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POScala: Correcting type inference of representation type over if statement
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  1. POScala: Correcting type inference of representation type over if statement
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    copied!<p>This is a follow-up to <a href="https://stackoverflow.com/questions/11274964/">two</a> <a href="https://stackoverflow.com/questions/11236122/">questions</a> on representation types, which are type parameters of a trait designed to represent the type underlying a bounded type member (or something like that). I've had success creating instances of classes, e.g <code>ConcreteGarage</code>, that have instances <code>cars</code> of bounded type members <code>CarType</code>.</p> <pre><code>trait Garage { type CarType &lt;: Car[CarType] def cars: Seq[CarType] def copy(cars: Seq[CarType]): Garage def refuel(car: CarType, fuel: CarType#FuelType): Garage = copy( cars.map { case `car` =&gt; car.refuel(fuel) case other =&gt; other }) } class ConcreteGarage[C &lt;: Car[C]](val cars: Seq[C]) extends Garage { type CarType = C def copy(cars: Seq[C]) = new ConcreteGarage(cars) } trait Car[C &lt;: Car[C]] { type FuelType &lt;: Fuel def fuel: FuelType def copy(fuel: C#FuelType): C def refuel(fuel: C#FuelType): C = copy(fuel) } class Ferrari(val fuel: Benzin) extends Car[Ferrari] { type FuelType = Benzin def copy(fuel: Benzin) = new Ferrari(fuel) } class Mustang(val fuel: Benzin) extends Car[Mustang] { type FuelType = Benzin def copy(fuel: Benzin) = new Mustang(fuel) } trait Fuel case class Benzin() extends Fuel </code></pre> <p>I can easily create instances of <code>Car</code>s like <code>Ferrari</code>s and <code>Mustang</code>s and put them into a <code>ConcreteGarage</code>, as long as it's simple:</p> <pre><code>val newFerrari = new Ferrari(Benzin()) val newMustang = new Mustang(Benzin()) val ferrariGarage = new ConcreteGarage(Seq(newFerrari)) val mustangGarage = new ConcreteGarage(Seq(newMustang)) </code></pre> <p>However, if I merely return one or the other, based on a flag, and try to put the result into a garage, it fails:</p> <pre><code>val likesFord = true val new_car = if (likesFord) newFerrari else newMustang val switchedGarage = new ConcreteGarage(Seq(new_car)) // Fails here </code></pre> <p>The switch alone works fine, it is the call to <code>ConcreteGarage</code> constructor that fails with the rather mystical error:</p> <pre><code>error: inferred type arguments [this.Car[_ &gt;: this.Ferrari with this.Mustang &lt;: this.Car[_ &gt;: this.Ferrari with this.Mustang &lt;: ScalaObject]{def fuel: this.Benzin; type FuelType&lt;: this.Benzin}]{def fuel: this.Benzin; type FuelType&lt;: this.Benzin}] do not conform to class ConcreteGarage's type parameter bounds [C &lt;: this.Car[C]] val switchedGarage = new ConcreteGarage(Seq(new_car)) // Fails here ^ </code></pre> <p>I have tried putting those magic <code>[C &lt;: Car[C]]</code> representation type parameters everywhere, but without success in finding the magic spot.</p>
 

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