Feature Request: go-like struct composition with splatting syntax?

[Borketh]

The Go language has this cool feature for struct composition where if you put one struct after another's values, its values are inherited into the other.

type foo1 struct {
    bar   string
}
type foo2 struct {
    baz   string
    foo1  // foo2 now also has the field "bar" inherited from foo1
}

This seems like it would fit well into Julia.

My suggested implementation would be:

struct foo1
    bar::String
end

struct foo2
    baz::String
    foo1...
end

Thanks for taking the time to read this! It would be yet another pretty nifty trick that Julia could use.

[jpsamaroo]

I would imagine this could be pretty easily implemented with a macro.

Also, if you want to put the fields of foo1 into foo2, why not just include an instance of foo1 in foo2 directly? The struct layout and performance should (probably) be identical, assuming that both structs are immutable.

[Borketh]

@jpsamaroo because the point is to get foo2.bar, not foo2.[foo1].bar

@vtjnash invalid?

[vtjnash]

Duplicate of #4935

[JeffBezanson]

I wouldn't say this is an exact duplicate. The proposal there is to allow adding fields via explicit subtyping, while the proposal here is syntactic sugar for adding fields in a specific place, with no effect on subtyping.

[vtjnash]

That seems almost worse then somehow, since you can't write code against these fields, as they don't participate in dispatch anywhere, and there's no name for them.

[quinnj]

I think you're way over-complicating the issue here @vtjnash; I think this is purely a syntax sugar request; at type-definition time, look up the "splatted type"'s fields, and insert them into the current type definition as if the user had typed them.

[Borketh]

Can we actually discuss this without shutting me down please?
Marking this as invalid, duplicate of something vaguely related, and closing it without giving it a mote of consideration doesn't speak well of a best practice for anything.

[Borketh]

Here's a more concrete example

struct Child
    name::String
    age::Int
    hobbies:Vector{String}
end

struct Student
    Child...
    currentclass::String
    grade::Union{Int, Char}
end

Bobbert = Child("Bobbert Smythe", 9, ["Gaming", "Sewing", "Horseback Riding"])
BobbertStudent = Student("Bobbert Smythe", 9, ["Gaming", "Sewing", "Horseback Riding"], "Math", 76)

# this would make sense with regular splat syntax too

Rowena = Child("Rowena Ravenclaw", 16, ["Magic", "Ravens"])
RowenaStudent = Student(Rowena..., "Charms", 'A')

[JeffBezanson]

@AstroFloof Yes I think this absolutely can be discussed. Thank you for the clarifying example; it always helps to have something more than foo and bar :)