Why does my abstract function get E08180 on the return value?

← Functions and Methods · Ref: Q1072

An abstract function's return value must be declared with ? (unset marker). It cannot be left bare, and it cannot be given a default value either.

BEFORE (E08180 error):

  transformer() as abstract
    -> input as String
    <- rtn as String

The problem: 'rtn as String' has no initial value and no ? marker. The compiler cannot guarantee it will be set.

AFTER (? marker):

  transformer() as abstract
    -> input as String
    <- rtn as String?

NOT A FIX (default value, E07100):

  transformer() as abstract
    -> input as String
    <- rtn as String: String()    // ERROR E07100

An initialiser on the return value counts as an implementation, and an abstract function may not have one: E07100 (defined as default/abstract but an implementation has been provided). The '?' form is the only one that works; the functions that implement the abstract one decide what value to return.

For dynamic functions implementing the abstract, the return variable inherits the declaration:

  myFn <- (prefix) is transformer as function
    rtn: prefix + input

See Q51 for abstract function types. See Q52 for dynamic functions.

Example

defines module qa.functions.abstractreturn

  defines function

    //Abstract with ? marker on return
    transformer() as abstract
      -> input as String
      <- rtn as String?

  defines program

    AbstractReturnDemo()
      stdout <- Stdout()

      //Dynamic function implementing the abstract
      prefix <- "Hello: "
      myFn <- (prefix) is transformer as function
        rtn: prefix + input

      result <- myFn("World")
      stdout.println(result)

Common mistakes

E08180 — Abstract function return values need ? (unset marker). Use '<- rtn as String?'; a default initialiser is not allowed either (E07100).

Incorrect:

    transformer() as abstract
      -> input as String
      <- rtn as String

Correct:

    transformer() as abstract
      -> input as String
      <- rtn as String?

E07100 — An initialiser on an abstract function's return value counts as an implementation, so it triggers E07100 - defined as default/abstract but an implementation has been provided. Declare the return as 'String?' instead.

Incorrect:

    transformer() as abstract
      -> input as String
      <- rtn as String: String()

Correct:

    transformer() as abstract
      -> input as String
      <- rtn as String?

Coming from another language?

Java: abstract methods declare return type only. Python: no return type. EK9: abstract function return variables must be declared with ? (an initialiser counts as a body, E07100).

Keywords: function, unset, abstract, E08180, return, initialise