Values

Programs work with values: pieces of information like a name, a price, or whether a customer is logged in. In this chapter you’ll learn how to give values names and what kinds of values there are.

Giving a value a name

Write a name, then =, then the value:

name = "Amina"
age = 34

say name
say age

Amina
34

After this line runs, name means "Amina" everywhere below it. Using a name instead of repeating the value makes programs easier to read and to change.

Names can use letters, digits and _, but must start with a letter or _. Good names say what the value means: total_price is clearer than tp. Names are case-sensitive, so total and Total are different names.

Values don’t change

In Quill, a value created with = can’t change. Once name is "Amina", it stays "Amina". Creating the same name again is a mistake:

name = "Amina"
name = "Omar"

Problem on line 2 of example.quill:

    name = "Omar"
    ^^^^
name already exists. It was created on line 1.
Values in Quill can't change once they're created, so give this one a different name.

This might seem strict, but it makes programs much easier to understand: when you read name, you know it is the value from the line that created it. When you do need a value that changes, like a counter, you say so with var. You’ll learn how in Values That Change.

Create before you use

Quill reads a program from top to bottom, so a name must be created before it’s used:

say greeting
greeting = "Hello"

Problem on line 1 of example.quill:

    say greeting
        ^^^^^^^^
greeting is used here, but it isn't created until line 2.
Quill reads from top to bottom, so create values before using them.

Kinds of values

Every value has a kind. Quill starts with three:

city = "Dubai"
guests = 120
confirmed = true

say city
say guests
say confirmed

Dubai
120
true

A list and a record gather several values into one. They have their own chapter, Lists and records.

A name keeps its kind for the whole program. Quill knows the kind of every value before the program runs, which is how it catches mistakes like adding text to a number. You’ll see that in the next chapter.

Quill never turns one kind into another on its own: "5" is text, while 5 is a number.

Try it

1. Create values for a product: its name, its price, and whether it’s in stock. Show each one with say.
2. Try to create the same name twice. Read the error.
3. Use a name before creating it. What does Quill say?