Scala类型不匹配；找到：布尔值必需：List [Int]

Question

I have tried to resolve my sbt testing with simple example

package example
object Lists {
    def sum(xs: List[Int]): Int = {

        xs match {
      case x :: tail => x + sum(tail)
      case Nil => 0
     }
    }
    def max(xs: List[Int]): Int = {
      xs match {
        case Nil => throw new java.util.NoSuchElementException()
        case List(x) => x
        case x :: y :: rest => max ((if (x > y) x else y) :: rest)  
    }
    }

  }

The second file can be downloaded from http://www.filedropper.com/listssuite_1

When I try test with sbt

[error] /home/milenko/dom1/example/src/test/scala/example/ListsSuite.scala:122: type mismatch;
[error]  found   : Boolean
[error]  required: List[Int]
[error]     assert(sum(List() === 0))
[error]                       ^
[error] /home/milenko/dom1/example/src/test/scala/example/ListsSuite.scala:134: type mismatch;
[error]  found   : Boolean
[error]  required: List[Int]
[error]       assert(max(List(1, -4, 2, 6) === 6))
[error]                                    ^
[error] two errors found
[error] (test:compileIncremental) Compilation failed
[error] Total time: 1 s, completed 17/03/2017 10:43:00

I do not understand why this happens.How it comes that compiler found my type as Boolean?

Answer 1

This is faulty:

assert(sum(List() === 0))

because the type of the parameter of sum is boolean ( List() === 0 )

Try: assert(sum(List()) === 0) instead.

Same goes for the max function.

Answer 2

List() === 0 is an Expression of type Boolean because it could be either true (if the empty list is equal to the number zero) or false. Or more accurately it can only ever be false because the empty list is never equal to 0. You then try to calculate the sum of that Boolean, which is a type error.

The same goes for line 134.