derive_more

Rust derive macros for some common traits for general types.

The traits that can be derived currently are From and infix arithmetic traits (Add, Sub, Mul, Div, Rem, BitAnd, BitOr, BitXor, Shl, Shr).

Installation

This library heavily uses Macros 1.1, which is to stabilized in Rust 1.15 (the next Rust release). To use it before that time you have to install the nightly or beta channel.

After doing this, add this to Cargo.toml:

[dependencies]
derive_more = "0.4.0"

And this to the top of your Rust file:

#[macro_use]
extern crate derive_more;

Explanation of traits

This is a basic explanation of how the traits will be implemented, but the example code below might do a better job explaining. There are three different types of traits at the moment, From, Add-like and Mul-like.

From

The From trait only works for tuple structs with one element (newtypes) or enums containing these tuple structs. The types wrapped by these tuple structs can than simply be converted by using the .into() method. For enums no from code will be generated for types that occur multiple times since this would be ambiguous.

Add-like

The first group of arithmetic traits are the Add-like traits. These are the traits that operate on two arguments of the same type, they include Add, Sub, BitAnd, BitOr and BitXor. A simple example would be that one apple plus two apples logically equals three apples.

These traits work for both structs and enums. For structs they simply do the respective operation on each pair of fields separately. For enums it will generate code that returns a Result, as adding different enum options together will not work. When adding the same ones together the same approach is done as for structs. Adding two unit types (e.g. None) will result in an error.

Mul-like

The second group of arithmetic traits are the Mul-like traits. These are the traits that can take arguments with different types, normally a user defined type and a primitive numeric type (e.g u64). These traits include the other types mentioned above, Mul, Div, Rem, Shl and Shr. A simple reason why these are different than the Add-like traits is doing arithmetic on meters, one meter times two would be two meters, but one meter times one meter would be one square meter. As this second case clearly requires more knowledge about the types this is not implemented.

Currently these traits can only be derived for structs. An extra requirement is that the other argument implements the trait for all types present in that struct, and it must also define Copy.

Example usage

It can simply be used like this:

#[macro_use]
extern crate derive_more;

#[derive(From, Add)]
struct MyInt(i32);

#[derive(Add)]
struct NormalStruct{int1: u64, int2: u64}

#[derive(From, Add)]
enum MyIntEnum{
    Int(i32),
    Bool(bool),
    UnsignedOne(u32),
    UnsignedTwo(u32),
    Nothing,
}

#[derive(Mul)]
struct DoubleUInt(u32, u32);

With this code and Rust its built in type inference the following can be done now (this sample also requires some other derives such as Eq):

fn main() {
    let my_enum_val = (MyIntEnum::SmallInt(5) + 6.into()).unwrap();

    if my_enum_val == 5.into() {
        println!("The content of my_enum_val is 5")
    }
    assert_eq!(DoubleUInt(5, 6) * 10, DoubleUInt(50, 60));
}

For more example usage check out the tests.

Generated code

The code that is generated by using these derives actually looks like this:

struct MyInt(i32);
impl ::std::convert::From<i32> for MyInt {
    fn from(a: i32) -> MyInt { MyInt(a) }
}
impl ::std::ops::Add for MyInt {
    type Output = MyInt;
    fn add(self, rhs: MyInt) -> MyInt {
        MyInt(self.0.add(rhs.0))
    }
}


struct NormalStruct{int1: u64, int2: u64}

impl ::std::ops::Add for NormalStruct {
    type Output = NormalStruct;
    fn add(self, rhs: NormalStruct) -> NormalStruct {
        NormalStruct{int1: self.int1.add(rhs.int1),
                     int2: self.int2.add(rhs.int2),}
    }
}


enum MyIntEnum {
    SmallInt(i32),
    BigInt(i64),
    TwoInts(i32, i32),
    UnsignedOne(u32),
    UnsignedTwo(u32),
    Nothing,
}

impl ::std::convert::From<i32> for MyIntEnum {
    fn from(a: i32) -> MyIntEnum { MyIntEnum::SmallInt(a) }
}

impl ::std::convert::From<i64> for MyIntEnum {
    fn from(a: i64) -> MyIntEnum { MyIntEnum::BigInt(a) }
}

impl ::std::ops::Add for MyIntEnum {
    type Output = Result<MyIntEnum, &'static str>;

    fn add(self, rhs: MyIntEnum) -> Result<MyIntEnum, &'static str> {
        match (self, rhs) {
            (MyIntEnum::SmallInt(__l_0), MyIntEnum::SmallInt(__r_0)) => Ok(MyIntEnum::SmallInt(__l_0.add(__r_0))),
            (MyIntEnum::BigInt(__l_0), MyIntEnum::BigInt(__r_0)) => Ok(MyIntEnum::BigInt(__l_0.add(__r_0))),

            (MyIntEnum::TwoInts(__l_0, __l_1),
             MyIntEnum::TwoInts(__r_0, __r_1)) => Ok(MyIntEnum::TwoInts(__l_0.add(__r_0), __l_1.add(__r_1))),

            (MyIntEnum::UnsignedOne(__l_0), MyIntEnum::UnsignedOne(__r_0)) => Ok(MyIntEnum::UnsignedOne(__l_0.add(__r_0))),
            (MyIntEnum::UnsignedTwo(__l_0), MyIntEnum::UnsignedTwo(__r_0)) => Ok(MyIntEnum::UnsignedTwo(__l_0.add(__r_0))),

            (MyIntEnum::Nothing, MyIntEnum::Nothing) => Err("Cannot add unit types together"),
            _ => Err("Trying to add mismatched enum types"),
        }
    }
}


struct DoubleUInt(u32, u32);

impl <T> ::std::ops::Mul<T> for DoubleUInt where T:
        ::std::ops::Mul<u32, Output = u32> +
        ::std::marker::Copy {

    type Output = DoubleUInt;
    fn mul(self, rhs: T) -> DoubleUInt {
         DoubleUInt(rhs.mul(self.0), rhs.mul(self.1))
    }
}

For more usage examples look at the tests.

Licence

MIT