Set a default port only if one has not already been provided.

Only assigns if the target is currently UNSET:

  port :=? 8080

If port is already SET, the assignment is skipped. No if-check needed. See Q1038, Q1091.

Example

defines module qa.controlflow.guardedassignment

  defines program

    GuardedAssignmentDemo()
      stdout <- Stdout()

      //Port is unset — :=? assigns the default
      port <- Integer()
      port :=? 8080
      stdout.println(`Port: ${port}`)

      //Port is already set — :=? is skipped
      port :=? 9090
      stdout.println(`Still: ${port}`)

Common mistakes

E01073 — EK9 has no null. Use :=? for conditional assignment — it only sets the value if the target is currently unset.

Incorrect:

      if port == null
        port = 8080

Correct:

      port :=? 8080

Other ways to ask this

I need to assign a fallback value but only when the variable is unset
In Ruby I'd use ||=. Write the EK9 guarded assignment equivalent
Given an optional port configuration, apply a default if nothing was set
Use :=? to conditionally initialise a variable that might already have a value

Coming from another language?

Ruby: port ||= 8080. Kotlin: port = port ?: 8080. JavaScript: port ??= 8080. EK9: port :=? 8080.

Keywords: assignment, fallback, :=?, default, guarded, conditional