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Simon Warta
Simon Warta

Reputation: 11408

How to use macros to add an enum case?

I have an

enum EntryPoints {
    A(),
    B(),
}

Is it possible to create a macro to adds an enum case, like this?

use some_lib::make_callback;

enum EntryPoints {
    A(),
    B(),
    make_callback!(),
}

which expands to smething like

enum EntryPoints {
    A(),
    B(),
    SomeLibCallback(data: u64),
}

All my attempts to create a very simple example lead to syntax errors and I don't find anything on the web.

Upvotes: 1

Views: 294

Answers (1)

PitaJ
PitaJ

Reputation: 15012

You can't call macros within the body of an enum declaration, but you can pass the enum declaration into the macro instead:

macro_rules! make_callback {
    { enum $name:ident { $($inner:tt)* } } => {
        enum $name {
            SomeLibCallback(u64),
            $($inner)*
        }
    }
}

make_callback! { enum EntryPoints {
    A(),
    B(),
} }

// expansion
enum EntryPoints {
    SomeLibCallback(u64),

    A(),
    B(),
}

{ $($inner:tt)* } essentially just says to match any arbitrary sequence of tokens within the set of curly braces, and $($inner)* then expands that sequence back into the new enum body.

Upvotes: 4

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