Expanded enums and unions

meetings/working-groups/discriminated-unions/enums-and-unions.md

@@ -0,0 +1,131 @@
+# Enums and unions
+
+## Abstract
+
+This proposal separates two high-level concepts into two separate syntaxes: `enum` and `union`. Unions are a type declaration that represent the union of existing types. Enums are a compositional type syntax. They allow declaring a sequence of nested types, each of which are unioned into the parent type. This is also known as an "algebraic data type". Union declarations take a list of existing types as input, while the enhanced enum syntax takes a list of type names, parameter lists, and bodies.
+
+## Example
+
+Union declaration:
+
+```C#
+union StringOrInt
+{
+  string,
+  int
+}
+```
+
+Enum declaration:
+
+```C#
+enum JsonValue
+{
+  Number(double value),
+  Bool(bool Value),
+  String(string Value),
+  Object(ImmutableDictionary<string, JsonValue> Members),
+  Array(ImmutableArray<JsonValue> Elements),
+  Null
+}
+```
+
+## Syntax
+
+Expanded enum:
+```bnf
+<type-declaration> ::= "enum" <type-name> "{" <variant-list> "}"
+<variant-list>     ::= <variant> { "," <variant> } [ "," ]
+<variant>          ::= <variant-name> [ "(" <field-list> ")" ]
+<field-list>       ::= <field> { "," <field> }
+<field>            ::= <type> <field-name>
+<type>             ::= <simple-type> | <generic-type>
+<simple-type>      ::= "double" | "bool" | "string" | <type-name>
+<generic-type>     ::= <type-name> "<" <type-arguments> ">"
+<type-arguments>   ::= <type> { "," <type> }
+<type-name>        ::= identifier
+<variant-name>     ::= identifier
+<field-name>       ::= identifier
+```
+
+Union:
+```bnf
+<union-declaration> ::= "union" <type-name> "{" <union-type-list> "}"
+<union-type-list>   ::= <type> { "," <type> } [ "," ]
+<type>              ::= <simple-type> | <generic-type>
+<simple-type>       ::= "double" | "bool" | "string" | "int" | <type-name>
+<generic-type>      ::= <type-name> "<" <type-arguments> ">"
+<type-arguments>    ::= <type> { "," <type> }
+<type-name>         ::= identifier
+```
+
+## Semantics
+
+An enum has the same semantics as declaring all the cases as nested records, then making a union e.g.,
+
+```C#
+union JsonValue
+{
+  Number,
+  Bool,
+  String,
+  Object,
+  Array,
+  Null;
+
+  record Number(double value);
+  record Bool(bool Value);
+  record String(string Value);
+  record Object(ImmutableDictionary<string, JsonValue> Members);
+  record Array(ImmutableArray<JsonValue> Elements);
+  record Null;
+}
+```
+
+## Expansion options
+
+The above grammar doesn't provide more than the minimal set of syntax. There are a few small additions that could be easily made.
+
+### Allow extra members after a semicolon
+
+In both enums and unions, after a semicolon additional members could be provided. Fields, auto-properties, and any other syntax that create additional fields would be prohibited.
+
+### Allow extra members in a partial declaration
+
+All enums and unions would have a so-called "primary" declaration that lists the union/enum members. Other partial declarations could be allowed, which would allow extra members. Again, no fields would be allowed.
+
+### Allow record bodies in enums
+
+Enums could be expanded to allow each declaration to have a body, e.g.
+
+```C#
+enum JsonValue
+{
+  Number(double Value)
+  {
+    public override string ToString() => Value.ToString();
+  },
+  Bool(bool Value)
+  {
+    public override string ToString() => Value.ToString();
+  },
+  String(string Value)
+  {
+    public override string ToString() => Value.ToString();
+  },
+  Object(ImmutableDictionary<string, JsonValue> Members)
+  {
+    public override string ToString() => Members.ToString();
+  },
+  Array(ImmutableArray<JsonValue> Elements)
+  {
+    public override string ToString() => Elements.ToString();
+  },
+  Null
+  {
+    public override string ToString() => "null";
+  }
+}
+```
+
+If partial declarations of enums are allowed, the type members could also themselves be partial.