While Expression

Syntax

while_expr ::= 'while' expr_c block_expr

Note that in contrast to most imperative languages like C/C++ or Java, a while-construct is not a statement but an expression and, thus, yields a value.

Typing

The condition expr_c of an while-expression must be of type bool. The body block_expr must be of type unit () or of type <no-return>.

Semantics

If expr_c evaluates to true, the body expression block_expr evaluates and the while-expression is evalulated again. Additionally, impala implicitly declares two continuations which are usable inside block_expr:

1. continue of type fn() -> !

   Invoking this continuation jumps to the next iteration of this loop.

2. break of type fn() -> !

   Invoking this continuation jumps to the pointer after the loop.

These continuations are first-class citizens and can be passed to other functions.

Examples

fn main(mut x: int) -> () {
    while x < 23 {
        ++x
    }

    while x < 42 {
        let outer_break = break;
        let outer_continue = continue;
        while x < 42 {
            if x == 1 {
                continue()
            }
            if x == 2 {
                outer_continue()
            }
            if x == 3 {
                break()
            }
            if x == 4 {
                outer_break()
            }
        }
        ++x
    }