From 14c4550aff926f53f256e831a7d99bffffd21dfd Mon Sep 17 00:00:00 2001
From: Melody Horn <melody@boringcactus.com>
Date: Sat, 31 Oct 2020 22:38:29 -0600
Subject: settle on a tagged union plan (for now)

---
 language/scanning.rst        |  2 +-
 language/source-file.rst     |  2 +-
 language/type-definition.rst | 52 ++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 52 insertions(+), 4 deletions(-)

(limited to 'language')

diff --git a/language/scanning.rst b/language/scanning.rst
index ed85ead..c45e6b8 100644
--- a/language/scanning.rst
+++ b/language/scanning.rst
@@ -15,7 +15,7 @@ Scanning
         ``char``, ``const``, ``continue``, ``default``, ``do``, ``double``,
         ``else``, ``enum``, ``extern``, ``float``, ``for``, ``fragile``,
         ``function``, ``if``, :crowbar:ref:`include <IncludeStatement>`, ``int``, ``long``, ``return``,
-        ``short``, ``signed``, ``sizeof``, ``struct``, ``switch``,
+        ``short``, ``signed``, ``sizeof``, ``struct``, ``switch``, ``union``,
         ``unsigned``, ``void``, or ``while``.
     
     identifier
diff --git a/language/source-file.rst b/language/source-file.rst
index 6522ea8..2fc3ba0 100644
--- a/language/source-file.rst
+++ b/language/source-file.rst
@@ -2,7 +2,7 @@ Source Files
 ------------
 
 .. crowbar:element:: HeaderFile <- IncludeStatement* HeaderFileElement+
-.. crowbar:element:: HeaderFileElement <- TypeDefinition / FunctionDeclaration / ConstantDefinition / UninitializedVariableDeclaration
+.. crowbar:element:: HeaderFileElement <- TypeDefinition / FunctionDeclaration / ConstantDefinition / VariableDeclaration
 
     A Crowbar header file defines an API boundary, either at the surface of a library or between pieces of a library or application.
     :crowbar:ref:`IncludeStatement`\ s can only appear at the beginning of the header file, and header files cannot define behavior directly.
diff --git a/language/type-definition.rst b/language/type-definition.rst
index 02616b8..88c4a04 100644
--- a/language/type-definition.rst
+++ b/language/type-definition.rst
@@ -5,7 +5,7 @@ Defining Types
 
     Crowbar has three different kinds of user-defined types.
 
-.. crowbar:element:: StructDefinition <- 'struct' identifier '{' VariableDeclaration+ '}' ';'
+.. crowbar:element:: StructDefinition <- 'struct' identifier '{' VariableDeclaration+ '}'
 
     A ``struct`` defines a composite type with several members.
 
@@ -13,7 +13,7 @@ Defining Types
 
         define struct layout in memory
 
-.. crowbar:element:: EnumDefinition <- 'enum' identifier '{' EnumMember (',' EnumMember)* ','? '}' ';'
+.. crowbar:element:: EnumDefinition <- 'enum' identifier '{' EnumMember (',' EnumMember)* ','? '}'
                      EnumMember <- identifier ('=' Expression)?
 
     An ``enum`` defines a type which can take one of several specified values.
@@ -21,3 +21,51 @@ Defining Types
     .. todo::
         
         define enum value assignment, type-related behavior
+
+.. crowbar:element:: UnionDefinition <- RobustUnionDefinition / FragileUnionDefinition
+
+    Unions as implemented in C are not robust by default, and care must be taken to ensure that they are only used robustly.
+    However, for the purpose of interoperability with C, Crowbar unions may be defined as robust or as fragile.
+
+.. crowbar:element:: RobustUnionDefinition <- 'union' identifier '{' VariableDeclaration UnionBody '}'
+                     UnionBody <- 'switch' '(' identifier ')' '{' UnionBodySet+ '}'
+                     UnionBodySet <- CaseSpecifier+ (VariableDeclaration / ';')
+
+    A robust union, or simply union, in Crowbar is what is known more broadly as a tagged union.
+    The top-level variable declaration must have a type which is some ``enum``, and its name must be the same as the identifier in the ``switch``.
+
+    For example::
+
+        enum TokenType {
+            Identifier,
+            Constant,
+            Operator,
+            Whitespace,
+        }
+
+        union Token {
+            enum TokenType type;
+
+            switch (type) {
+                case Identifier: (const char) * name;
+                case Constant: int value;
+                case Operator: char op;
+                case Whitespace: ;
+            }
+        }
+    
+    defines a ``union Token`` type, where the ``type`` field controls which of the other fields in the union is valid.
+
+    .. todo::
+
+        go into more depth about how tagged unions work
+
+.. crowbar:element:: FragileUnionDefinition <- 'fragile' 'union' identifier '{' VariableDeclaration+ '}'
+
+    A fragile union, like a struct, is a simple bag of fields.
+    However, unlike a struct, only the most recently assigned field is valid at any given time.
+    As such, in non-trivial cases no compiler can predict which field is or is not valid, and any statement which reads a field of a fragile union must itself be a :crowbar:ref:`FragileStatement`.
+
+    .. todo::
+
+        explain memory layout of fragile unions
-- 
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