Unions

Tagged unions are the main way to achieve polymorphism in niva

Unions are types that can be represented by one of several options at a time.

Declaration of union is like declaration of type + switch

union Shape =
    | Rectangle width: Int height: Int
    | Circle    radius: Int

Pattern match

To work with a union, you first need to understand what form it is in.
Switch syntax is used for this.

constructor Float pi = 3.14

Shape getArea -> Float = 
    | this // "this" is Shape here
    | Rectangle => width * height |> toFloat
    | Circle => Float pi * radius * radius

Exhaustive

If you don't check all form options, this will be a compile time error.

Shape getArea -> Float = 
    | this // "this" is Shape here
    | Rectangle => width * height |> toFloat
//  | Circle => Float pi * radius * radius

Instantiate

It is impossible to instantiate the root of the union, since then it is not clear what form it will correspond to.
But you can create branches, each root branch is no different from a regular type declaration.

x = Rectangle width: 2 height: 3
x getArea echo

// TODO as note
Note that you don't need the root qualifier like Shape.Rectangle. This is debatable, but I like the branches to be no different from the declaration of ordinary top-level types

Single line syntax

union Color = Red | Blue
union Shape = Circle r: Int | Rectangle w: Int h: Int

Fields for Roots

Unlike other ML languages, union roots can contain fields

union Person name: String = 
| Student
| Teacher

Both Student and Teacher have a name field here.
This simplifies 2 things at once:

There is no need to repeat common fields for each union's branch.
You don't need to pattern match the Person to gain access to the name field

Printer printPerson: p::Person = 
    p name echo

Include one union into another

// TODO tree picture

// TODO replace with car models

union Sas = Sus | Ses
union Sus =
    | Sos 
    | ^Sas // include Sas as Sus branch

So now u can pattern match it like

| sas
| Sos => ...
| Sus => ...
| Ses => ...

Or on more general Sas

| sas
| Sos => ...
| Sas => ...

You will get a compilation error if you inadvertently check out more general branches before more specific ones

| sas
| Sos => ...
| Sas => ... // Sas is more general than Sus
| Sus => ... // So Sus is unreachable here

// TODO error text

Last modified: 12 June 2024