E50040: cannot be abstract

What went wrong

You've used the 'abstract' modifier on a construct that doesn't support it. Not all EK9 constructs can be abstract.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 103 (What are abstract classes and methods in EK9?)
• ek9 -q 115 (How do dynamic classes work in EK9?)
• ek9 -q 111 (What are components in EK9?)
• ek9 -q 117 (How do I create a singleton in EK9?)
• ek9 -q 1290 (What does EK9 error E50040 mean when I write an abstra...)

Constructs that can be abstract

• Classes: 'MyClass as abstract'
• Functions: 'myFunction() as abstract'
• Methods in classes/traits

Constructs that cannot be abstract

• Records (they are data-focused, always concrete)
• Program and service methods/operations (entry points; nothing can extend them, so nothing could implement the abstract member - 'override' on the same methods is E05100)
• Generic function definitions (must have implementation)
• Constructors (they must be concrete)
• Operators (implementation required)

How to fix it

1. Remove 'abstract' if not supported by this construct
2. If abstraction is needed, change to a construct that supports it
3. For records, use composition instead of inheritance

See also

• E50040: invalid abstract modifier (this error)
• E05100: 'override' on a program or service method
• E06100: abstract generic function (specific to generics)

Examples

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

Fails with E50040: cannot be abstract

#!ek9
defines module error.e50040.bad

  defines class

    Shape
      area() as abstract
        <- rtn as Float?

Fixed: Mark the class abstract, as its method is

#!ek9
defines module error.e50040.good.fix

  defines class

    Shape as abstract
      area() as abstract
        <- rtn as Float?

Fixed: Only an open or abstract type may have an abstract method

#!ek9
defines module error.e50040.good

  defines class

    Shape as abstract
      area() as abstract
        <- rtn as Float?

    Square extends Shape
      side as Float: 1.0

      override area()
        <- rtn as Float: side * side

      default operator ?

Questions and answers

These Q&A entries show E50040 and its fix:

• Q103: What are abstract classes and methods in EK9?
• Q111: What are components in EK9?
• Q115: How do dynamic classes work in EK9?
• Q117: How do I create a singleton in EK9?
• Q1234: Create me a generic Processor class of type T with an abstract process method, then extend it inline using a dynamic class with String as the type parameter.
• Q1290: What does EK9 error E50040 mean when I write an abstract method in a component and how do I fix it?
• Q1291: Why does E50040 fire on my abstract method in a class but not in a trait?