[scala] What is the equivalent of this program using type parameters?
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[scala] What is the equivalent of this program using type parameters?
by Mushtaq Ahmed
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Problem statement:
- Allow addition of KG + Gram, Meter + Kilometer but prevent addition of Meter and Miles - Allow conversion from Meter to Miles, KG to pounds but prevent conversion from Miles to Grams
- Allow multiplication of any two units (return type Double) I came up with the following program using abstract type members, but still struggling to find solution using type parameters/generics. Will appreciate any help/guidance. Thanks.
trait AbstractMeasure { type Quantity <: AbstractQuantity trait AbstractQuantity { this: Quantity =>
type Unit <: AbstractUnit trait AbstractUnit { val amount: Double implicit val implicitThis = this def +(that: Unit) = make(amount + that.to[this.type].amount)
def *(that: AbstractMeasure#AbstractQuantity#AbstractUnit) = amount * that.amount def *(that: Double) = make(amount * that)
def to[T<: Quantity#AbstractUnit](implicit m: Manifest[T]) = { val mappedUnit = convert(make(1)).find(_.getClass == m.erasure).get
mappedUnit * amount } }
} } object Singleton { type U = AbstractMeasure#AbstractQuantity#AbstractUnit def convert: Map[U,List[U]] = Map( Meter(1) -> List(Meter(1), KM(0.001) , Mile(0.00062), Yard(1.093)), KM(1) -> List(Meter(1000), KM(1) , Mile(0.62), Yard(1093)),
Mile(1) -> List(Meter(1609), KM(1.069) , Mile(1), Yard(1760)), ....
) def make(x: Double)(implicit o: U): U = o match { case Meter(_) => Meter(x) case KM(_) => KM(x)
case Mile(_) => Mile(x) ... } } object Length extends AbstractMeasure {
type Quantity = LengthQuantity trait LengthQuantity extends AbstractQuantity object Metric extends LengthQuantity { type Unit = LengthUnit trait LengthUnit extends AbstractUnit case class Meter(amount: Double) extends LengthUnit case class KM(amount: Double) extends LengthUnit
} object English extends LengthQuantity { type Unit = LengthUnit abstract class LengthUnit extends AbstractUnit case class Mile(amount: Double) extends LengthUnit
case class Yard(amount: Double) extends LengthUnit } } object Weight extends AbstractMeasure { type Quantity = WeightQuantity trait WeightQuantity extends AbstractQuantity
object Metric extends WeightQuantity { type Unit = WeightUnit trait WeightUnit extends AbstractUnit case class Gram(amount: Double) extends WeightUnit
case class KG(amount: Double) extends WeightUnit } object English extends WeightQuantity { type Unit = WeightUnit trait WeightUnit extends AbstractUnit
case class Pound(amount: Double) extends WeightUnit case class Ounce(amount: Double) extends WeightUnit } } import Length.Metric._, Length.English._, Weight.Metric._, Weight.English._
println(KM(4) + Meter(5)) => KM(4.005) println(KM(5) + Mile(3)) //compile time error println(KM(5).to[Yard]) => Yard(5465.0) println(KG(2).to[Mile]) //compile time error
println(Meter(3) * Gram(9)) => 27 |
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Re: [scala] What is the equivalent of this program using type parameters?
by Luc Duponcheel
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Ahmed,
your example sounds similar to the Mars Climate Orbiter one which is the stereotypical example to make use of type members in the first place why do you want a solution using type parameters? I'm not sure you'll find one that is more elegant than one using type members Luc On Fri, Oct 16, 2009 at 2:38 PM, Mushtaq Ahmed <mushtaq.a@...> wrote: Problem statement: -- __~O -\ <, (*)/ (*) reality goes far beyond imagination |
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[scala] Re: What is the equivalent of this program using type parameters?
by Jesper Nordenberg
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Mushtaq Ahmed wrote:
> Problem statement: > - Allow addition of KG + Gram, Meter + Kilometer but prevent addition of > Meter and Miles > - Allow conversion from Meter to Miles, KG to pounds but prevent > conversion from Miles to Grams > - Allow multiplication of any two units (return type Double) > > I came up with the following program using abstract type members, but > still struggling to find solution using type parameters/generics. Will > appreciate any help/guidance. Thanks. See: http://svn.assembla.com/svn/metascala/src/metascala/Units.scala /Jesper Nordenberg |
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Re: [scala] What is the equivalent of this program using type parameters?
by Mushtaq Ahmed
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why do you want a solution using type parameters? I always found parameterized types (especially with variance annotations) difficult to apply. This is an attempt to translate a known solution to a familiar problem... in the hope that the process will make these concepts easier to understand, maybe.
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Re: [scala] Re: What is the equivalent of this program using type parameters?
by Mushtaq Ahmed
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Thanks for the pointer, Jesper. It seems too challenging for my abilities. Will keep it as a reference to gain some wisdom.
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Re: [scala] Re: What is the equivalent of this program using type parameters?
by Jim McBeath-3
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On Fri, Oct 16, 2009 at 10:42:14PM +0530, Mushtaq Ahmed wrote:
> See: > http://svn.assembla.com/svn/metascala/src/metascala/Units.scala > /Jesper Nordenberg > > Thanks for the pointer, Jesper. It seems too challenging for my > abilities. Will keep it as a reference to gain some wisdom. See <http://jim-mcbeath.blogspot.com/2008/11/practical-church-numerals-in-scala.html>, for a slightly simpler example with detailed explanation. By the way, Jesper, I believe your definition of / in Units.scala needs a / not * near the end of the line. -- Jim |
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[scala] Re: What is the equivalent of this program using type parameters?
by Jesper Nordenberg
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Jim McBeath wrote:
> By the way, Jesper, I believe your definition of / in Units.scala > needs a / not * near the end of the line. Good catch :) /Jesper Nordenberg |
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