Why are trait bounds represented by "+"?

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Why are trait bounds represented by +?
When there is a trait A and a trait B, and the generics argument T has a trait boundary of A + B, then T must be A∩B, so it might as well be expressed as A * B.
Why did you use the + operator?

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Old thread:

Why do you ask?

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I have a math degree, so I understand what you're getting at. But don't you think someone who doesn't would be confused by a multiplication symbol?

Conceptually, you're adding another requirement to the list of requirements.

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Last time @cuviper found the commit.

Here's the meeting minutes.

Multiple type bounds

N: Current syntax uses whitespace as a separator, ambiguous with rooted paths
N: Propose <T:Foo+Bar> as alternative
N: Graydon?
G: a bit gnarly but tolerable
N: Good enough!
B: We're done here. Thanks

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Speaking as a mathematical midget I start thinking:

Were I to be looking for a job I might want a salary S and a bonus B and a C and X weeks of holiday. The are the traits I might be looking for. These things all add up so: A + B + C + X. So use of "+" in requiring multiple traits seems reasonable.

Which reminds me. Ever since school day I have always wondered why logical "and" is "." and logical "or" is "+". Always seemed backwards to me.

Except that in binary and modulo 2:
0 + 0 = 0
0 + 1 = 1
1 + 0 = 1
1 + 1 = 0
So the arithmetic plus, is actually logical "or" and the symbol for both is "+".

Very confusing.

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Note that the choice of an "and" or "or" of any kind depends on the context in which you use it. Compare these two ways of computing the same thing:

let has_a = input & A == A;
let has_b = input & B == B;
let has_all = has_a && has_b;

let wanted = A | B;
let has_all = input & wanted == wanted;

In one case we use an "and" to combine the two parts, and in another case, we use an "or".

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Where does anyone denote set intersection with A * B? The characteristic function of the intersection is the product of characteristic functions, but other than that, I can't think of any examples for that notation. If I saw A * B, I'd think of the cartesian product, which has nothing to do with multiple trait bounds. A + B also isn't particularly related to set intersection, since it would mean a disjoint sum. But at least + has the correct informal meaning "both of those things", while * has no relevant intuitions.

+ is also commonly expected to be a commutative and associative operator, and compound trait bounds certainly behave this way. * is more often used in mathematics in cases where the order matters, making it a bit more ambiguous.

& or && would probably be the most intuitive connective, but it would introduce parsing ambiguities with reference types. , would be another nice connective, in which case generic parameters and where-bounds would need a different separator. ; would likely work for that, but it's a bit less intuitive and pretty for writing sequences of parameters.

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