diff --git a/Sources/Common/DataTypes.swift b/Sources/Common/DataTypes.swift
index be94e7f..6b27f95 100644
--- a/Sources/Common/DataTypes.swift
+++ b/Sources/Common/DataTypes.swift
@@ -162,7 +162,7 @@ public struct P4Struct: P4Type {
     }
   }
 
-  public func def() -> any P4DataValue {
+  public func def() -> P4DataValue? {
     return P4StructValue(withType: self)
   }
 }
@@ -203,7 +203,7 @@ public class P4StructValue: P4DataValue {
       }
 
       // Now that we know that the field names match, do the values match?
-      if !op(left_field_value.dataValue(), right_field_value.dataValue()) {
+      if !op(left_field_value?.dataValue(), right_field_value?.dataValue()) {
         return false
       }
     }
@@ -297,19 +297,21 @@ public class P4StructValue: P4DataValue {
   }
 
   public let stype: P4Struct
-  public let values: [P4Value]
+  public let values: [P4Value?]
 
   public convenience init(withType type: P4Struct) {
     self.init(withType: type, andInitializers: [])
   }
 
   public init(withType type: P4Struct, andInitializers initializers: [P4Value?]) {
-    let values = zip(0..<type.fields.count(), type.fields.fields).map { (index, field) in
+    let values: [P4Value?] = zip(0..<type.fields.count(), type.fields.fields).map { (index, field) in
       // If there is an initializer for the field, then use it.
       if index < initializers.count, let initializer = initializers[index] {
         initializer
       } else {
-        // Otherwise, set a default!
+        // Otherwise, try to set a default!
+        // Note: If the field type does not have a default, then the value
+        // will be a none. Pretty cool!
         field.type.def()
       }
     }
@@ -361,7 +363,7 @@ public struct P4Boolean: P4Type {
     default: false
     }
   }
-  public func def() -> any P4DataValue {
+  public func def() -> P4DataValue? {
     return P4BooleanValue(withValue: false)
   }
 }
@@ -434,7 +436,7 @@ public struct P4Int: P4Type {
     default: false
     }
   }
-  public func def() -> any P4DataValue {
+  public func def() -> P4DataValue? {
     return P4IntValue(withValue: 0)
   }
 }
@@ -506,7 +508,7 @@ public struct P4String: P4Type {
     default: false
     }
   }
-  public func def() -> any P4DataValue {
+  public func def() -> P4DataValue? {
     return P4StringValue(withValue: "")
   }
 }
@@ -587,7 +589,7 @@ public struct P4Array: P4Type {
     }
   }
 
-  public func def() -> P4DataValue {
+  public func def() -> P4DataValue? {
     return P4ArrayValue(withType: self.vtype, withValue: [])
   }
 }
@@ -686,8 +688,11 @@ public struct P4Set: P4Type {
     }
   }
 
-  public func def() -> P4DataValue {
-    return P4SetValue(withValue: P4Value(self.stype.baseType().def(), self.stype))
+  public func def() -> P4DataValue? {
+    if let base_type_default = self.stype.baseType().def() {
+      return P4SetValue(withValue: P4Value(base_type_default, self.stype))
+    }
+    return .none
   }
 }
 
@@ -784,7 +789,7 @@ public struct P4HitMiss: P4Type {
     }
   }
 
-  public func def() -> any P4DataValue {
+  public func def() -> P4DataValue? {
     return P4TableHitMissValue.Miss
   }
 
diff --git a/Sources/Common/P4Types.swift b/Sources/Common/P4Types.swift
index ff91952..0d515ae 100644
--- a/Sources/Common/P4Types.swift
+++ b/Sources/Common/P4Types.swift
@@ -141,8 +141,11 @@ public struct P4QualifiedType: CustomStringConvertible {
     return self.base_type
   }
 
