wren/doc/site/functions.markdown

^title Functions ^category types

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:

:::wren
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:

:::wren
class Blondie {
  callMe(fn) {
    // Call it...
  }
}

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

:::wren
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:

:::wren
var someFn = // Get a function...
blondie.callMe(someFn)

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:

:::wren
var someFn = Fn.new {
  System.print("Hi!")
}

As you can see it takes a block argument too! All the constructor does it return that, 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:

:::wren
class Blondie {
  callMe(fn) {
    fn.call()
  }
}

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.

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

blondie.callMe(FakeFn.new())

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:

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

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

:::wren
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:

:::wren
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:

:::wren
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:

:::wren
var counter = Counter.create
System.print(counter.call()) // Prints "1".
System.print(counter.call()) // Prints "2".
System.print(counter.call()) // Prints "3".