Helios language/

Container types

Helios has 3 container types:

  • List (linked list)
  • Map (association list of key-value pairs)
  • Option (equivalent to Maybe in Haskell)
  • Tuple (fixed list with heterogenous item types)


Helios has a builtin linked list type, similar to Haskell's List. The syntax for a list type is []ItemType where ItemType is a parameter type that represents the type of the contained items. The ItemType can be any type except a function type.

List literals have a syntax similar to Go:

my_ints = []Int{1, 2, 3, 4, 5};

x: Int = some_ints.get(2); ...   // x == 3

Note: lists aren't indexed with [...]. Instead the get method can be used. Indices are 0-based.

More information about lists can be found here.


A Map in Helios is internally represented as a list of key-value pairs. Both key and value can have any type except a function type. Uniqueness of keys isn't guaranteed.

A Map has a type syntax and a literal syntax similar to Go:

// either side of the colon can be an arbitrary expression 
//  that evaluates to the correct type
my_map = Map[String]Int{
    "zero": 0,
    "one":  1,
    "two":  2
}; ... 

print(my_map.get("zero").show()); ... // prints '0'

More information about maps can be found here.


The Option type is a builtin enum with type syntax Option[SomeType]. It is internally defined as:

enum Option[SomeType] {
    Some { some: SomeType }

An Option is instantiated like any other enum:

some_int = Option[Int]::Some{42};

none_int = Option[Int]::None; ...

If you expect Some, you can assign, and even destructure, using the correct type annotation. Helios will automatically turn the assignment into a runtime type assertion (any enum can take advantage of this):

Option[Int]::Some{my_int} = option; ...

More information about Option can be found here.


A tuple is a collection of two or more items which can have different types.

my_tuple = (1, "hello", true)

Tuples are convenient when returning multiple values from a function:

func my_pair(a: Int) -> (Int, Int) {
    (a+1, a+2)

Tuples can contain anything, including functions. The contents of a tuple can be accessed through destructuring, or via getters:

(my_number: Int, my_string: String, my_bool: Bool) = my_tuple;

my_number_alt: Int = my_tuple.first;
my_string_alt: String = my_tuple.second;
my_bool_alt: Bool = my_tuple.third

Note: tuples in Helios are limited to 5 entries. The getters are named first, second, third, fourth and fifth.

Note: although tuples can be used as fields in structs/enums this is not recommended as it can become unclear what the meaning is of the tuples items, and there is also a performance penalty to doing so.

Nested literal constructors

If a literal List, Map, or Option contains other literal constructors, the types of those literal constructors can be omitted.

struct Pair {
  a: Int
  b: Int
list = []Pair{{0, 0}, {1, 1}, {2, 2}};
map = Map[Pair]Pair{{0, 0}: {0, 0}, {1, 1}: {1, 1}, {2, 2}: {2, 2}};
nested_list = [][]Pair{{{0, 0}, {1, 1}, {2, 2}}, {{0, 0}, {1, 1}, {2, 2}}};
option = Option[Pair]::Some{{0, 0}}