Holy traits vs "boolean traits"

[gdalle]

See also

Why traits?

I’ve been trying to teach my students traits, only to realize that there is something I don’t fully understand. Is there any different between these two approaches to dispatch? struct A end struct B end ## Holy trait trait1(::A) = Val(true) trait1(::B) = Val(false) f(x) = f(x, trait1(x)) f(x, ::Val{true}) = π f(x, ::Val{false}) = "pi" ## Type-inferrable if-else trait2(::A) = true trait2(::B) = false g(x) = trait2(x) ? π : "pi" Both make @code_warntype happy, and while I know the first one …