a classy little scripting language


No self-respecting language today can get by without functions—first class little bundles of code. Since Wren is object-oriented, most of your code will live in methods on classes, but free-floating functions are still eminently handy.

Functions are objects like everything else in Wren, instances of the Fn class.

Block arguments #

Most of the time you create a function just to pass it to some method. For example, if you want to filter a list by some criteria, you’ll call its where method, passing in a function that defines the predicate you’re filtering on.

Since that’s the most common usage pattern, Wren’s syntax optimizes for that. Taking a page from Ruby, a function is created by passing a block argument to a method. At its simplest, it looks like this:

blondie.callMe {
  System.print("This is the body!")

Here we’re invoking the callMe method on blondie. We’re passing one argument, a function whose body is the following block—everything between that pair of curly braces.

Methods that take a block argument receive it as a normal parameter. callMe could be defined like so:

class Blondie {
  callMe(fn) {
    // Call it...

var blondie = Blondie.new()

A method can take other arguments in addition to the block. They appear before the block just like a regular argument list. For example:

blondie.callMeAt(867, 5309) {
  System.print("This is the body!")

Of course, you don’t have to use a block argument to pass a function to a method. If you already have a function object, you can pass it like a regular argument:

var someFn = // Get a function...

Block arguments are purely sugar for creating a function and passing it in one little blob of syntax. There are some times when you want to create a function but don’t need to pass it to a method. For that, you can call the Fn class’s constructor:

var someFn = Fn.new {

As you can see it takes a block argument too! All the constructor does is return that argument, so this exists purely as a convenience method for you.

Calling functions #

Once you have a function, how do you invoke it? Like everything in Wren, you do so by calling a method on it:

class Blondie {
  callMe(fn) {

Functions expose a call() method that executes the body of the function. This method is dynamically-dispatched like any other, so you can define your own “function-like” classes and pass them to methods that expect “real” functions.

class FakeFn {
  call() {
    System.print("I'm feeling functional!")


Function parameters #

Of course, functions aren’t very useful if you can’t pass values to them. The functions that we’ve seen so far take no arguments. To change that, you can provide a parameter list surrounded by | immediately after the opening brace of the body, like so:

blondie.callMe {|first, last|
  System.print("Hi, " + first + " " + last + "!")

Here we’re passing a function to callMe that takes two parameters, first and last. They are passed to the function when it’s called:

class Blondie {
  callMe(fn) {
    fn.call("Debbie", "Harry")

It’s an error to call a function with fewer arguments than its parameter list expects. If you pass too many arguments, the extras are ignored.

Returning values #

The body of a function is a block. If it is a single expression—more precisely if there is no newline after the { or parameter list—then the function implicitly returns the value of the expression.

Otherwise, the body returns null by default. You can explicitly return a value using a return statement. In other words, these two functions do the same thing:

Fn.new { "return value" }

Fn.new {
  return "return value"

Closures #

As you expect, functions are closures—they can access variables defined outside of their scope. They will hold onto closed-over variables even after leaving the scope where the function is defined:

class Counter {
  static create() {
    var i = 0
    return Fn.new { i = i + 1 }

Here, the create method returns the function created on its second line. That function references a variable i declared outside of the function. Even after the function is returned from create, it is still able to read and assign toi:

var counter = Counter.create()
System.print(counter.call()) //> 1
System.print(counter.call()) //> 2
System.print(counter.call()) //> 3

Classes → ← Variables