Functions

Functions in Do are first-class values. They close over their lexical scope and can be stored in variables, passed as arguments, and returned from other functions.

def

Define named functions with def:

def greet name
  echo "Hello, $name!"

greet Alice
# prints: Hello, Alice!

Functions with no arguments must have () after their name:

def hello()
  echo Hello, world!

Implicit Return

A function returns the result of its final statement:

Statement	Result
command	function return value
let	value of right-hand side
bind	value of scrutinee
if (with final else)	result of branch
if (without final else)	nil
try/catch	result of try (no error) or invoked catch

All other statements have a nil result.

Explicit Return

Use return for early exit:

def abs x
  if (x < 0)
    return (-x)
  x

Parameters

Positional Parameters

def add a b
  (a + b)

Key Parameters

Key parameters use key: param syntax in the definition:

def create_user name age: user_age
  echo "$name is $user_age"

create_user Alice age: 30

Ditto Key Shorthand

The :key shorthand declares a key parameter bound to a variable of the same name:

def create_user :name :age
  echo "$name is $age years old"

create_user name: Alice age: 30

The shorthand also works at call sites to pass a variable as a key argument:

let name = "Alice"
let age = 30
create_user :name :age
# equivalent to: create_user name: $name age: $age

Default Values

Both positional and key parameters support defaults:

def greet name = "World"
  echo "Hello, $name!"

greet()        # Hello, World!
greet Alice    # Hello, Alice!

def connect :host = "localhost" :port = 8080
  echo "Connecting to $host:$port"

connect()                      # localhost:8080
connect port: 3000             # localhost:3000
connect host: example.com      # example.com:8080

Defaults are evaluated on every invocation of the function, so the following function always returns a fresh empty array if called with no arguments:

def default_empty arg = []
  arg

Defaults are evaluated left-to-right and may reference earlier parameters:

def window start end = (start + 10)
  start..end

assert_eq (window 5) (5..15)
assert_eq (window 5 20) (5..20)

Variadic Parameters

Use ... to accept extra arguments:

def log level ...rest
  echo "[$level]" ...rest

log INFO hello world
# prints: [INFO] hello world

The rest parameter receives a record of the extra arguments, in invocation order. Read a key argument by indexing, as in rest[:name:]. Iteration yields key/value tuples, where the key is the positional argument index (0-origin) for positional arguments.

def echo_all ...args
  for k v = args
    echo "$k: $v"
echo_all foo bar: 1 baz
# prints:
# 0: foo
# bar: 1
# 1: baz

Open in playground

* accepts only extra positional arguments, and ** only extra key arguments. *args receives a tuple and **kwargs a record. Both may appear, * first, but neither combines with .... An extra argument that no rest parameter accepts is an error:

def run cmd *args **opts
  echo $cmd $args $opts

run ls -l -a color: always
# prints: ls ("-l", "-a") (color: "always")

Open in playground

Without a name, * and ** accept the arguments without binding them.

Argument Spreading

Spread collections or iterators into a call:

let args = [1, 2, 3]
func ...args
# equivalent to: func 1 2 3

let kwargs = {name: "Alice", age: 30}
func ...kwargs
# equivalent to:
# func name: "Alice" age: 30

Vertical Parameter Layout

Parameters in def can use vertical layout:

pub def build
  :from
  :pull = true
  :tag
  ...args
do
  echo "Building $tag from $from"

do Blocks

Anonymous functions (blocks and lambdas) are created with do:

Statement Context

In statement context, do without a following newline creates a one-statement block:

let greet = do echo "hello"
greet()  # prints: hello

With parameters:

let double = do |x| echo (x * 2)
double 5  # prints: 10

If an immediate newline and indented block follows, it creates a multi-statement block:

let process = do |x|
  let doubled = (x * 2)
  echo "Result: $doubled"
  doubled

Expression Context

In expression context, do creates a lambda where the body is an expression:

assert_eq ((do |x| x * 2) 5) 10

Non-Local Flow Control

break, continue, and return work through do blocks:

• break exits the innermost enclosing loop
• continue skips to the next iteration of the innermost enclosing loop
• return exits the innermost enclosing def

This allows natural flow control in callbacks and higher-order functions:

def validate_record record
  for field = ["id", "name", "email"]
    record.get field else: do
      return {valid: false, missing: field}
  {valid: true}

A non-local branch will only be effective for the duration of the statement that introduces its containing closure; after this it will propagate a runtime error. The compiler requires that a closure containing non-local branches be in argument position to reduce the likelihood of this sort of error:

# Valid: closure is passed as an argument
def validate record
  record.get "name" else: do
    return false
  true

# Invalid: closure is bound to a variable first
def bad_example()
  # Compiler will reject this line
  let closure = do return false
  record.get "name" else: $closure

Public Functions

Use pub def to export a function from a module:

pub def helper x
  (x + 1)

See Modules for details on the module system.

Decorators

Function defs may be preceded by decorators:

#[memoize]
def fib n
  if (n < 2)
    n
  else
    (fib(n - 1) + fib(n - 2))

See Decorators for syntax, ordering, and examples.