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fixes for tour/Lower type bounds #865

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Merged
merged 2 commits into from
Aug 15, 2017
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fixes for tour/Lower type bounds #865

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017f106
tour/lower-type-boudns: swallows aren't mammals
lierdakil Aug 15, 2017
c48688c
tour/lower-type-boudns: Return type annotation on Node.prepend
lierdakil Aug 15, 2017
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14 changes: 7 additions & 7 deletions _tour/lower-type-bounds.md
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Original file line number Diff line number Diff line change
Expand Up @@ -42,7 +42,7 @@ To fix this, we need to flip the variance of the type of the parameter `elem` in

```tut
trait Node[+B] {
def prepend[U >: B](elem: U)
def prepend[U >: B](elem: U): Node[B]
}

case class ListNode[+B](h: B, t: Node[B]) extends Node[B] {
Expand All @@ -58,13 +58,13 @@ case class Nil[+B]() extends Node[B] {

Now we can do the following:
```tut
trait Mammal
case class AfricanSwallow() extends Mammal
case class EuropeanSwallow() extends Mammal
trait Bird
case class AfricanSwallow() extends Bird
case class EuropeanSwallow() extends Bird


val africanSwallowList= ListNode[AfricanSwallow](AfricanSwallow(), Nil())
val mammalList: Node[Mammal] = africanSwallowList
mammalList.prepend(new EuropeanSwallow)
val birdList: Node[Bird] = africanSwallowList
birdList.prepend(new EuropeanSwallow)
```
The `Node[Mammal]` can be assigned the `africanSwallowList` but then accept `EuropeanSwallow`s.
The `Node[Bird]` can be assigned the `africanSwallowList` but then accept `EuropeanSwallow`s.