Values That Change

So far, every value has stayed the same once it was created. But some values need to change: a visitor counter, a running total, a game score. In this chapter you’ll learn how Quill handles change.

var and set

Mark a value with var when you create it, and you can change it later with set:

var visits = 0
say visits

set visits = visits + 1
say visits

set visits = visits + 1
say visits

0
1
2

Read set visits = visits + 1 as: “work out visits + 1, then make that the new value of visits”.

Why two words?

Most languages let you change any value at any time. Quill makes you mark changes for two reasons:

1. You can see every change. Search for set to find every place a value changes. A value without var never changes, so you never need to check.
2. Mistakes are caught. Changing a value that wasn’t meant to change is usually a bug.

The README calls this “every mutation is visible when you read the code”.

Values without var can’t be set

name = "Amina"
set name = "Omar"

Problem on line 2 of example.quill:

    set name = "Omar"
        ^^^^
name can't change, because it isn't marked var.
It was created on line 1 as a value that stays the same. To let it change, write line 1 as:

    var name = "Amina"

set only changes, it never creates

A value must be created with var before set can change it. When you misspell its name, Quill suggests the one you meant:

var visits = 0
set vistis = 2

Problem on line 2 of example.quill:

    set vistis = 2
        ^^^^^^
There's no value called vistis.
set changes a value that was created with var, so create it first.

Did you mean:  set visits = 2

Creating a var a second time is also a mistake. Use set to change it:

var count = 0
count = 5

Problem on line 2 of example.quill:

    count = 5
    ^^^^^
count already exists. It was created on line 1 with var.
To change it, use set instead of creating it again.

Did you mean:  set count = 5

A value keeps its kind

A var can change its value, but not its kind. A Number stays a Number:

var total = 0
set total = "10"

Problem on line 2 of example.quill:

    set total = "10"
                ^^^^
total holds a Number, so it can't be set to Text ("10").
A value keeps the same kind for the whole program.

Did you mean:  set total = 10

A running total

Here’s a typical use: adding up a bill one item at a time.

var total = 0
set total = total + 3.50   -- coffee
set total = total + 4.25   -- cake
set total = total + 3.50   -- another coffee

say "Total: {total}"

Total: 11.25

Try it

1. Make a var called score starting at 0. Add 10 three times, then say the score.
2. Try to set a value that wasn’t created with var. Read the error and fix your program the way it suggests.
3. Make a var holding text, and try to set it to a number.