Apply a default session timeout only if no timeout was set.

← Control Flow · Ref: Q1225

The :=? guarded assignment operator assigns a value ONLY if the target variable is currently unset. If the variable already has a value, the assignment is skipped.

  timeout <- Duration()
  defaultTimeout <- PT30M
  timeout :=? defaultTimeout
  //timeout is now 30 minutes because it was unset

This is an ASSIGNMENT operator — it sets a variable.

See Q1038 for :=? with config fallback chains.

Example

defines module qa.flow.guardassign.duration

  defines function

    loadTimeoutFromConfig()
      <- rtn <- Duration()
      //Simulates: no timeout in configuration

    loadTimeoutFromProfile()
      <- rtn <- PT15M

  defines program

    GuardAssignDurationDemo()
      stdout <- Stdout()

      // === TIMEOUT IS UNSET — :=? assigns the default ===

      timeout <- Duration()
      defaultTimeout <- PT30M
      timeout :=? defaultTimeout
      stdout.println(`Session timeout: ${timeout}`)

      // === TIMEOUT IS ALREADY CONFIGURED — :=? is skipped ===

      customTimeout <- PT1H
      customTimeout :=? defaultTimeout
      stdout.println(`Custom timeout preserved: ${customTimeout}`)

      // === FALLBACK CHAIN — first set value wins ===

      sessionTimeout <- Duration()
      sessionTimeout :=? loadTimeoutFromConfig()
      sessionTimeout :=? loadTimeoutFromProfile()
      sessionTimeout :=? defaultTimeout
      stdout.println(`Effective timeout: ${sessionTimeout}`)

Common mistakes

E01073 — EK9 has no null. Use :=? to conditionally assign — it only sets the value when the variable is currently unset. See ek9 -h E01072 for details.

Incorrect:

      if timeout == null
        timeout := defaultTimeout

Correct:

      timeout :=? defaultTimeout

Other ways to ask this

How do I conditionally assign a Duration only when the variable is unset?
A web application has an optional session timeout — apply a default if not configured.
In Java I'd check if timeout == null before assigning a Duration default. What does EK9 use?
Migrating from C# where I use '??' for nullable TimeSpan — what is the EK9 pattern?

Coming from another language?

Java: if (timeout == null) timeout = Duration.ofMinutes(30). Python: timeout = timeout or timedelta(minutes=30). C#: timeout = timeout ?? TimeSpan.FromMinutes(30). Go: if timeout == 0 { timeout = 30 * time.Minute }. EK9: timeout :=? defaultTimeout — one operator, correct tri-state semantics.

Keywords: session, web, guarded, duration, timeout, default, assignment, :=?