Rust if, match extension library

or-rs is a library that extends the syntax to allow different types to be returned in each branch of Rust's if, match via or_gen! macro.

Usage

Currently, the or_gen! macro supports Rust's if and match statement.

if expression

#![feature(proc_macro_hygiene)] // for now, you have to add this unstable feature flag

use macros::or_gen;
use or::enums::*;

fn main() {
    // you have to add a type annotation **explicitly** for `x`.
    #[or_gen]
    let x: Or3<i32, String, f32> = if true {
        3                   // this branch returns `i32`
    } else if false {
        "hello".to_string() // this branch returns `String`
    } else {
        3.0                 // this branch returns `f32`
    };

    // Check if `x` is of type t2(=String)
    if x.is_t2() {
        // here `x` is String
    } else {
        // here `x` is not String
    }
}

match expression

#![feature(proc_macro_hygiene)] // for now, you have to add this unstable feature flag

use macros::or_gen;
use or::enums::*;

fn main() {
    // you have to add a type annotation **explicitly** for `x`.
    #[or_gen]
    let x: Or3<i32, f32, String> = match 42 {
        42 => 3,                   // this branch returns `i32`
        10 => 3.2,                 // this branch returns `f32`
        _  => "hello".to_string(), // this branch returns `String`
    };

    // map works **only** when the inner value of Or is i32.
    let y = x.map_t1(|a| a * a);
    assert_eq!(y.clone().as_t1().unwrap(), 9);

    // map works **only** when the inner value of Or is f32.
    let z = y.map_t2(|a| a.floor());
    assert_eq!(z.as_t1().unwrap(), 9);
}

Background

In Rust, each branch of an if, match expression must return a unified type. This means that the following code will fail to compile:

let s = if true {
    3
} else if false {
    "tofs".to_string()
} else {
    3.0 // ERROR! `if` and `else` have incompatible types.
};

However, with this library's or_gen! macro and Or type, you can compile code like the following!

use macros::or_gen;
use or::enums::*;

#[or_gen]
let s: Or3<i32, String, f32> = if true {
    3
} else if false {
    "tofs".to_string()
} else {
    3.0
};
// -> OK: This compiles!

Under the hood, the or_gen! macro converts the above if expression into code like this at compile time:

let s: Or3<i32, String, f32> = if true {
    Or3::<i32, String, f32>::T1(3)
} else if false {
    Or3::<i32, String, f32>::T2("tofs".to_string())
} else {
    {
        Or3::<i32, String, f32>::T3(3.0)
    }
};

As each branch of the if returns an Or3 enum type, so the above code compiles successfully. Currently, Or types from Or2 to Or9 are provided.

Limitation

Currently, this library has some limitations.

• The number of type arguments in Or type and the number of branches in if(match) must match.
• You cannot use return within each branch of if(match).
• The or_gen! macro can only be used with let statements.
  • For example, you cannot use the macro for a single if(match) expression to return the Or type as a function's return value.