"T is not a legal path" is it bug?

Question
1

scala 3.3.0

type ValuesOf[T<:{type Values}] = T#Values

T#Values cant compile

T is not a legal path
since it is not a concrete type

is that bug? how i get the type T#Values now

1 Like
2

This isn’t a bug but a type safety improvement in Scala 3 (see the docs)

Either T needs to have a class/trait/enum/object as its upper bound, or you need to have a term parameter.

e.g.

// option 1
class Foo { type Values }

type ValuesOf[T <: Foo] = T#Values

or

// option 2
def valuesOf[T<:{type Values}](t: T): t.Values = ???
3

Option 1 still doesn’t work for me.

-- Error: ----------------------------------------------------------------------
1 |type ValuesOf[T <: Foo] = T#Values
  |                          ^
  |                          T is not a legal path
  |                          since it is not a concrete type
4

:open_mouth: oh sorry! it only works directly from a class then
type T = Foo#Values

the main restriction is that the compiler needs a proof that if a value of the type exists then its safe to access the type - this is trivial for class types as they are concrete. I think the explanation for parameters is because if the method gets called, then the types of its arguments were checked for soundness.

5

Option 1 still doesn’t work for me too ,unless’ - source: 3.0 migration 'is added
I’m not sure if the grammar is properly designed, but I will have a lot of code to modify and adjust :rofl:

The current compatibility approach is like this

    type ValOf[T <: {type Values} ] = T match
      case JString => JString#Values
      case JInt => JInt#Values
      //case [t]:T => [t]#Values
      //case  {type Values} => T#Values
      case _ => NotImplementedError

There is no scalability in writing like this. Do you have any other good ideas to solve this problem

6

Are you porting code from scala 2? or starting from scratch? It seems like you need a different design but I can’t suggest much unless I know the purpose of type Values, and the API you want to make