A function in YarnSpinner is the same notion as in any other programming language, or in math: it takes a certain number of arguments, and then computes and returns the result. A function call is indicated by the name of the function, followed by its arguments in parentheses. The parentheses are required, even when there are no arguments:

<<set $roll_2d6 = dice(6) + dice(6)>>
<<set $random = random()>>

There are around 20 built-in functions in Jenny, listed below; and it is also possible to add user-defined functions as well.

Built-in functions

User-defined functions

In addition to the built-in functions, you can also define any number of user-defined functions which can later be used in your yarn scripts. The syntax for these functions is exactly the same as for the built-in functions: it consists of a function name, followed by the arguments in parentheses.

Each user-defined function has a fixed signature, declared at the time when the function is added to the YarnProject. A function must have a fixed number of arguments of specific types, and a fixed return type.

All user-defined functions must be added to the YarnProject before they can be used. A compile error will be raised if the parser encounters an unknown function, or if the number or types of arguments do not match.

User-defined functions can be used for a variety of purposes, such as:

  • implement functionality that is currently missing in Jenny;

  • interface with the game engine;

  • provide access to “variables” stored outside of Jenny;

  • etc.

title: Blacksmith
// This example showcases several hypothetical user-defined functions:
// - broken(slot): checks whether the item in the given slot is broken;
// - name(slot): gives the name for an item in a slot, e.g. "sword" or "bow";
// - money(): returns the current amount of money that the player has.
// At the same time, functions `round()` and `plural()` are built-in.

<<if broken("mainhand")>>
  <<local $repair_cost = round(value("mainhand") / 5)>>

  Blacksmith: Your {name("mainhand")} seems to be completely broken!
  Blacksmith: I can fix it for just {plural($repair_cost, "% coin")}
  -> Ok, do it  <<if money() >= $repair_cost>>
  -> I'll be fine...

See also

  • FunctionStorage – document describing how to add user-defined functions to a YarnProject.