Why can't I use compound assignment like '+=' in a switch pre-flow in EK9?

← Control Flow · Ref: Q836

A switch pre-flow written WITHOUT a separate control expression must assign to a simple variable, and that variable becomes the subject of the switch ('switch result += 3' switches on result). A compound assignment on a property access such as 'scoring.prop1 += 3' has no variable to be the subject, so the compiler reports E07340 ('failed to find subject of flow'). Declare a variable and name it as the control: 'switch controlScore <- scoring.prop1 + 3 with controlScore'.

See Q69 for multi-case switch. See Q144 for no break/continue/return.

Example

defines module qa.controlflow.switch.preflow

  defines record
    Scoring
      prop1 <- 0

      Scoring()
        -> initial as Integer
        prop1 :=: initial

      override operator ? as pure
        <- rtn as Boolean: prop1?

  defines function

    classifyScore()
      <- rtn as String: String()

      scoring <- Scoring(5)
      result <- 0

      switch controlScore <- scoring.prop1 + 3 with controlScore
        case > 1
          result += 6
        default
          result += 10

      rtn: $result

  defines program

    SwitchPreFlowDemo()
      stdout <- Stdout()
      stdout.println(classifyScore())

Common mistakes

E07340 — A compound assignment on a property in switch pre-flow gives the switch no variable subject (E07340). Declare a variable and name it with 'with' instead. See ek9 -h E07340 for details.

Incorrect:

      switch scoring.prop1 += 3

Correct:

      switch controlScore <- scoring.prop1 + 3 with controlScore

Coming from another language?

Java: switch has no pre-flow. C: assignment in switch is bad practice. Kotlin: when has no pre-flow. EK9: a pre-flow with no separate control must assign to a simple variable (the subject); property targets need a declaration plus 'with'.

Keywords: switch, pre-flow, assignment, declaration, compound, E07340