Control Flow

Sun provides C-style control flow statements for branching and iteration.

Conditionals

If-Else

Use if and else for conditional execution:

function abs(x: i32) i32 {
    if (x < 0) {
        return -x;
    } else {
        return x;
    }
}

The condition must be wrapped in parentheses. The else branch is optional:

function clamp_positive(x: i32) i32 {
    if (x < 0) {
        return 0;
    }
    return x;
}

Else-If Chains

Chain multiple conditions with else if:

function sign(x: i32) i32 {
    if (x > 0) {
        return 1;
    } else if (x < 0) {
        return -1;
    } else {
        return 0;
    }
}

Loops

For Loop

The for loop uses traditional C-style syntax with initialization, condition, and increment:

for (initialization; condition; increment) {
    // body
}

Basic example - sum numbers 0 to 4:

function main() i32 {
    var sum: i32 = 0;
    for (var i: i32 = 0; i < 5; i = i + 1) {
        sum = sum + i;
    }
    return sum;  // Returns 10
}

Step by 2:

var sum: i32 = 0;
for (var i: i32 = 0; i < 10; i = i + 2) {
    sum = sum + i;
}
// sum = 0 + 2 + 4 + 6 + 8 = 20

Countdown:

for (var i: i32 = 5; i > 0; i = i - 1) {
    println_i32(i);
}
// Prints: 5 4 3 2 1

Nested loops:

function main() i32 {
    var count: i32 = 0;
    for (var i: i32 = 0; i < 3; i = i + 1) {
        for (var j: i32 = 0; j < 4; j = j + 1) {
            count = count + 1;
        }
    }
    return count;  // Returns 12 (3 * 4)
}

Each part of the for loop header is optional. An empty condition creates an infinite loop: for (;;) { ... }

While Loop

The while loop executes as long as its condition is true:

function main() i32 {
    var sum: i32 = 0;
    var i: i32 = 0;
    while (i < 5) {
        sum = sum + i;
        i = i + 1;
    }
    return sum;  // Returns 10
}

Convergence example:

function main() f64 {
    var x: f64 = 1.0;
    while (x > 0.01) {
        x = x / 2.0;
    }
    return x;  // Returns a value <= 0.01
}

Break and Continue

Break

Exit a loop early using break:

function find_first_multiple_of_7(limit: i32) i32 {
    var result: i32 = -1;
    for (var i: i32 = 1; i < limit; i = i + 1) {
        if (i % 7 == 0) {
            result = i;
            break;
        }
    }
    return result;
}

Continue

Skip to the next iteration using continue:

function sum_even_numbers(limit: i32) i32 {
    var sum: i32 = 0;
    for (var i: i32 = 0; i < limit; i = i + 1) {
        if (i % 2 != 0) {
            continue;  // Skip odd numbers
        }
        sum = sum + i;
    }
    return sum;
}

Nested Loop Control

break and continue only affect the innermost loop:

function main() i32 {
    var count: i32 = 0;
    for (var i: i32 = 0; i < 5; i = i + 1) {
        for (var j: i32 = 0; j < 5; j = j + 1) {
            if (j == 2) {
                break;  // Only breaks inner loop
            }
            count = count + 1;
        }
    }
    return count;  // Returns 10 (5 outer * 2 inner iterations)
}

For-In Loop

The for-in loop iterates over objects that implement the iterator pattern (have hasNext() and next() methods):

for (var element: Type in iterator) {
    // body
}

Basic example - sum using a custom iterator:

class RangeIterator implements IIterator<i32> {
    var current: i32;
    var end: i32;

    function init(start: i32, stop: i32) {
        this.current = start;
        this.end = stop;
    }

    function hasNext() bool {
        return this.current < this.end;
    }

    function next() i32 {
        var result = this.current;
        this.current = this.current + 1;
        return result;
    }
}

function main() i32 {
    var iter = RangeIterator(0, 5);
    var sum: i32 = 0;
    for (var x: i32 in iter) {
        sum = sum + x;
    }
    return sum;  // Returns 10 (0+1+2+3+4)
}

The in keyword is contextual - you can still use in as a variable name: var in: i32 = 5;

Break and continue work with for-in loops:

function sum_until_five(iter: RangeIterator) i32 {
    var sum: i32 = 0;
    for (var x: i32 in iter) {
        if (x == 5) {
            break;  // Exit early
        }
        if (x == 3) {
            continue;  // Skip 3
        }
        sum = sum + x;
    }
    return sum;
}

Nested for-in loops:

function main() i32 {
    var sum: i32 = 0;
    var outer = RangeIterator(1, 4);
    for (var i: i32 in outer) {
        var inner = RangeIterator(1, 4);
        for (var j: i32 in inner) {
            sum = sum + i * j;
        }
    }
    return sum;  // (1*1+1*2+1*3) + (2*1+2*2+2*3) + (3*1+3*2+3*3) = 36
}

Comparison with Other Languages

Feature	Sun	C/C++	Rust
For loop syntax	`for (init; cond; incr)`	`for (init; cond; incr)`	—
For-in loop	`for (var x: T in iter)`	— (range-for in C++11)	`for x in iterator`
While loop	`while (cond)`	`while (cond)`	`while cond`
Break/Continue	✓	✓	✓
Labeled breaks	Not yet	✗ (goto)	✓
Loop expressions	✗	✗	✓

Functions & Lambdas Match Expressions