two type parameters with the same name

Question

I am wondering why the two type parameters (named \"A\") with the same name (\"A\") is allowed as per the example below. I know this is a POOR naming of type parameters, do

Answer 1

I'm not expert of Scala, but your code behave exactly what I would expected.

First, you need to know that the type parameter of method is not need bound to class.

For example, the following is valid Scala.

class Test1 {
    def test[A] (x: A) = println(x)
}

And the following is also a valid Scala code, the only different is that this one does not use the type A at all.

class Test2[A] {
    def test (x: Int) = println(x)
}

So I think it is clear now, first you created a instance of MyTest[Int], which is fine.

scala> val test = new MyTest[Int]
test: MyTest[Int] = MyTest@308ff65f

Then you called checkString[A, Int] without provide type parameter A, since it is a generic function, the compiler must inference what type is A.

scala> test.checkString("String",1)
Value is a String
x is not a String
res7: java.lang.String = String

Note:

In this time point, Scala already knows that x must be a Int and its type is fixed, since you provide it by MyTest[Int]. So the following code will yield compile error.

scala> val t = new MyTest[Int]
t: MyTest[Int] = MyTest@cb800f

scala> t.checkString ("A", "B")
:8: error: type mismatch;
 found   : java.lang.String("B")
 required: t.MyType
       t.checkString ("A", "B")

Now the complier looks at the arguments you provided, and found its is

checkString ("String", 1)

which is corresponding to

checkString (value: A, x: Int)

So now compiler knows type A in checkString[A, Int] must be a string, and if you do all of this by hand, your code will look like this.

scala> val test = new MyTest[Int]
test: MyTest[Int] = MyTest@5bda13

scala> test.checkString[String]("String", 1)
Value is a String
x is not a String
res1: String = String

scala> test.checkString[Int] (3, 4)
Value is not a String
x is not a String
res4: Int = 3

scala> test.checkString[Int] ("String", 4)
:8: error: type mismatch;
 found   : java.lang.String("String")
 required: Int
       test.checkString[Int] ("String", 4)
                          ^