Programming in Scala Forum - 2nd Edition Page 429

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2nd Edition Page 429 - Structural Subtyping

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sanjiv sahayam

Posts: 2
Nickname: 53683
Registered: Jan, 2008

2nd Edition Page 429 - Structural Subtyping

Posted: Mar 30, 2011 6:31 PM

Hi,

While trying out the example for structural subtyping on
page 429, I came across the following anomaly:

class Food

abstract class Animal {
type SuitableFood <: Food
def eat(food:SuitableFood)
}

class Grass extends Food
class Cow extends Animal {
type SuitableFood = Grass
override def eat(food:Grass) {}
}

class Meat extends Food
class Lion extends Animal {
type SuitableFood = Meat
override def eat(food: Meat) {}
}

scala> val leo = new Lion
leo: Lion = Lion@284d0371

scala> val molly = new Cow
molly: Cow = Cow@6a5c775d

scala> val herbs:List[Animal { type SuitableFood = Grass }] = Nil
hebs: List[Animal{type SuitableFood = Grass}] = List()

scala> molly :: herbs
res15: List[Animal{type SuitableFood = Grass}] = List(Cow@6a5c775d)

scala> leo :: res15
res16: List[Animal{type SuitableFood >: Grass with Meat <: Food}] = List(Lion@284d0371, Cow@6a5c775d)

How am I able to add a Lion (who eats meat) to a List[Animal{type SuitableFood = Grass}] ?

Is my assumption that only animals that eat grass should be allowed to the List correct?

If I do the above with a mutable list the types work "correctly" and I can't add a Lion to a List[Animal{type SuitableFood = Grass}]:

val buf = new ArrayBuffer[Animal {type SuitableFood = Grass}]()
buf: scala.collection.mutable.ArrayBuffer[Animal{type SuitableFood = Grass}] = ArrayBuffer()

scala> buf += molly
res18: buf.type = ArrayBuffer(Cow@6a5c775d)

scala> buf += leo
<console>:22: error: type mismatch;
found : leo.type (with underlying type Lion)
required: Animal{type SuitableFood = Grass}
buf += leo

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