# Variable bindings, constants and statics

⭐️ In Rust variable are **immutable by default**, so we call them **Variable bindings**. To make them mutable, `mut` keyword is used.

⭐️ Rust is a **statically typed** language; It checks data type at compile time. But it **doesn’t require you to actually type it when declare variable bindings**. On that case, the compiler checks the usage and sets a better data type for it. But for **constants and statics you must annotate the type**. Types come after a colon(:)

* **Variable bindings**

```rust
let a = true;
let b: bool = true;

let (x, y) = (1, 2);

let mut z = 5;
z = 6;
```

* **Constants**

```rust
const N: i32 = 5;
```

* **Statics**

```rust
static N: i32 = 5;
```

**let** keyword is used in binding expressions. We can bind a name to a value or a function. Also because the left-hand side of a let expression is a ‘pattern’, you can bind multiple names to set of values or function values.

**const** keyword is used to define constants. It lives for the entire lifetime of a program but have no fixed address in memory. **static** keyword is used to define ‘global variable’ type facility. There is only one instance for each value, and it’s at a **fixed location in memory**.

💡 **Always use const**, instead of static. It’s pretty rare that you actually want a memory location associated with your constant, and using a const allows for optimizations like constant propagation not only in your crate but also in downstream crates.

💡 Usually statics are placed at top of the code file, outside the functions.