Expressions and Variables

The Yarn language is a full programming language, which means it has support for writing complex expressions that let you control how the dialogue in your game works. In this document, you’ll learn how to write expressions, and where you can use them.

Expressions and if statements

An if statement is used to control whether a collection of content is shown or not. When you write an if statement, you provide an expression, which is evaluated; if that expression evaluates to true, then the contents of the if statement are run.

For example, consider this code:

<<if $gregg_day_intro is 0>>
    Gregg: Sup duder. 
    Mae: hey.
<<endif>>

The expression being used here is $gregg_day_intro is 0. In this expression, the value of the variable $gregg_day_intro is being compared to the number 0, using the is operator. If they’re the same, the expression evaluates to true, and the lines are run.

Variables

You can store information about the state of your game in variables. Variables store information; more specifically, they can store text, numbers, true or false values, or null (which represents ‘no value’).

To set the value of a variable, you use the set keyword. For example:

Mae: Oh god. Steve Scriggins.
<<set $suspect_steve to 1>>
Lori: Yeah. Him.

This sets the variable $suspect_steve to the number 1.

If a variable hasn’t been set, it will have the value null.

Variables and your game

Yarn Spinner doesn’t manage the storage of information in variables itself. Instead, your game provides a variable storage object to Yarn Spinner before you start running dialogue.

When Yarn Spinner needs to know the value of a variable, it will ask the variable storage object you’ve given it. When Yarn Spinner wants to set the value of a variable, it will provide the value and the name of the variable. In this way, your game has control over how data is stored.

Values

In addition to variables, expressions work with values. These are things like numbers, strings, and true and false.

Any value can be stored in a variable, in addition to being used in an expression.

Yarn Spinner also has a special value, called null. This value represents “no value”; any variable that hasn’t yet been assigned a value will be null.

Operators

To work with variables and values, you combine them with operators. Operators are things like the arthimetic operations + (add) and - (subtract), comparison operators like < (less than) and == (equal to), as well as logical operations like and and or.

To see the complete list of operators in Yarn Spinner, see Syntax Quick Reference.

Inline expressions

An expression can be embedded inside a line. When you do this, the result of that expression is embedded in the text of the line, which is shown to the user.

For example, if you store the number of times the player has spoken to a character in the variable $times_spoken, you can display it like this:

Player: Hi!
Character: You've spoken to me {$times_spoken} times today!