
}

def withBrand(b:String) = new
DefaultBuilder[TRUE,self.HAS_MODE,self.HAS_DOUBLE_STATUS](self) {
override protected[BuilderPattern] val theBrand:Option[String] = Some(b);
}

def withMode(m:Mode) = new
DefaultBuilder[self.HAS_BRAND,TRUE,self.HAS_DOUBLE_STATUS](self) {
override protected[BuilderPattern] val theMode:Option[Preparation] = Some(m);
}

On the other hand, while the Phantom Types are very cool, going further down
the abstract member path might be more practical--- at least for the
required fields:

case class OrderOfScotch (val brand:String, val mode:Preparation,
val glass:Option[Glass], val double:Boolean);
abstract case class ScotchBuilder {
self :ScotchBuilder =>
val theBrand:String;
val theMode:Preparation;
val theGlass:Option[Glass]=None;
val double:Boolean;

// optional fields can be like before
def withGlass(g:Glass)=new ScotchBuilder{
val theBrand=self.theBrand;
val theMode=self.theMode;
val double=self.double;
override val theGlass=Some(g);
}

def build=new OrderOfScotch(theBrand,theMode,theGlass,double);
}

def main(argv:Array[String]){
val b=new ScotchBuilder { val theBrand="Glen Ordinal"; val
theMode=Neat; val double=false; }
println(b);
println(b.build);
}

-Henry Ware

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