In Lily, functions are first-class entities. They can be passed around as arguments, returned, stored in vars, and so forth. There are several kinds of functions in the language, as well as a handful of keywords that can augment them.

The most common kind of function is a native function created through the define keyword:


# Function(Integer, Integer => Integer)
define add(left: Integer, right: Integer): Integer {
    return left + right

The above defines add as taking two Integer values and producing an Integer as output.

In the event that a function has no arguments, or produces no output, that entire section can be omitted:

# Function( => Integer)
define return_10: Integer {
    return 10

# Function(Integer)
define return_nothing(a: Integer) {

# Function()
define no_op {

Like many curly-brace family languages, functions are invoked by passing their arguments like so: add(5, 10) to receive 15.

Omitting a return type doesn't mean that a function doesn't return anything. A function that doesn't mention a return value will actually the value unit of the class Unit. Since all functions actually return a value (even if it's just unit), it's possible to chain functions that otherwise could not be chained:

define no_op { }

print(no_op()) # unit

Functions can also be used as arguments:

define square(input: Integer): Integer {
    return input * input

define apply_action(a: Integer, fn: Function(Integer => Integer)): Integer
    return fn(a)

print(apply_action(10, square)) # 100

The define keyword has a number of modifiers that are available to it ( public, forward, static, etc. ). For simplicity, Lily requires that modifiers are introduced in alphabetical order.


One problem with the above example is that square is relatively simple. One is likely to assume that square will take an input, multiply it by itself, and return that input. But suppose that the codebase is large, and square is somewhere else. If there is an issue, the source to square must be tracked down.

An alternative is to use a lambda. Lambdas are nameless functions that can be used where a function is needed. A lambda begins with (|, followed by argument names, until | is seen. From there, everything until there is a matching ) for the (| is the body of the lambda. Here's the above, rewritten to use a lambda.

define apply_action(a: Integer, fn: Function(Integer => Integer)): Integer
    return fn(a)

print(apply_action(10, (|a| a * a) )) # 100

Now the intent is much clearer, because a is being multiplied by itself.

Another feature of lambdas is that they are values, and the var they're assigned to can opt to have a different lambda. On the other hand, lambdas are quite restricted, and not eligible for many of the function features listed below.

Lambdas don't require type information. In the above example, the lambda uses inference to determine that a should be of type Integer. Since lambdas exist primarily to return some kind of a value, the return type of a lambda is the last expression that run. If the last expression is part of a block such as if or match, then the return type is instead unit.


One limitation of the above kinds of declarations is that, outside of global variables, they're limited strictly to what they've been given. This is where closures come in handy.

Here's an example of a function that returns an ever-increasing Integer value:

define get_counter: Function( => Integer) {
    var counter = 0

    define counter_fn: Integer {
        counter += 1
        return counter

    return counter_fn

var c = get_counter()
var results = [c(), c(), c()]

# Result:
# [0, 1, 2]

Lambdas can also be closures:

define list_total(l: List[Integer]): Integer {
    var total = 0

    l.each(|e| total += e )

    return total

The above would be much improved if it used generics to allow for any type of List. Or, even better, if it used List.fold to avoid making a closure at all.


Foreign functions are functions that are introduced from a non-Lily library. These functions, like those created by define, cannot be reassigned.


Functions in Lily have a number of different features available to them. All function kinds except for lambdas can make use of all of these features.


Using the forward keyword before define denotes that the function will be declared at some point in the future. Instead of giving the define a body, it must instead have the triple dot (...) token:

forward define add(Integer, Integer): Integer { ... }

# (more definitions)

define add(x: Integer, y: Integer): Integer {
    return x + y

Functions declared with forward are not allowed to specify names for their variables, and are not allowed to use keyword arguments.

While there are unresolved forward declarations, it is a syntax error to attempt to import a file, declare a class property, or declare a var. This is done to prevent the resolving function from using variables that are not properly initialized.

Additionally, if a class or module finishes with incomplete forward declarations, a syntax error is generated immediately.


Adding ... to the end of a type denotes that the function can receive a variable number of arguments of that type. The function receives the arguments as a List of the type provided. If no arguments were passed, the List will be empty.

