Helios language/ User-defined types/


A struct in Helios is a named grouping of types (sometimes called a product type). They are similar to structs in other languages (e.g. C, Go and Rust):

// example of a Rational (fractional type)
struct Rational {
    top:    Int
    bottom: Int

Note: a struct can't be empty and must have at least one field.

Instantiating a struct

A struct can be instantiated using the following literal syntax:

const x: Rational = Rational { 1, 3 }

The fields can also be named:

const x: Rational = Rational { bottom: 3, top: 1 }

CIP 68 tags

Regular Helios structs are internally implemented as data-lists.

This internal data-list format isn't convenient for datums that are intended for public reading/writing. For such applications it is recommended to use the CIP 68 data-map format.

Helios will automatically use the CIP 68 format internally if any struct field is tagged. The tags are internally converted into the data-map keys.

struct TaggedRational {
    top:    Int "@top"     // the tag can be any valid utf-8 string
    bottom: Int "@bottom"

Any missing tags default to the field name:

struct TaggedRational {
    top:    Int "@top"
    bottom: Int         // data-map key will be "bottom"

Field tagging isn't available for enum variants.