user2374587 - 1 year ago 135

Scala 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'm using the ":paste" method because :load doesn't 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 class when Prof. is using...

Any helps?

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Answer Source

You get that error because not every type `T`

has `>`

,`<`

etc. defined.

What you probably wanted is `T`

to be `Ordered`

or be implicitly convertible to something that is `Ordered`

, and therefore have all of them 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`

(a view bound) says that type `T`

must be convertible to `S`

, so it has to be either a subtype of `S`

or have implicit conversion defined.

Accepter answer to this queston explains it in more detail.

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