Crate impl_enum

Macro for generating methods on an enum that match on the enum and call the same method on each variant.

Example

// The variant of the writer is dynamically selected with an environment variable.
// Using the macro, we get the convenience of a trait object with the performance of an enum.

use std::env;
use std::fs::File;
use std::io::Cursor;
use std::io::Write;

#[impl_enum::with_methods {
    fn write_all(&mut self, buf: &[u8]) -> Result<(), std::io::Error> {}
    pub fn write(&mut self, buf: &[u8]) -> Result<usize, std::io::Error> {}
}]
enum Writer {
    Cursor(Cursor<Vec<u8>>),
    File(File),
}

fn get_writer() -> Writer {
    if let Ok(path) = env::var("WRITER_FILE") {
        Writer::File(File::create(path).unwrap())
    } else {
        Writer::Cursor(Cursor::new(vec![]))
    }
}

fn main() {
    let mut writer = get_writer();
    writer.write_all(b"hello!").unwrap();
}

The macro generates an impl block for the Writer enum equivalent to

impl Writer {
    fn write_all(&mut self, buf: &[u8]) -> Result<(), std::io::Error> {
        match self {
            Self::Cursor(cursor) => cursor.write_all(buf),
            Self::File(file) => file.write_all(buf),
        }
    }

    pub fn write(&mut self, buf: &[u8]) -> Result<usize, std::io::Error> {
        match self {
            Self::Cursor(cursor) => cursor.write(buf),
            Self::File(file) => file.write(buf),
        }
    }
}

This would be simple enough to write manually in this case, but with many variants and methods, maintaining such an impl can become tedious. The macro is intended to make such an enum easier to work with.

Variants with named fields and multiple fields are also supported, the method is always called on the first field and the rest are ignored. Enums with variants with no fields are currently not supported.

Attribute Macros

with_methods

Generates an impl block for an enum containing the given methods, where the method is a simple match over all the variants, calling the same method on the matched variant's first field.