Time：2021-9-22

## Explanation content

1. Type alias
example:

``````type Kilometers = i32;
let x: i32 = 5;
let y: Kilometers = 5;
println!("x + y = {}", x + y);``````

Note: in the example, kilmeters is a synonym for I32. Values of the kilometers type are treated exactly as I32 types.
The main purpose of type aliases is to reduce duplication.

(1) Consider the following types:

``Box<dyn Fn() + Send + 'static>``

Such as code:

``````let f: Box<dyn Fn() + Send + 'static> = Box::new(|| println!("hi"));
fn takes_long_type(f: Box<dyn Fn() + Send + 'static>) {
// --snip--
}
fn returns_long_type() -> Box<dyn Fn() + Send + 'static> {
// --snip--
}``````

Use alias, code:

``````type Thunk = Box<dyn Fn() + Send + 'static>;
let f: Thunk = Box::new(|| println!("hi"));
fn takes_long_type(f: Thunk) {
// --snip--
}
fn returns_long_type() -> Thunk {
// --snip--
}``````

(2) Consider the following examples:

``````use std::io::Error; // The STD:: IO:: error structure in the standard library represents all possible I / O errors
use std::fmt;
pub trait Write {
fn write(&mut self, buf: &[u8]) -> Result<usize, Error>;
fn flush(&mut self) -> Result<(), Error>;
fn write_all(&mut self, buf: &[u8]) -> Result<(), Error>;
fn write_fmt(&mut self, fmt: fmt::Arguments) -> Result<(), Error>;
}``````

Add the following type alias declaration:

``type Result<T> = std::result::Result<T, std::io::Error>;// STD:: IO:: error is placed in result < T, E >``

The code can become:

``````pub trait Write {
fn write(&mut self, buf: &[u8]) -> Result<usize>;
fn flush(&mut self) -> Result<()>;
fn write_all(&mut self, buf: &[u8]) -> Result<()>;
fn write_fmt(&mut self, fmt: Arguments) -> Result<()>;
}``````