Container types

Helios has 3 container types:

  • List (linked list)
  • Map (association list of key-value pairs)
  • Option (equivalent to Maybe in Haskell)


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 item type 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; ...

More information about Option can be found here.