17. Functions


Functions are first class objects in Fantom modeled via the Func class. Functions have a signature formally defined as a set of parameter types and a return type. Functions which don't return an object have a Void return type. Functions are created one of three ways:

  1. Methods: all methods are really wrappers for a function
  2. Closures: closures are expressions which result in a function
  3. Bind: the Func.bind method is used to create a new function by binding arguments to an existing function

Function Signatures

The Func class is a built-in generic type, with a custom parameterization syntax:

// format
|A a, B b ... H h -> R|

// examples
|Int a, Int b->Str|  // function which takes two Int args and returns a Str
|Int, Int->Str|      // same as above omitting parameter names
|->Bool|             // function which takes zero args and returns Bool
|Str s->Void|        // function which takes one Str arg and returns void
|Str s|              // same as above, omitting optional void return
|->Void|             // function which takes no arguments and returns void
|->|                 // shortcut for above

The examples above are type signatures much like you'd use Str or Str[].

Calling Functions

Functions are just objects like everything else in Fantom. All functions subclass the Func class which provides methods for invoking the function. The most basic method is Func.callList which takes a list of arguments and returns the result (or null if the return if void):

// func is a function which takes two Int args and returns an Int
|Int a, Int b->Int| func
func.callList([7, 8])

The Func class also supports the call method for calling with different arity (zero through eight). For example to call the function above with two arguments:

func.call(7, 8)

Using the call arity versions provides better performance in most cases because it skips packaging up the arguments in a list.

Fantom also supports a bit of syntax sugar to call a function like a normal method call using the () operator. For example we could call the function above using this syntax:

func(7, 8)    // syntax sugar for func.call(7, 8)

Type Compatibility

Functions have some special rules when it comes to type compatibility. The axiom for type compatibility is that type A is compatible for type B if A can be used whenever B is expected. Most of the time this means A extends from B through inheritance. For example Int is type compatible with Num because anyplace Num is expected, we can pass an Int.

A type declaration for a function means "these are the are the arguments I'm going to pass to this function and the result I expect back". So function type A is compatible with function type B if A can accept the arguments which B declares it is going to pass and returns an expected type. Specifically, function type A is compatible with function type B if these rules apply:

  1. A declares the same number or less parameters than B
  2. Each parameter type in A is compatible with its respective parameter type in B
  3. A returns a type compatible with B (or if B returns void, then A can return anything)

The following table illustrates some examples which shows what Type.fits would report:

Num  fits  Int  =>  false
Int  fits  Num  =>  true

|Int a|  fits  |Int a|  =>  true
|Num a|  fits  |Int a|  =>  true
|Int a|  fits  |Num a|  =>  false

|Int a|  fits  |Int a, Int b|  =>  true
|Int a, Int b|  fits  |Int a|  =>  false

|->Void|  fits  |->Int|    =>  false
|->Int|   fits  |->Void|   =>  true
|->Int|   fits  |->Num|    =>  true
|->Num|   fits  |->Int|    =>  false

The first two items in the table above are for reference - Int fits a Num, but not vise versa. Next let's look closely at this example:

|Num a|  fits  |Int a| =>  true

What this shows is that if a function type is declared to take an Int, we can pass a function that accepts a Num. That may seem counterintuitive at first, but remember that functions are the flip side of normal type checking. Here is a concrete example of that concept in terms a typical Java or C# programmer might find more natural:

class WidgetEvent {}
class ButtonEvent : WidgetEvent {}
addButtonListener(|ButtonEvent event| callback)

In the code above ButtonEvent is a subclass of WidgetEvent. We've got a method which registers a callback to invoke when a button is pressed - the argument passed to the callback will be a ButtonEvent. However, if we happen to have a function that accepts any WidgetEvent, then it will quite happily accept ButtonEvent arguments:

anyWidgetCallback := |WidgetEvent event| {...}


This is what is meant by functions being the "flip side" of normal type checking. Where normal type checking accepts any specialization of a type, function type checking accepts any generalization of a function.

Arity Compatibility

Next let's look at how arity (number of parameters) figures into functional type compatibility by dissecting these examples:

|Int a|  fits  |Int a, Int b|  =>  true
|Int a, Int b|  fits  |Int a|  =>  false

Here we see that a function that accepts one Int is compatible with a function type that generates two Ints. This is an ability of all functions in Fantom - to accept more arguments than they will use. It is kind of like default parameters in reverse. We use this technique all the time in the core classes. For example the Map.each method is used to iterate the key/value pairs:

// actual signature of Map.each
Void each(|V value, K key| c)

// iterate with function that only accepts value
map.each |Obj value| { echo(value) }

// or iterate with function that accepts both value and key
map.each |Obj value, Obj key| { echo("$key = $value") }

Many of the APIs which accept a function will pass multiple parameters, but you don't actually have to use all of those parameters.


In Fantom, all methods wrap a function accessed via the Method.func method. The Func for a method serves as its reflective handle. This relationship between functions and methods is a key aspect of how Fantom bridges object oriented and functional programming (the flip side is that all functions are an object).

Mapping static methods to functions is straight forward:

static Int add(Int a, Int b) { return a + b }

func := type.method("add").func
nine := func(7, 2)

One gotcha to be aware of - you can't access the Method.func method without parenthesis, and then use the parenthesis to invoke the function because the parenthesis will bind to the Method.func call:

type.method("add").func               // returns Func
type.method("add").func()             // same as above

type.method("add").func().call(7,2)   // invoke function
type.method("add").func()(7,2)        // same as above

Instance methods map to a function where the first argument is the implicit this parameter. If you've ever used Python this concept is pretty much in your face with the explicit self argument. Fantom lets you use instance methods like Java or C#, but we still need to map those OO methods to functions. Let's consider this example:

m := Str#replace
f := m.func

// note method params does not include the
// implicit this, but the function params does
m.params  => [sys::Str from, sys::Str to]
f.params  => [sys::Str this, sys::Str from, sys::Str to]

// both of these result in "hello!"
s1 := "hi!".replace("hi", "hello")
s2 := f("hi!", "hi", "hello")

The code above gets the Str.replace instance method as a function. The replace method takes two string arguments, but when flattened into a function it takes three string arguments because we have to account for the implicit this argument.

Immutable Functions

An immutable function is one proven to be thread safe. You can check immutability at runtime via Obj.isImmutable and attempt to convert a function via Obj.toImmutable. Immutable functions are often required when working with actors.

Immutability works as follows:

  • Method functions are always immutable - see Method.func
  • Closures which only capture final, const variables are always immutable; toImmutable always returns this
  • Closures which capture non-final or non-const variables are always mutable; toImmutable always throws NotImmutableErr
  • Closure which capture non-final variables which aren't known to be immutable until runtime (such as Obj or List) will return false for isImmutable, but will provide a toImmutable method which attempts to bind to the current variables by calling toImmutable on each one
  • Functions created by Func.bind are immutable if the original function is immutable and every bound argument is immutable

The definition of a final variable is a variable which is never reassigned after it is initialized. Any variable which is reassigned is considered a non-final variable.