How do I provide a default value when something might be unset in EK9?

EK9 provides two operators for default values:

?? VALUE COALESCING

Returns the left value if SET, otherwise the right:

  displayName <- userName ?? "Guest"
  timeout <- configuredTimeout ?? defaultTimeout

:=? GUARDED ASSIGNMENT

Only assigns if the target is currently UNSET:

  serverHost <- String()
  serverHost :=? "localhost"
  serverHost :=? "other"     has no effect — already set to localhost

WHEN TO USE EACH

?? when creating a new variable with a fallback.
:=? when conditionally initialising an existing variable.

COMBINING WITH GUARDS

  if connection <- tryConnect(primaryHost)
    useConnection(connection)
  else
    fallbackHost <- primaryHost ?? backupHost
    if backup <- tryConnect(fallbackHost)
      useConnection(backup)

Example

defines module qa.controlflow.defaultvalue

  defines function

    lookupSetting() as pure
      -> settingName as String
      <- rtn as String: String()

      if settingName == "host"
        rtn: "production.example.com"

  defines program

    DefaultValueDemo()
      stdout <- Stdout()

      // ?? for inline defaults
      configuredHost <- lookupSetting("host")
      activeHost <- configuredHost ?? "localhost"
      stdout.println(`Host: ${activeHost}`)

      // ?? when lookup returns unset
      missingPort <- lookupSetting("port")
      activePort <- missingPort ?? "8080"
      stdout.println(`Port: ${activePort}`)

      // :=? for conditional initialisation
      logLevel <- String()
      logLevel :=? "INFO"
      stdout.println(`Log level: ${logLevel}`)

      // :=? has no effect when already set
      logLevel :=? "DEBUG"
      stdout.println(`Still: ${logLevel}`)

Common mistakes

E01010 — 'null' does not exist in EK9 and the ? symbol is the isSet suffix, not a conditional. Use ?? for value coalescing: left ?? right returns left if set, else right.

Incorrect:

      activeHost <- configuredHost != null ? configuredHost : "localhost"

Correct:

      activeHost <- configuredHost ?? "localhost"

E01010 — The ? is the isSet suffix, not part of a conditional expression. Use :=? for guarded assignment — it only assigns if the target is currently unset.

Incorrect:

      logLevel <- logLevel ? logLevel : "INFO"

Correct:

      logLevel :=? "INFO"

Other ways to ask this

How do I fall back to a default in EK9?
What is the EK9 equivalent of a default value expression?
How does ?? work for defaults in EK9?
How do I use :=? for conditional initialisation?

Coming from another language?

EK9 uses ?? for value coalescing and :=? for guarded assignment. Both handle unset values without any concept of null.

Keywords: fallback, coalescing, guarded, unset, default