proper use of scala traits and types

What are you trying to do with that line of code? You already have a shape, the one passed in called a. Just remove that line and call a.sides().

Stylistically, there are several problems. First of all, class names should start with a capital letter. Second, sides seems like an immutable property, not a mutating method, so it should be declared and overridden with no parentheses. You also need override modifiers in your subclass. Last, you can do without the empty braces: {4} should just be 4.

There is several methods to do this. One is a complex one using reflection, second is little bit simplier, using a builder and third is most straightforward for your use case. Just change definition of someFunction to

def someFunction(a: ()=>Shape)={ val aShape = a() aShape.sides } 

so someFunction(Square) return 4 and someFunction(Triangle) returns 3 . Note this work only with case classes because real thing, we are passing here is not class itself, but it's auto-generated companion object

But more often there no need to define classes, you could write inside any context except top level thing just like

def square() = new Shape{ def sides() = 4 } def triangle() = new Shape{ def sides() = 3 } 

Next thing: methods with empty parameter list are generally reading as method that have side effects. So it is more convenient to define your type like

sealed trait Shape{ def sides:Int } 

and if you define your builders like

def square = new Shape{ def sides = 4 } def triangle = new Shape{ def sides = 3 } 

you should use them as someFunction(square _) telling, that you gonna use method call and not the value it's returning

And last thing is: if you really need the code, that creates some object, but it could contain complex computations, resource handling or some probable exception, so you want to hold over it's execution until it' really needed, you could use call-by-name parameters which is equivalent to R , which i assume you are familiar with

Unless shape has an apply function, you cannot call () on a shape object.

If you want to assign aShape to a, simply write val aShape = a.

But since I do not see the added value, you might as well call the sides function directly on a:

def someFunction(a:shape) = { val sides = a.sides // use sides } 

This is the closest translation for what you wrote:

sealed trait Shape { def sides: Int } case object Square extends Shape { override val sides = 4 } case object Triangle extends Shape { override val sides = 3 } def someFunction(a: Shape) = val shapeSides = a.sides 

Some notes:

• Classes in scala should be CamelCase
• Your subclasses have no instance members, so you can use a singleton object
• You if you have a: Shape it means that a is a Shape, and you haven't defined anything that would let you call () on it.
• You can omit braces when there's only one expression inside
• You can override a def with val if it's static