compiler, runtime: Make Default Values Optional

For types, make it optional to return a default value. While default
values for some types are meaningful, they are not meaningful for all
types (e.g., Action, Control, Parser, Parser state, etc.).

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

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
   }
 
 }

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 {

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
   }
 }