-  public func def() -> P4Value {
-    return P4Value(self.base_type.def(), self)
+  public func def() -> P4Value? {
+    if let default_value = self.base_type.def() {
+      return P4Value(default_value, self)
+    }
+    return .none
   }
 
   public func eq(_ rhs: P4QualifiedType) -> Bool {
diff --git a/Sources/Common/Protocols.swift b/Sources/Common/Protocols.swift
index c5b3f28..d28b43c 100644
--- a/Sources/Common/Protocols.swift
+++ b/Sources/Common/Protocols.swift
@@ -36,7 +36,7 @@ public protocol EvaluatableStatement {
 
 public protocol P4Type: CustomStringConvertible {
   func eq(rhs: any P4Type) -> Bool
-  func def() -> P4DataValue
+  func def() -> P4DataValue?
 }
 
 public protocol P4DataValue: CustomStringConvertible {
diff --git a/Sources/P4Compiler/Statement.swift b/Sources/P4Compiler/Statement.swift
index 87c42a1..7e1d386 100644
--- a/Sources/P4Compiler/Statement.swift
+++ b/Sources/P4Compiler/Statement.swift
@@ -194,17 +194,18 @@ extension VariableDeclarationStatement: CompilableStatement {
         Error(withMessage: "Could not parse a P4 type from \(typeref.text!)"))
     }
 
-    var initializer: EvaluatableExpression = declaration_p4_type.def()
+    var initializer: EvaluatableExpression? = .none
+
     // If there is an initializer, it must be an expression.
-    if let rvalue = maybe_rvalue {
-      guard rvalue.nodeType == "expression" else {
+    if let initializer_expression = maybe_rvalue {
+      guard initializer_expression.nodeType == "expression" else {
         return Result.Error(
           ErrorOnNode(
             node: node,
             withError: "initial value for declaration statement is not an expression"))
       }
 
-      let maybe_parsed_rvalue = Expression.Compile(node: rvalue, withContext: context)
+      let maybe_parsed_rvalue = Expression.Compile(node: initializer_expression, withContext: context)
       guard
         case .Ok(let parsed_rvalue) = maybe_parsed_rvalue
       else {
@@ -217,10 +218,22 @@ extension VariableDeclarationStatement: CompilableStatement {
         return Result.Error(
           Error(
             withMessage:
-              "Cannot initialize \(parsed_variablename) (with type \(declaration_p4_type)) from rvalue with type \(parsed_rvalue.type())"
+              "Cannot initialize \(parsed_variablename) (with type \(declaration_p4_type)) from expression with type \(parsed_rvalue.type())"
           ))
       }
     }
+
+    // If there is no initializer, then it must be defaultable.
+
+    if initializer == nil {
+      initializer = declaration_p4_type.def()
+    }
+
+    guard let initializer = initializer else {
+        return Result.Error(
+          ErrorOnNode(node: node, withError: "No initializer for declaration"))
+    }
+
     return Result.Ok(
       (
         VariableDeclarationStatement(
diff --git a/Sources/P4Lang/Control.swift b/Sources/P4Lang/Control.swift
index 6796ae1..f47d125 100644
--- a/Sources/P4Lang/Control.swift
+++ b/Sources/P4Lang/Control.swift
@@ -64,8 +64,8 @@ public struct Action: CustomStringConvertible, P4Type, P4DataValue {
     }
   }
 
-  public func def() -> any Common.P4DataValue {
-    return Action()
+  public func def() -> P4DataValue? {
+    return .none
   }
 
   public var description: String {
@@ -284,15 +284,8 @@ public struct Control: P4Type, P4DataValue, Equatable, CustomStringConvertible {
       withActions: self.actions, withApply: self.apply)
   }
 
-  public func def() -> any P4DataValue {
-    return Control(
-      named: Identifier(name: ""),
-      withParameters: ParameterList(),
-      withTable: Table(
-        withName: Identifier(name: "empty"),
-        withPropertyList: TablePropertyList(
-          withActions: TableActionsProperty(), withKeys: TableKeys())),
-      withActions: Actions(withActions: []), withApply: ApplyStatement())
+  public func def() -> P4DataValue? {
+    return .none
   }
 
