Syntax of Hetu Script Language

Hetu's grammar is close to most modern languages, with a few key characteristics:

Declaration keyword first

Declarations starts with a keyword before the identifier: var, final, const, fun, construct, get, set, class, type, etc.


Semicolon is optional. In most cases, the interpreter will know when a statement is finished. In rare cases, the lexer will implicitly add "end of statement token" (a semicolon in default lexicon) to avoid ambiguities. For example, before a line when the line starts with one of '++, --, (, [, {', or after a line when the line ends with 'return'.

Type annotation writes after identifiers

Type annotation is optional. Type is annotated with a colon after the identifier like typescript/kotlin/swift.

Some keyword are different

Use when instead of switch, with more complex usage to match the condition value. This is borrowed from kotlin.


Normal comments:

// This is a comment.
/* These are multi-line comments:
another line.*/

Documentation comments:

/// This is a documentation comment
fun myFunc() -> int {
  // some code

Private members

You can specify a private member of a class/namespace/source by making its name starts with '_'.

class Person {
  var _name
  construct (name) {
    _name = name
  fun greeting {
    print('Hi, I\'m ', _name)
final p = Person('jimmy')
// print(p._name) // Error!

Expression, statement

In hetu, variable declaration and variable assignment are both expression, which have the value of the assignment expression. It's possible to write code like:

if (a = fetch()) {
  // ...

The if statement will run if the value of a if truthy.

Block of code

For blocks of code (function body), Hetu will implicitly return the last expression.

The function below:

fun test(n) {
  var x = n * 2

will return the value of x.