E07040: 'by' variable is not supported on a trait only on a class

What went wrong

You're using 'by' delegation syntax in a TRAIT, but delegation only works in classes.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 770 (Why can't I use 'by' delegation inside a trait?)

How to fix it

1. Use delegation in a class instead
2. Traits define interfaces only
3. Classes provide implementations via delegation

Common causes

• Treating trait like a class
• Trying to delegate trait implementation within trait
• Misunderstanding what traits can contain

See also

• E07040: 'by' delegation in trait (this error)
• E07030: default operator in trait
• E07070: constructor in trait

Examples

Every example is compiled by the EK9 compiler: the first fails exactly as captioned, the others fix it.

Fails with E07040: 'by' variable is not supported on a trait only on a class

#!ek9
defines module error.e07040.bad

  defines trait
    Reader
      read()
        <- rtn as String: "read"

    Writer
      write()
        -> text as String
        <- rtn as Boolean: text?

    //A trait has no properties, so it has nothing to delegate 'by'
    ReaderWriter with trait of Reader by reader, Writer

Fixed: Delegate 'by' a property in a class

#!ek9
defines module error.e07040.good

  defines trait
    Reader
      read() as abstract
        <- rtn as String?

  defines class
    FileReader with trait of Reader
      override read()
        <- rtn as String: "read from file"

    //A class can delegate a trait 'by' one of its properties
    Document with trait of Reader by reader
      reader as Reader: FileReader()

      default operator ?

Questions and answers

These Q&A entries show E07040 and its fix:

• Q770: Why can't I use 'by' delegation inside a trait?