 }
diff --git a/Sources/P4Lang/Declarations.swift b/Sources/P4Lang/Declarations.swift
index 3fd37e1..0e1c59d 100644
--- a/Sources/P4Lang/Declarations.swift
+++ b/Sources/P4Lang/Declarations.swift
@@ -43,7 +43,7 @@ public struct Declaration: P4Type {
     }
   }
 
-  public func def() -> any Common.P4DataValue {
+  public func def() -> P4DataValue? {
     /// TODO: Is a default of the extern'd type the right way to go?
     return self.identifier.type.baseType().def()
   }
@@ -107,11 +107,8 @@ public struct FunctionDeclaration: P4Type, P4DataValue {
     }
   }
 
-  public func def() -> any Common.P4DataValue {
-    return FunctionDeclaration(
-      named: Identifier(name: ""), ofType: P4QualifiedType(P4Boolean()),
-      withParameters: ParameterList([]),
-      withBody: .none)
+  public func def() -> P4DataValue? {
+    return .none
   }
 
   public var description: String {
diff --git a/Sources/P4Lang/Parser.swift b/Sources/P4Lang/Parser.swift
index b213a50..e56b353 100644
--- a/Sources/P4Lang/Parser.swift
+++ b/Sources/P4Lang/Parser.swift
@@ -101,8 +101,8 @@ public class ParserState: P4Type, P4DataValue, Equatable, CustomStringConvertibl
   /// Construct a ParserState
   public init() {}
 
-  public func def() -> any P4DataValue {
-    return ParserState()
+  public func def() -> P4DataValue? {
+    return .none
   }
 }
 
@@ -353,8 +353,8 @@ public struct Parser: P4Type, P4DataValue {
     return "Parser \(self.name) with parameters: \(parameters) and states: \(self.states)"
   }
 
-  public func def() -> any P4DataValue {
-    return Parser(withName: Identifier(name: ""))
+  public func def() -> P4DataValue? {
+    return .none
   }
 }
 
diff --git a/Tests/p4rseTests/ValueTypeParserTests.swift b/Tests/p4rseTests/ValueTypeParserTests.swift
index cd8dbae..fd99d36 100644
--- a/Tests/p4rseTests/ValueTypeParserTests.swift
+++ b/Tests/p4rseTests/ValueTypeParserTests.swift
@@ -103,7 +103,7 @@ import TreeSitterP4
     #RequireErrorResult(
       Error(
         withMessage:
-          "{86, 27}: Failed to parse a statement element: Cannot initialize where_to (with type Boolean) from rvalue with type String"
+          "{86, 27}: Failed to parse a statement element: Cannot initialize where_to (with type Boolean) from expression with type String"
       ),
       Program.Compile(simple_parser_declaration)))
 }
@@ -123,7 +123,7 @@ import TreeSitterP4
     #RequireErrorResult(
       Error(
         withMessage:
-          "{77, 29}: Failed to parse a statement element: Cannot initialize where_from (with type String) from rvalue with type Boolean"
+          "{77, 29}: Failed to parse a statement element: Cannot initialize where_from (with type String) from expression with type Boolean"
       ),
       Program.Compile(simple_parser_declaration)))
 }
@@ -258,7 +258,7 @@ import TreeSitterP4
     #RequireErrorResult(
       Error(
         withMessage:
-          "{49, 22}: Failed to parse a statement element: Cannot initialize where_to (with type Boolean) from rvalue with type Int"
+          "{49, 22}: Failed to parse a statement element: Cannot initialize where_to (with type Boolean) from expression with type Int"
       ),
       Program.Compile(simple_parser_declaration, withGlobalInstances: test_types)))
 }