What is the default value of an unset variable?

  Absent: object does not exist (a Dict lookup miss). Note an empty Optional is NOT absent: it is a real object that is present but unset, so use '?:' (unset-or-absent) to default it.
  Present but unset: object exists, no meaningful value. String() is unset. ? returns false.
  Present and set: object has valid value. String("hello") is set. ? returns true.

  name <- String()
  stdout.println(`Name is set: ${name?}`)    // false: present, but unset

  name :=? "default"    assigns because unset
  name :=? "other"      no-op because already set

  if record <- database.find(key)
    stdout.println(`Found: ${record}`)

  parsed <- Integer("not a number")
  if parsed?    false — invalid input produces unset, no exception

  Absent: field omitted. Unset: field as null. Set: field has value.

#!ek9
defines module qa.unset.variables

  defines function

    savedNickname() as pure
      <- rtn as String: String()

  defines program
    UnsetVariablesDemo()
      stdout <- Stdout()

      // Present but unset
      name <- String()
      stdout.println(`Name is set: ${name?}`)

      // Present and set
      greeting <- "Hello"
      stdout.println(`Greeting is set: ${greeting?}`)

      // Assign-if-unset - only assigns if unset
      nickname <- savedNickname()
      nickname :=? "default"
      stdout.println(`Nickname after guard: ${nickname}`)

      nickname :=? "other"
      stdout.println(`Nickname after second guard: ${nickname}`)

      // Guard in control flow
      items <- List() of String
      items += "first"

      // Collections are always set when created
      stdout.println(`Items is set: ${items?}`)

      emptyList <- List() of Integer
      stdout.println(`Empty list is set: ${emptyList?}`)

      // Unset integer
      count <- Integer()
      stdout.println(`Unset count: ${count?}`)
      //Known to be unset here, so ':=' - a ':=?' would always apply (E08097)
      count := 42
      stdout.println(`Count after assignment: ${count}`)