Zig: Allow equality operator (== and !=) for ?T and T when equality operator is supported for T and T

[mdsteele]

This test passes just fine:

test "compare optional with non-optional" {
    const foo: *i32 = @intToPtr(*i32, 4);
    const bar: ?*i32 = @intToPtr(*i32, 4);
    assert(foo == bar);
}

While this one fails to compile, with the error message "error: operator not allowed for type '?i32'":

test "compare optional with non-optional" {
    const foo: i32 = 4;
    const bar: ?i32 = 4;
    assert(foo == bar);
}

At first, I assumed this was a bug, but reading #658, it sounds like this was a deliberate decision?
(Because ?i32 == i32 would have to do two comparisons behind the scenes, while ?*i32 == *i32 can just do one.)

Still, this seems like a surprising inconsistency -- in every other language I can think of with a generic option type, either (1) Option<T> supports == iff T supports ==, or (2) Option<T> doesn't support == at all. Here, we have ?T supporting == for one primitive type (pointers) but not for an even more primitive type (i32).

[andrewrk]

I think this makes sense especially considering #1059 and #1953 which create a scenarios where pointer types are in the second category.

[thejoshwolfe]

I'd like this feature here: https://github.com/thejoshwolfe/legend-of-swarkland/blob/8a90a00847cd20e11d761be9399633a9a1e17cf6/src/server/game_engine.zig#L412

[theInkSquid]

What should happen if one side is T, while the other is ?T ?

Should fn_returning_i32_not_an_option() == null be allowed?

Edit: I suppose the i32 could implicitly coerce to a ?i32

[thejoshwolfe]

Should fn_returning_i32_not_an_option() == null be allowed?

A similar question is whether 0 == null should be a compile error. I think there's no good reason for 0 == null, but @as(?i32, 0) == null should be allowed.

[andrewrk]

A similar question is whether 0 == null should be a compile error.

It's a bug if not.

[theInkSquid]

For example, when comparing two distinct optional values:

const x: ?i32 = createAnOptional();
const y: ?i32 = createAnOptional();
if (x == y) { 
    debug.warn("equal", .{});
}

It would be necessary to turn the x == y into something like the following (under the hood):

((x == null) == (y == null)) and (x == null or x.? == y.?)

But if one were to write this instead:

if (x == 10) { ... }

What should that expand to? Should the 10 be implicitly cast to an optional, or should comparing ?T with T be a special case, where it turns into something closer to this:

(lhs != null) and (lhs.? == rhs)

Which is likely a lot more efficient.
Would it be worth it to make this optimization? Or could LLVM just do this on its own?

[thejoshwolfe]

@theInkSquid this proposal is for comparing ?T and T. If we want to support comparing ?T and ?T, that's a separate proposal.

This proposal is to expand @as(?T, foo) == @as(T, bar) to foo != null and foo.? == bar.

What you're proposing for ?T and ?T might also be valuable, but it needs its own proposal.

[theInkSquid]

I see, thank you. Would it be appropriate to short-circuit evaluate the non-optional parameter? For example:

const x: ?i32 = null;
if (x == @compileError("err")) {...}

Should this be a compile error? Or, since it can be determined that because x is null (and in no case will it be equal to the right hand side), should it just evaluate to false?

I am guessing this should not short-circuit, since that would be a non-keyword operator affecting control flow. Also, it seems a bit useless and confusing, but I just wanted to check.
@andrewrk

[thejoshwolfe]

@theInkSquid that's an insightful question. There are two options.

For the expression lhs == rhs where lhs is of type ?T and rhs is of type T, the semantics should either be:

Option 1: no short circuit.

$lhs_value = evaluate lhs;
$rhs_value = evaluate rhs;
if $lhs_value == null {
    result = false;
} else {
    result = $lhs_value.? == $rhs_value;
}

Option 2: short circuit:

$lhs_value = evaluate lhs;
if $lhs_value == null {
    result = false;
} else {
    $rhs_value = evaluate rhs;
    result = $lhs_value.? == $rhs_value;
}

The OP posted an important motivation for this issue: the inconsistency of the == operator for optional pointers and optional everything-else. Following this logic, we should do option 1 with no short circuit, since we don't do that for pointer types.

EDIT: An even stronger argument for no short circuiting is that the == operator is supposed to be commutative after operand evaluation. Consider instead of ?T == T you had T == ?T. The == operator semantics require that the left hand side be evaluated before the right-hand-side. It would be very surprising if there was short circuiting depending on the ordering of the parameters and depending on whether T was a pointer type.

[mikdusan]

this proposal is for comparing ?T and T.

to clarify, ??T == T is to be a compile-error?

[andrewrk]

Yes

[theInkSquid]

How about when ?T == U when T and U coerce to the same type?

Like could I do this?

var x: ?i32 = ...;
var y: i16 = ...;
x == y

[theInkSquid]

Also, what about for other special cases where a type is comparable with some different other type? For example, you can compare a tagged union with an enum literal to check if that is the current value. Should an optional tagged union be comparable with an enum literal? That doesn't fall into the rigid ?T ==/!= T that was specified, but would make a lot of sense to have.

If something like opt_tagged_union == .enum_literal is allowed, then what mikdusan asked about would indeed by allowed by extension:

If we allow ?T == U if T and U can be compared, then in ??i32 == i32, T is ?i32 and U is i32, which are also comparable under this proposal.

Even more powerful, if we allow ?T == U, is that it could compare ?T == ?T, with this chain of logic:

?i32 == ?i32
T = i32, U = ?i32

now we check if i32 is comparable with ?i32, which it also is under this proposal.

@andrewrk