Variables

We can create variables in Disco as names to stand for certain values. Variable names can be any combination of lower and uppercase letters, digits, underscores (_), and single quotes ('), but they must start with a letter. For example, these are all valid variable names:

f
FROG
f_99x'
e1E10

On the other hand, these are not valid variable names:

_x
2nd_try
m&m's

To create a variable, there are two steps.

We must first declare the variable’s type, that is, we must say what type of value the variable will hold. We do this by writing the variable’s name, a colon, and a type, which is called a type signature. For example:
```
x : N
```
We can then define the variable to be equal to the value of some expression. For now, just think of an expression as any combination of numbers, operations, parentheses, and other variables. For example:
```
x = (3 + 77) * 5
```
If you wish, you can read more about definitions in the reference documentation.

Expressions are evaluated to find out what value they have, using the same order of operations you are used to from math. For example, to evaluate 3 + 7 * 5 Disco will first multiply 7 by 5, then add 3.

Once we have created a variable, we can use it anywhere, and it will be replaced by its value. For example:

Disco> x : N
Disco> x = (3 + 77) * 5
Disco> x
400
Disco> x + 7
407
Disco> x * x - 99
159901

Exercises

Define a natural number variable named vol and give it a value equal to the volume of a 6 by 7 by 8 box. Use it to evaluate the following expressions:
- vol + vol
- 3 vol^2 + 2 vol - 7
- (vol - 8)(vol + 8)