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Why "trait T; class C; class X extends (C with T)` cannot be compiled? "

Scala code:

trait T
class C
type W = C with T
class X extends W

W

is a type alias, but I want to define a class to extend it. Why and how to fix it?

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2 answers


I'm having a hard time structuring my answer in a pleasant manner, but here's an attempt to explain what's going on nonetheless:

You get a compilation error because the proposal extends

requires a class and traits, not types, and you are giving a type. Classes and traits should not be confused with types.

There are, of course, better explanations for this. But basically, a type indicates operations that can be applied to something (and sometimes other properties). Classes and traits define the behavior of their instances.

In most statically typed OO languages, every class / interface / property also has an associated type. However, the mutual meaning is usually wrong: not all types have a corresponding class / interface / trait. For example, yours C with T

is a type, but not a class or a trait (or even a combination of both).



extends

Classes and traits (separated by a character with

) are expected in sentences , but not one type. This has to do with what it extends

means: extending the behavior of that thing. As I said, types don't define behavior.

In most cases, the syntax A with B

represents a type, which is a subtype of a type A

and a type B

. However, it extends

with

accepts a different meaning in the clause and simply acts as a separator for arguments extends

(just as it ,

acts as a separator for method invocation arguments).

If you write class X extends C with T

it will work, because it means class X extends C, T

if you want.

NTN

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String is final class, cannot be extended.



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