Refactor Parsing/Runtime

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

Sources/P4Runtime/Program.swift

@@ -0,0 +1,47 @@
+// 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 P4Lang
+import Common
+
+extension VariableDeclarationStatement: EvaluatableStatement {
+  public func evaluate(execution: ProgramExecution) -> ProgramExecution {
+    guard case .Ok(let initial_value) = self.initializer.evaluate(execution: execution) else {
+      return execution.setError(error: Error(withMessage: "Could not evaluate \(self.initializer)"))
+    }
+    let new_scopes = execution.scopes.declare(identifier: self.identifier, withValue: initial_value)
+    execution.scopes = new_scopes
+    return execution
+  }
+}
+
+extension ExpressionStatement: EvaluatableStatement {
+  public func evaluate(execution: ProgramExecution) -> ProgramExecution {
+    return execution
+  }
+}
+
+// Variables are evaluatable because they can be looked up by identifiers.
+extension TypedIdentifier: EvaluatableExpression {
+    public func type() -> any Common.P4Type {
+      return self.parsed_type
+    }
+
+    public func evaluate(execution: Common.ProgramExecution) -> Result<P4Value> {
+      return execution.scopes.lookup(identifier: self)
+    }
+}