frank orchid
pub const Content = struct {
here: T
}
and I am curious if there is a way to instantiate a Content struct with a user defined struct T? So if the user had a struct called Jake where Jake is
pub const Jake = struct {
age: u8,
friend: u8
}
I would like a Content struct so that i could do content.here.age
sharp patrol
pub fn Content(comptime T: type) type {
return struct {
here: T
};
}
var jake_content = Content(Jake){ .here = .{ .age = 42, .friend = 255 } };
This is the challenge im having trying to apply your solution to my current project
Sorry - im on my phone and can’t format the code block
const std = @import("std");
const headers = @import("headers.zig");
const start_line = @import("start_line.zig");
pub const Message = struct { start_line: start_line.Type, headers: headers.Headers, body: ??? };
pub fn Build(start_line: start_line.Type, headers: headers.Headers, comptime Body: type) ??? {
return struct { start_line: start_line.Type, headers: headers.Headers, body: Body };
}
What do I do for the ???
I guess how do you define a type (in my case a struct) where a field type can be set yo anything?
inland pecan
you can turn Message into a function like mike suggested
the second ???, put Message(Body)
then in the return expression, put Message(Body){ .start_line = ... }
sharp patrol
++ gracefu that's what I'd do
with generics you kind of have to make any builder/creation types and functions generic up until the point where you're able to specify the concrete type
if the part of code that knows what the concrete type of Body should be doesn't share a call graph ancestor with Builder, but you do have a finite set of types it could be, then maybe what you need is to make that member a tagged union
another option is type erasure, but that's a massive leap forward in complexity--only worth pursuing if the set of types for Body is effectively boundless and you can't make generics work because you can't get a consistent comptime call tree from point of instantiation to point of use
frank orchid
I’ll have to explore this more - body can be any user defined struct though so maybe I need to look into type erasure?
sharp patrol
Do you need to support arbitrarily changing the type of Body for the same Message object, or can you have users work with a Message(SomeBodyType)?
I really wanna emphasize that type erasure can be a giant PITA, especially if you need to be able to serialize messages since std.json doesn't have a way to deal with type erased fields