whiteout

documentation

whiteout provides macros that erase the type of any value into an impl Trait for a given trait. This gives you a value that can only be use with the methods of that trait, and whose type is unnameable.

#[macro_use] extern crate whiteout;
fn main() {
    let a = erase!(10, std:ops::Add<i64, Output=i64>);
    assert_eq!(a + 10, 20);
}

Since we sometimes want to use these values together, whiteout provides both a one-time macro and a macro that produces a function which returns a consistent unnameable type, allowing multiple erased values to be used in conjunction. See the documentation for more info.

lib.rs:

whiteout provides macros that erase the type of any value into an impl Trait for a given trait.

Examples

Single values can be erased using the erase! macro.

let a = erase!(10, std::ops::Add<i64, Output=i64>);
let b = erase!(5, std::ops::Add<i64, Output=i64>);
assert_eq!(a + 10, 20);
assert_eq!(b + 10, 15);
// This fails, the types are opaque
// assert_eq!(a + b, 15);

These erased values can't be used together, though; they have different anonymized types. Therefore, you sometimes need the eraser! macro.

#[macro_use]
extern crate whiteout;

// Define a custom trait into which types will be erased.
trait MyTrait: 
    std::ops::Add<Self, Output=Self>  // Allow the operation we need
    + std::convert::From<i32>  // Allow converting from concrete values
    + std::fmt::Debug  // Allow printing (for use with assert!())
    + PartialEq  // Allow comparison (for use with assert_eq!())
    {}

// Implement MyTrait for all possible types.
impl<T> MyTrait for T 
    where T: std::ops::Add<Self, Output=Self> 
    + std::convert::From<i32> 
    + std::fmt::Debug
    + PartialEq
    {}

// Create an eraser function for the custom trait
eraser!(erase_my_trait, MyTrait);

fn main() {
    // Use the eraser function. 
    // If we used erase!(10, MyTrait); for these
    // they would be of different types.
    let a = erase_my_trait(10);
    let b = erase_my_trait(5);
    assert_eq!(a + b, 15.into());
}