# Function(Integer...) => Integer
define sum(numbers: Integer...): Integer
    var total = 0
    numbers.each(|e| total += e )
    return total

# sum() # 0
# sum(1, 2, 3) # 6


Adding * before a type, then = <value> after it denotes that the parameter is optional. Optional arguments may be a simple value, or an expression. Required arguments must not come after an optional argument.

The expressions of optional arguments, if run, are always run from left to right. As a result, it's permissible for a parameter to depend on another to the left of it.

# Function(*Integer => Integer)
define optarg(a: *Integer = 10): Integer { return a + 10 }

optarg(100) # 110
optarg() # 20

# Function(*Integer, *Integer, *Integer => Integer)
define my_slice(source: List[Integer],
                start: *Integer = 0,
                end: *Integer = source.size()): List[Integer]
    return source.slice(start, end)

my_slice([1, 2, 3], 1)    # [2, 3]
my_slice([4, 5, 6], 0, 1) # [4]

The calling function runs the optional argument expressions each time they are needed. As a result, each invocation will receive fresh versions of a default argument that do not carry over into the next invocation.

# Function(*List[Integer] => List[Integer])
define optarg_list(x: *List[Integer] = []): List[Integer]
    return x

optarg_list()             # [0]
optarg_list([1, 2])       # [1, 2, 2]
optarg_list()             # [0]
optarg_list([10, 20, 30]) # [10, 20, 30, 3]

Mixing variable and optional arguments is permissible. By default, the vararg parameter receives an empty List if no values are passed. Mixing these two features allows a different default value:

# Function(Integer, *Integer, *Integer... => Integer)
define optarg_sum(a: Integer,
                  b: *Integer = 10,
                  args: *Integer... = [20, 30]): Integer
    var total = a + b

    for i in 0...args.size() - 1:
        total += args[i]

    return total

optarg_sum(5)              # 65
optarg_sum(5, 20)          # 75
optarg_sum(10, 20, 30, 40) # 100


Placing :<name> before the name of a parameter will allow the function to be called using keyword arguments. Keyword arguments allow calling a function with arguments in a different order than the function's parameters. The function can then be called either with positional arguments or keyword arguments.

# Function(Integer, Integer, Integer => Integer)
define simple_keyarg(:first x: Integer,
                     :second y: Integer,
                     :third z: Integer): List[Integer]
    return [a, b, c]

simple_keyarg(1, 2, 3)                         # [1, 2, 3]

simple_keyarg(1, :second 2, :third 3)          # [1, 2, 3]

simple_keyarg(:third 30, :first 10, :second 5) # [10, 5, 30]

It isn't necessary to name all arguments:

# Function(Integer, Integer)
define tail_keyarg(x: Integer, :y y: Integer) {}

tail_keyarg(10, 20)

tail_keyarg(10, :y 20)

Calling a function with keyword arguments has some restrictions:

# simple_keyarg(:first 1, 2, 3)
# Syntax error: Positional argument after keyword argument

# simple_keyarg(1, :first 1, 2, 3)
# Syntax error: Duplicate value provided to the first argument.

# simple_keyarg(1, 2, 3, :asdf 1)
# Syntax error: 'asdf' isn't a valid keyword.

Keyword arguments are evaluated and contribute to type inference in the order that they're provided:

var keyorder_list: List[Integer] = []

define keyorder_bump(value: Integer): Integer
    return value

define keyorder_check(:first  x: Integer,
                      :second y: Integer,
                      :third  z: Integer): List[Integer]
    return keyorder_list

keyorder_check(:second keyorder_bump(2),
               :first  keyorder_bump(1),
               :third  keyorder_bump(3))
               # [2, 1, 3]

A function call with keyword arguments is verified at parse-time. The vm does not understand keyword arguments, and the type system does not carry keyword information either.

Despite the above, keyword arguments can be mixed together with optional arguments and variable arguments:

define optkey(:x x: *Integer = 10,
              :y y: *Integer = 20): Integer
    return x + y

optkey()        # 30
optkey(50, 60)  # 110
optkey(:y 170)  # 180
optkey(4, :y 7) # 11

define varkey(:format fmt: String,
              :arg args: *String...=["a", "b", "c"]): List[String]
    return args

varkey("fmt")                # ["fmt", "a", "b", "c"]
varkey("fmt", "1", :arg "2") # ["fmt", "1", "2"]