Can guard expressions be used in switch and while, not just if?

← Control Flow · Ref: Q975

Yes. Guard declarations work IDENTICALLY in if, switch, while, do-while, and try. The same 'v <- expression' pattern means 'declare v, check if SET, only proceed if SET' everywhere.

IN IF:

  if name <- findUser(id)
    stdout.println(name)

IN SWITCH:

  switch record <- database.get(id) with record
    case .type == "USER"
      processUser(record)
    default
      processOther(record)

IN WHILE:

  while item <- iterator.next()
    process(item)

IN TRY:

  try connection <- openDatabase()
    query(connection)
  catch
    -> ex as Exception
    logError(ex)

The guard pattern is universal — one syntax to learn, works everywhere. This eliminates 90-95% of null pointer exceptions across ALL control flow.

See Q971 for if guard details. See Q75 for switch guard details.

Example

defines module qa.controlflow.guardcontexts

  defines function

    <?-
      Returns a set or unset value based on input.
    -?>
    fetchValue() as pure
      -> key as String
      <- rtn as String: String()

      if key?
        rtn: "Value for " + key

  defines program

    GuardContextsDemo()
      stdout <- Stdout()

      // Guard in IF
      if greeting <- fetchValue("hello")
        stdout.println(`IF guard: ${greeting}`)

      // Guard in IF with else
      if missing <- fetchValue(String())
        stdout.println("Should not print")
      else
        stdout.println("IF guard: skipped because unset")

      // Guard in SWITCH
      switch found <- fetchValue("test") with found
        case == "Value for test"
          stdout.println(`SWITCH guard: exact match`)
        default
          stdout.println(`SWITCH guard: ${found}`)

Common mistakes

E01073 — 'null' does not exist in EK9; check a value with the '?' isSet operator, not '!= null'. See ek9 -h E01073 for details.

Incorrect:

if key != null

Correct:

if key?

Other ways to ask this

Where can I use the <- guard pattern in EK9?
Does the guard declaration work in switch statements?
Show me guards in while loops in EK9
Is the if v <- expr guard universal across EK9 control flow?

Coming from another language?

Go: if err := f(); err != nil — only works in if. Rust: if let — only works in if/while. Swift: if let — only in if. EK9: guards work in if, switch, while, do-while, and try — truly universal.

Keywords: while guard, switch guard, guard, universal, try guard