control_flow.md

id	title
control_flow	Control Flow

Skip includes the usual control flow constructs including if, for/in, while, do and loop. Unlike most languages, control flow constructs in Skip are expressions and they produce values just like other expressions. Control flow expressions may be used in any context an expression is expected.

If-else

if/else evaluate one of 2 possible expressions.

fun maybeAdd1(condition: Bool, x: Int): Int {
  y = (if (condition) 1 else 0);
  x + y
}

The else clause may be omitted, in which case the result of the if expression must be an expression of void type.

fun maybeAdd1(condition: Bool, x: Int): Int {
  y = 0;
  if (condition) {
    !y = 1
  };
  x + y
}

Loops

The for/in expression enables iteration over all of the elements of a collection or sequence.

fun findMax(values: Sequence<Int>): Int {
  max = Int::min;
  for (value in values) {
    if (value > max) {
      !max = value
    }
  };
  max
}

Within the body of a for/in expression, a continue expression terminates the current iteration and advances to the next iteration through the loop body. A break expression within the body of a for/in expression terminates the iteration. The break expression includes an argument expression. The result of a for/in expression which terminates due to a break results in the value of the break argument; the value of a for/in expression which terminates by completing the entire iteration is either void or the result of the else clause on the for/in expression:

fun getAge(name: String, people: Sequence<Person>): Int {
  for (person in people) {
    if (person.name == name) {
      break person.age
    }
  } else -1 // Return -1 if the person is not found.
}

Similarly, while and do loops may include an else clause:

fun getAgeWhile(name: String, people: Sequence<Person>): Int {
  iter = people.values();
  current = iter.next();
  while (current.isSome()) {
    person = current.fromSome();
    if (person.name == name) {
      break person.age
    };
    !current = iter.next();
  } else -1 // Return -1 if the person is not found.
}

The loop expression loops forever:

fun getAgeLoop(name: String, people: Sequence<Person>): Int {
  iter = people.values();
  loop {
    current = iter.next();
    if (current.isNone()) {
      break -1
    };
    person = current.fromSome();
    if (person.name == name) {
      break person.age
    }
  }
}