Why must the promote operator #^ return a different type than the class?

← Operators and Expressions · Ref: Q839

The promote operator #^ converts an object to a DIFFERENT type. Returning the same type as the enclosing class is not promotion — it is a copy or identity, which is meaningless.

WHAT PROMOTE DOES

Promotion converts one type to another:

  Temperature -> String (displaying degrees)
  Distance -> Float (extracting raw measurement)
  Score -> Integer (extracting numeric score)

CORRECT PATTERN

  operator #^ as pure
    <- rtn as String: `${degrees} degrees`

The return type (String) differs from the class (Temperature).

INCORRECT PATTERN

  operator #^ as pure
    <- rtn as C5: this

Returning the SAME type (C5) from #^ on class C5 triggers E07420. This is not promotion.

USAGE

  reading <- Temperature(98.6)
  promoted <- #^reading  // promoted is now a String

See Q238 for operator overview. See Q242 for conversion operators.

Example

defines module qa.operators.promote.different.type

  defines class

    <?-
      Temperature with promote operator returning String (a different type).
      This is the correct pattern for promotion.
    -?>
    Temperature
      degrees <- 0.0

      Temperature()
        -> d as Float
        degrees :=: d

      //Promote to String — returns DIFFERENT type (correct)
      operator #^ as pure
        <- rtn as String: `${degrees} degrees`

      default operator ?

  defines program

    PromoteDemo()
      stdout <- Stdout()
      reading <- Temperature(98.6)
      promoted <- #^reading
      stdout.println(promoted)

Common mistakes

E07420 — The promote operator #^ must return a DIFFERENT type than the enclosing class. Returning Temperature from a Temperature class is identity, not promotion. Return String, Integer, Float, or another distinct type. See ek9 -h E07420 for details.

Incorrect:

      operator #^ as pure
        <- rtn as Temperature: this

Correct:

      operator #^ as pure
        <- rtn as String: `${degrees} degrees`

Other ways to ask this

What triggers E07420 MUST_NOT_RETURN_SAME_TYPE?
Why does my promote operator fail with same type error?
How do I correctly implement the #^ promote operator in EK9?

Coming from another language?

Java: No promote operator. Implicit conversions via widening (int to long). Explicit casts for narrowing. Python: No operator-based promotion; uses __int__(), __float__(), __str__() dunder methods with no return type enforcement. Rust: From/Into traits enforce different types by design. Kotlin: toInt(), toString() methods with no operator syntax. EK9: #^ operator with compile-time enforcement that return type differs from enclosing class.

Keywords: conversion, E07420, operator, return, promote, #^, type, different