Can I use a guard (including '?=') inside a switch or loop expression in EK9?

← Control Flow · Ref: Q837

Yes. A body-skipping guard ('<-' declaration, '?=' guarded assignment, ':=?' assign-if-unset) works in an expression-form switch/for/while/do-while/try, just as in statement form. The guard gates the whole construct: if the guarded value is unset the body is skipped, and the left-hand side takes the returning variable's INITIAL value - 'the initialiser is the guard's else'.

BECAUSE OF THAT, THE RETURN MUST BE INITIALISED

Declare the return with an initial value ('<- rtn as T: default'). If you declare it uninitialised ('<- rtn as T?') and set it only inside the (skippable) body, the compiler rejects it with E08050 RETURN_NOT_ALWAYS_INITIALISED, because the skip path would leave the LHS unset.

THIS EXAMPLE

A switch expression with a '?=' guard and an initialised return. The guard sets 'temperature', the switch runs, and 'resultText' gets a case value; had the guard left 'temperature' unset, 'resultText' would be the initial "Unknown".

NOT THE STREAM FORM

This holds for switch/for/while/do-while/try, which all declare a returning param. A STREAM expression does not: it ends in '| collect as T', so there is nowhere to write the initial value that would be the result on the skip path. A guarded stream expression is therefore rejected with E08056:

  collected <- for limit <- getLimit() then i in 1 ... 3 | collect as List of Integer  //E08056

The stream STATEMENT form is fine with a guard - there is no result to leave undeclared. See Q1381.

See Q759 for guard patterns. See Q69 for multi-case switch. See Q1381 for guards on stream pipelines.

Example

defines module qa.controlflow.guard.expression

  defines function

    currentTemperature() as pure
      -> country as String
      <- temperature as Integer?

      if country == "GB"
        temperature :=? 18
      else if country == "DE"
        temperature :=? 35
      else
        temperature :=? 25

  defines program

    GuardDemo()
      stdout <- Stdout()

      //Valid: a '?=' guard in a switch expression. The guard gates the switch; if 'temperature' were left
      //unset the body would be skipped and 'resultText' would take the return's initial value ("Unknown").
      temperature as Integer?
      resultText <- switch temperature ?= currentTemperature("GB") with temperature
        <- result as String: "Unknown"
        case < 12
          result: "Cold"
        case < 25
          result: "Moderate"
        default
          result: "Warm"

      stdout.println(resultText)

Common mistakes

E08050 — A body-skipping guard ('<-', '?=', ':=?') can skip the switch body entirely, and on that path the result is the returning variable's INITIAL value. Declared uninitialised ('<- result as String?') there would be nothing to return on the skip path, so the initialiser is what makes a guarded switch EXPRESSION legal. The stream form of this trap is E08056 - see QA1381, which has an actual pipeline. See ek9 -h E08050 for details.

Incorrect:

        <- result as String?

Correct:

        <- result as String: "Unknown"

E08050 — In an expression form, a guard can skip the body, so the return must be initialised at declaration. Give it a default ('<- result as String: "Unknown"'); an uninitialised '<- result as String?' set only in the body is E08050 RETURN_NOT_ALWAYS_INITIALISED.

Incorrect:

        <- result as String?

Correct:

        <- result as String: "Unknown"

Coming from another language?

Kotlin 'when' and Rust 'match' are expressions but have no guard-gating pre-flow. EK9 unifies it: the same guard syntax works in statement AND expression forms, and the mandatory return initialiser is the value the LHS receives when the guard skips.

Keywords: initialised return, guard, conditional, switch, E08050, guarded assignment, expression