Undefined Optionals

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1

Hello, I have two questions about how optional values and undefined interact:

  1. Will @as(?u32, @as(u32, undefined)) != null always be true?
  2. Will @as(?u32, undefined) != null always be true?

I am pretty sure that Q.2 is not guaranteed, but how about Q.1?

2

The result of anything you interpret undefined as is undefined(not defined) and the equality of that value to any other value is also undefined(not defined).
1&2. It may always be true, always be false, and(sic) sometimes be true. The ?u32 you receive has no meaningful value and it is undefined to branch on.

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3
  1. Yes. Only the u32 part is undefined. The ? part is defined not null.
test {
    try std.testing.expect(@as(?u32, @as(u32, undefined)) != null);
}
test {
    // will fail
    try std.testing.expect(@as(?u32, @as(u32, undefined)) == null);
}
  1. No. It is not == null either. It is Illegal to read undefined.
test {
    // will fail
    try std.testing.expect(@as(?u32, undefined) != null);
}
test {
    // will fail
    try std.testing.expect(@as(?u32, undefined) == null);
}

But if you make sure that optional is runtime-known you skip compile-time safety check:

test {
    var optional: ?u32 = undefined;
    try std.testing.expect(optional != null); // unchecked Illegal Behavior
    optional = 0; // suppress compiler error
}
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4

Undefined is not the same as null. There are better explanations, but the way I understand it is that when the variable is set as undefined, the caller must guarantee that it had been set prior to using it.

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5

You can think of ?u32 as Optional = struct { is_null: bool, val: u32 }

  1. is equivalent to Optional{ .is_null = false, .val = undefined }
  2. is equivalent to @as(Optional, undefined)
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