Learning Zig from the official language reference.
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zig-langref/while_with_optionals.zig

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const std = @import("std");
const testing = std.testing;
var numbers_left: u32 = undefined;
// when the while (foo) |bar| form is used, bar must be an optional type
test "while null capture" {
var sum1: u32 = 0;
numbers_left = 3;
while (eventuallyNullSequence()) |value| {
sum1 += value;
}
testing.expectEqual(sum1, 3);
var sum2: u32 = 0;
numbers_left = 3;
while (eventuallyNullSequence()) |value| {
sum2 += value;
} else {
testing.expectEqual(sum2, 3);
}
}
fn eventuallyNullSequence() ?u32 {
return if (numbers_left == 0) null else blk: {
numbers_left -= 1;
break :blk numbers_left;
};
}