# Variable and Constant Declarations

## Constant Declaration

Whenever declaring a new constant, you must specify the type of the constant and then use `=` operator to assign values:

```
Num foo = 123
```

```javascript
const foo = 123;
```

Constants forbidden reassignment:

```
Num foo = 123
foo = 456
```

The sample code above throws error:

```
ParserError: Constant `foo` cannot be reassigned
```

## Variable Declaration

Wyrd introduce a keyword `mutable` in order to declare variables:

```
mutable Num foo = 123
```

It will compiled into JavaScript with `let` variable declaration:

```javascript
let foo = 123;
```

Hence, reassignment is now permitted:

```
mutable Num foo = 123
foo = 456
```

```javascript
let foo = 123;
foo = 456;
```

And of course, if you already declared a constant, redeclared as mutable variable throws error:

```
Num foo = 123
mutable Num foo = 456
```

```
ParserError: Constant `foo` cannot be redeclared as variable
```

## "maybe" Types Declaration

There are cases where you might need a variable to assign value represents the concept of **empty**. In Wyrd, everything about empty is represented as a primitive type of value `Null`. 

On the other hand, Wyrd introduced `maybe` types which can store either `Null` value or declared type. However, since the declared variable should have mutability, not only the keyword `maybe` involves, but also `mutable` keyword as well:

```
mutable maybe Num foo = 123
foo = Null
foo = 456
```

```javascript
let foo = 123;
foo = null;
foo = 456;
```

Since mutable variables can also be assigned `Null`, instead of directly assigned with `Null`, we can just omit the assignment and skip to next line, it will automatically assign the `maybe` type with `Null` value.

```
mutable maybe Num foo
```

```javascript
let foo = null;
```