Control Flow

Feng provides conditional branches, three loop forms, and break, continue, and return. Pattern matching is covered separately in Pattern Matching.

Conditional Branches

if score >= 90 {
  println("excellent");
} else if score >= 60 {
  println("pass");
} else {
  println("retry");
}

A condition must be bool. Feng does not implicitly treat a number, string, or object as a Boolean value.

if can also be used as an expression:

let status = if ready {
  "ready";
} else {
  "waiting";
};

The expression form requires an else, and every normally completing branch must produce a value of the same type.

while

var index = 0;
while index < 3 {
  println("{0}", index);
  index += 1;
}

The condition is reevaluated before each iteration.

Three-Clause for

for var index = 0; index < 10; index += 1 {
  if index == 3 {
    continue;
  }
  if index == 8 {
    break;
  }
  println("{0}", index);
}

The initializer, condition, and update clauses can each be omitted. for ;; { ... } is an infinite loop.

for/in

let values = [10, 20, 30];
var total = 0;

for let value in values {
  total += value;
}

for/in can iterate over arrays and over standard-library or user-defined iterators that implement the @iterable / @iterator protocols. The loop variable is a new binding on every iteration.

Control Transfer

• break exits the nearest loop.
• continue skips the current iteration of the nearest loop.
• return ends the current function and can carry a value when required by the function's return type.

Feng does not support loop labels, so one break or continue cannot cross multiple nested loops.