common, codegen: Implement Visitor And Use For CodeGen

Implement a generic visitor for components of a P4 program and use
it to start P4 code generation (according to the behavioral model).

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

Sources/P4Lang/Common.swift

@@ -0,0 +1,95 @@
+// p4rse, Copyright 2026, Will Hawkins
+//
+// This file is part of p4rse.
+//
+// This file is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+import Common
+
+public protocol Visitable {
+
+}
+
+public protocol LanguageVisitor<Context> {
+  associatedtype Context
+
+  // Program
+  func visit(_ program: Program, _ c: VisitorContext<Context>) -> Result<VisitorContext<Context>>
+
+  // Parser
+  func visit(_ parser: Parser, _ c: VisitorContext<Context>) -> Result<VisitorContext<Context>>
+  func visit(
+    _ parser_state: InstantiatedParserState, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+
+  // Statements
+  func visit(
+    _ variable_declaration: VariableDeclarationStatement, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ conditional: ConditionalStatement, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ block: BlockStatement, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ rtn: ReturnStatement, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ apply: ApplyStatement, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+
+  // Expressions
+  func visit(
+    _ keyset: KeysetExpression, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ select_case: SelectCaseExpression, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ select: SelectExpression, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ array_access: ArrayAccessExpression, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ field_access: FieldAccessExpression, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ function_call: FunctionCall, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ binary_operator: BinaryOperatorExpression, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+
+  // Declarations
+  func visit(_ decl: Declaration, _ c: VisitorContext<Context>) -> Result<VisitorContext<Context>>
+  func visit(
+    _ extern_decl: ExternDeclaration, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ func_decl: FunctionDeclaration, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+
+  // Control
+  func visit(_ action: Action, _ c: VisitorContext<Context>) -> Result<VisitorContext<Context>>
+  func visit(
+    _ table_key_entry: TableKeyEntry, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(
+    _ table_property_list: TablePropertyList, _ c: VisitorContext<Context>
+  ) -> Result<VisitorContext<Context>>
+  func visit(_ table: Table, _ c: VisitorContext<Context>) -> Result<VisitorContext<Context>>
+  func visit(_ control: Control, _ c: VisitorContext<Context>) -> Result<VisitorContext<Context>>
+}