Declare functions

// Declare our new function
function myFunction() {
  trace("Hello!");
}

// Call it
myFunction();

Declare function with arguments

// Declare our new function
function sayHelloTo(name:String) {
  trace('Hello ${name}');
}

// Call it
sayHelloTo("Mark");

See https://haxe.org/manual/types-function.html

Declare functions with default arguments

// Declare a function with one parameter which has a default value set
function sayHello(name:String = "Mark") {
  trace('Hello ${name}');
}

// Call it without any parameter and the 'default' one will be used
sayHello();

// Let's call it again with some parameter
sayHello("John");

See https://haxe.org/manual/types-function-default-values.html

Declare functions with optional arguments

// Declare a function with one optional parameter
function sayHello(?name:String) {
  if (name != null) {
    trace('Hello ${name}');
  } else {
    trace('No name');
  }
}

// Call it without any parameter and the 'default' one will be used
sayHello();

// Let's call it again with some parameter
sayHello("Sander");

See https://haxe.org/manual/types-function-optional-arguments.html

Declare a function with return type

// Declare our new function
function sum(a:Int, b:Int):Int {
  return a + b;
}

// Call it
var result = sum(2, 4);
trace(result);

See https://haxe.org/manual/types-function.html

Declare a function with parameterized arguments

// Declare our new function
function equals<T>(a:T, b:T):Bool {
  return a == b;
}

// Call it with integers
trace(equals(2, 2)); // true
trace(equals(2, 1)); // false

// Call it with strings
trace(equals("hello","hello")); // true
trace(equals("hello","world")); // false

See https://haxe.org/manual/type-system-type-parameters.html

Declare an inline function

// Declare our new function
inline function sum(a:Int, b:Int):Int {
  return a + b;
}

// Call it
var result = sum(2, 4);
trace(result);

In JavaScript this will be compiled (where possible) as:

var result = 6;
console.log(result);

See https://haxe.org/manual/class-field-inline.html