Unions

Unions

An union can be used to hold a value with different types. Unions can be used to avoid casting.

The syntactic structure of an union:

<memory_offset> ::= <int_number>
<union_value> ::= "let" <name> "=" <memory_offset> ";"
<union_declaration> ::= <modifier>? "union" <name> "{" <union_value>* "}"

A color union is a good example:

public union Color {
    let mut R: u8 = 0;
    let mut G: u8 = 1;
    let mut B: u8 = 2;
    let mut A: u8 = 3;

    let value: u32 = 0;
}

The value field can be used to convert all color values (RGBA) to a u32 integer without casting. An union allocates memory for the most sized variable type. In this example the whole union will allocate 4 bytes because the biggest type used in the union is u32.

Discriminated Unions

A discrimated union (DU) is a typed union. It can be used to create a hierarchical type structure easily. And Du's are generating a default constructor and a ToString method. You will see that discrimated unions are saving a lot of lines of code.

The structure of DU's:

<du_prop> ::= "mutable"? <name> ":" <type>
<du_prop_list> ::= <du_prop> "," <du_prop_list> | <du_prop>
<du_type_def> ::= "|" <name> "(" <du_prop_list> ")"
<du_definition> ::= "type" <name> "=" <du_type_def>* ";"

For example we will create a simple DU for an abstract syntax tree:

public type Expression = | Literal(Value : obj)
                  | Group(Inner : Expression)
                  | Binary(Lhs : Expression, Operator : string, Rhs : Expression);

The normal equivalent class structure would be:

public abstract class Expression {

}

public class Literal {
    let Value : obj;
}

implement Literal {
    public constructor(value : obj) {
       Value = value;
    }
}

public class Group {
    let Inner: Expression;
}

implement Group {
    public constructor(inner : Expression) {
       Inner = inner;
    }
}

public class Binary {
    let Lhs: Expression;
    let Rhs: Expression;
}

implement Binary {
    public constructor(lhs : Expression) {
       Lhs = lhs;
       Rhs = rhs;
    }
}

Exercices

1. What is the difference between a union and a discriminated union?

2. What is the profit of using unions?

3. What could be an usage screnario of using a discriminated union?