Scala REPL "error: value> is not a member of a parameter of type T"

Question

This is my file.

trait Set[T] { def contains(x: T): Boolean def incl(x: T): Set[T] def union(that: Set[T]): Set[T] } class Empty[T] extends Set[T] { override def toString = "." def contains(x: T): Boolean = false def incl(x: T): Set[T] = new NonEmpty[T](x, new Empty[T], new Empty[T]) def union(that: Set[T]): Set[T] = that } class NonEmpty[T](elem: T, left: Set[T], right: Set[T]) extends Set[T] { override def toString = "{" + left + elem + right + "}" def contains(x: T): Boolean = if (x < elem) left contains x else if (x > elem) right contains x else true def incl(x: T): Set[T] = if (x < elem) new NonEmpty(elem, left incl x, right) else if (x > elem) new NonEmpty(elem, left, right incl x) else this def union(that: Set[T]): Set[T] = ((left union right) union that) incl elem } 

I use the ": paste" method because: loading does not work. But I get the following error:

 <console>:25: error: value < is not a member of type parameter T if (x < elem) left contains x ^ <console>:26: error: value > is not a member of type parameter T else if (x > elem) right contains x ^ <console>:30: error: value < is not a member of type parameter T if (x < elem) new NonEmpty(elem, left incl x, right) ^ <console>:31: error: value > is not a member of type parameter T else if (x > elem) new NonEmpty(elem, left, right incl x) 

I'm sure this file is correct because it is from class examples, and it worked in the class when using Prof ....

Any help?

Answer 1

You get this error because not every type T has > , < , etc.

You probably need T be Ordered or to be implicitly convertible to something that Ordered , and therefore they are all defined.

This should fix the error messages:

 class NonEmpty[T <% Ordered[T]](elem: T, left: Set[T], right: Set[T]) extends Set[T] { override def toString = "{" + left + elem + right + "}" def contains(x: T): Boolean = if (x < elem) left contains x else if (x > elem) right contains x else true def incl(x: T): Set[T] = if (x < elem) new NonEmpty(elem, left incl x, right) else if (x > elem) new NonEmpty(elem, left, right incl x) else this def union(that: Set[T]): Set[T] = ((left union right) union that) incl elem } 

T <% S (associated with the view) says that the type T must be convertible to S , so it must be either a subtype of S or have an implicit conversion.

Accepter's answer to this question explains this in more detail.

Answer 2

Since the " scope " has been deprecated in Scala 2.11, an alternative implementation of a common binary search tree using the context boundary "below:

 object GenericBinarySearchTree { abstract class Set[T] { def incl(x: T): Set[T] def contains(x: T): Boolean def union(that: Set[T]): Set[T] } type L[X] = X => Ordered[X] class Empty[T : L] extends Set[T] { override def toString = "." def incl(x: T): Set[T] = new NonEmpty(x, new Empty, new Empty) def contains(x: T): Boolean = false def union(that: Set[T]): Set[T] = that } class NonEmpty[T : L](elem: T, left: Set[T], right: Set[T]) extends Set[T] { override def toString = "{" + left + elem + right + "}" def incl(x: T): Set[T] = { if(x > elem) new NonEmpty(elem, left, right.incl(x)) else if(x < elem) new NonEmpty(elem, left.incl(x), right) else this } def contains(x: T): Boolean = { if(x == elem) true else if(x > elem) right.contains(x) else left.contains(x) } def union(that: Set[T]): Set[T] = ((left union right) union that) incl elem } }