Loop while a connection value remains set, processing each iteration.

Fetch the first value before the loop, test it in the while condition, and fetch the next one at the end of the body:

  current <- source.getConnection()
  while current?
    stdout.println(current)
    current: source.getConnection()

The loop ends cleanly when getConnection() returns an unset value.

A while GUARD does not do this. In 'while conn <- supplier.getConnection() with counter < maxIters' the guard runs ONCE, before the loop: it takes one connection (and skips the loop entirely if that is unset), and only the 'with' condition is re-checked on each iteration. So conn stays the first connection for every pass. Use a guard to set up the loop, not to poll. See Q77 for while guard details. See Q971 for declaration guards.

Example

defines module qa.controlflow.whileguard

  defines class

    ConnectionSupplier
      remaining <- 0

      ConnectionSupplier()
        -> count as Integer
        this.remaining: count

      getConnection()
        <- rtn as String: String()
        if remaining > 0
          rtn: `Connection-${remaining}`
          remaining: remaining - 1

      override operator ? as pure
        <- rtn as Boolean: remaining?

  defines program

    WhileGuardDemo()
      stdout <- Stdout()

      source <- ConnectionSupplier(4)
      processed <- 0

      // Loop while getConnection() returns a SET value: fetch first, re-fetch at the end of the body
      current <- source.getConnection()
      while current?
        stdout.println(current)
        processed: processed + 1
        current: source.getConnection()

      stdout.println(`Processed ${processed} connections`)

      // A while GUARD runs ONCE, before the loop: conn is the first connection on every pass,
      // and only the 'with' condition is re-checked
      supplier <- ConnectionSupplier(2)
      counter <- 0
      maxIters <- 3
      while conn <- supplier.getConnection() with counter < maxIters
        stdout.println(`Guarded: ${conn}`)
        counter: counter + 1

Common mistakes

E07390 — Using a constant Boolean literal (true) as the while-guard's 'with' condition is a pointless expression; supply a real terminating condition instead. See ek9 -h E07390 for details.

Incorrect:

while conn <- supplier.getConnection() with true

Correct:

while conn <- supplier.getConnection() with counter < maxIters

E01073 — 'null' does not exist in EK9; use tri-state semantics (unset/set) with the '?' operator and guards instead. See ek9 -h E01073 for details.

Incorrect:

counter <> null

Correct:

counter < maxIters

Other ways to ask this

Write code to use a while guard that loops while a value is set
I have a supplier that eventually returns unset and need to loop until that happens
Given a function that returns set values for N calls then unset, process each value
In Java I'd use while ((val = getNext()) != null). Write the EK9 guard equivalent
Does a while guard call its function again on every iteration?

Coming from another language?

Java: while ((val = getNext()) != null) { process(val); }. Go: for val := getNext(); val != nil; val = getNext(). Rust: while let Some(v) = get_next(). EK9: val <- getNext(), then while val? with val: getNext() at the end of the body. A while guard (while val <- getNext() with cond) runs once, before the loop, so it does not poll.

Keywords: poll, while, loop, guard, isset, unset, declaration, connection