E07340: without a control, failed to find subject of flow

What went wrong

A flow construct has no control, and its pre-flow cannot stand in for one. A 'switch' without a control switches on its pre-flow variable, so a pre-flow like 'counter.value += 5' has no single subject. An 'if' (or 'else if'/'when') without a control decides on its pre-flow's isSet check, which only a guard ('<-', ':=?') performs; a blind assignment (':=', '=', ':', '+=', a typed declaration) checks nothing, so 'if item := findItem()' has no condition.

Worked examples

Compilable examples demonstrating this error and the correct fix:

ek9 -q 836 (Why can't I use compound assignment like '+=' in a switch...)
ek9 -q 1980 (What do <-, :=? and := each do in the head of an if, switch...)

How to fix it

Keep the assignment and add the set-check as the control: 'if item := findItem() then item?'
Or use a guard: '<-' for a new variable ('if item <- findItem()'), ':=?' to fill an unset one
In a 'switch', assign to a simple variable first, then switch on it

Common causes

Reading ':=' as a guard - it is a blind assignment with no isSet check
Compound or property assignment used as a switch subject

Examples

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

Fails with E07340: without a control, failed to find subject of flow

#!ek9
defines module error.e07340.bad

  defines function

    findItem()
      <- rtn as String: "found"

    blindAssignmentWithNoControl()
      <- rtn as String: String()
      selectedItem <- String()
      if selectedItem := findItem()
        rtn: selectedItem

    blindAssignmentInElseIfWithNoControl()
      -> flag as Boolean
      <- rtn as String: String()
      selectedItem <- String()
      if flag
        rtn: "flag"
      else if selectedItem := findItem()
        rtn: selectedItem

    typedDeclarationWithNoControl()
      <- rtn as String: String()
      if selectedItem as String: findItem()
        rtn: selectedItem

    compoundAssignmentWithNoControl()
      <- rtn as String: String()
      counter <- 1
      when counter += 2
        rtn: $counter

    blindAssignmentWithControlIsValid()
      <- rtn as String: String()
      selectedItem <- String()
      if selectedItem := findItem() with selectedItem?
        rtn: selectedItem

    guardsWithNoControlAreValid()
      <- rtn as String: String()
      existing <- String()
      if fresh <- findItem()
        rtn: fresh
      if existing := findItem() then existing?
        rtn: existing
      if existing :=? findItem()
        rtn: existing

Fixed: Use a guard, or say what to test

#!ek9
defines module error.e07340.good

  defines function

    findItem()
      <- rtn as String: "found"

    //A declaration guard: the body runs only when found is set
    withAGuard()
      <- rtn as String: String()
      if found <- findItem()
        rtn: found

    //An assignment needs a control saying what to test
    withAControl()
      <- rtn as String: String()
      selectedItem <- String()
      if selectedItem := findItem() then selectedItem?
        rtn: selectedItem

Questions and answers

These Q&A entries show E07340 and its fix: