diff --git a/Sources/Common/Execution.swift b/Sources/Common/Execution.swift
index 0ff24a6..a7f27cc 100644
--- a/Sources/Common/Execution.swift
+++ b/Sources/Common/Execution.swift
@@ -73,6 +73,13 @@ open class ProgramExecution: CustomStringConvertible {
     return new_pe
   }
 
+  public func declare(identifier: Identifier, withValue value: P4Value) -> ProgramExecution {
+    let new_pe = self
+    let new_scopes = self.scopes.declare(identifier: identifier, withValue: value)
+
+    new_pe.scopes = new_scopes
+    return new_pe
+  }
 }
 
 public typealias ValueScope = Scope<P4Value>
diff --git a/Sources/Common/ProgramTypes.swift b/Sources/Common/ProgramTypes.swift
index f8e41e9..0548738 100644
--- a/Sources/Common/ProgramTypes.swift
+++ b/Sources/Common/ProgramTypes.swift
@@ -72,29 +72,6 @@ public class Variable: TypedIdentifier {
   }
 }
 
-/// A base for all instances of P4 types
-open class P4ValueBase<T: P4Type>: P4Value {
-  public init() {}
-
-  public func type() -> P4Type {
-    return T.create()
-  }
-
-  public func eq(rhs: P4Value) -> Bool {
-    return false
-  }
-
-  public var description: String {
-    "Value of \(self.type()) type"
-  }
-
-}
-
-extension P4ValueBase: EvaluatableExpression {
-  public func evaluate(execution: ProgramExecution) -> Result<P4Value> {
-    return .Ok(self)
-  }
-}
 
 /// The type for a P4 struct
 public struct P4Struct: P4Type {
@@ -136,7 +113,19 @@ public struct P4StructField {
 }
 
 /// An instance of a P4 struct
-public class P4StructValue: P4ValueBase<P4Struct> {
+public class P4StructValue: P4Value {
+  public func type() -> any P4Type {
+    return P4Struct()
+  }
+
+  public func eq(rhs: any P4Value) -> Bool {
+    return true
+  }
+
+  public var description: String {
+    return "Struct"
+  }
+
   public let fields: [P4StructField]
   public init(withFields fields: [P4StructField]) {
     self.fields = fields
@@ -161,20 +150,24 @@ public struct P4Boolean: P4Type {
 }
 
 /// An instance of a P4 boolean
-public class P4BooleanValue: P4ValueBase<P4Boolean> {
+public class P4BooleanValue: P4Value {
+  public func type() -> any P4Type {
+    return P4Boolean()
+  }
+
   let value: Bool
 
   public init(withValue value: Bool) {
     self.value = value
   }
-  public override func eq(rhs: P4Value) -> Bool {
+  public func eq(rhs: P4Value) -> Bool {
     guard let bool_rhs = rhs as? P4BooleanValue else {
       return false
     }
     return self.value == bool_rhs.value
   }
 
-  public override var description: String {
+  public var description: String {
     "\(self.value ? "true" : "false") of \(self.type()) type"
   }
 }
@@ -196,18 +189,22 @@ public struct P4Int: P4Type {
 }
 
 /// An instance of a P4 integer
-public class P4IntValue: P4ValueBase<P4Int> {
+public class P4IntValue: P4Value {
+  public func type() -> any P4Type {
+    return P4Int()
+  }
+
   let value: Int
   public init(withValue value: Int) {
     self.value = value
   }
-  public override func eq(rhs: P4Value) -> Bool {
+  public func eq(rhs: P4Value) -> Bool {
     guard let int_rhs = rhs as? P4IntValue else {
       return false
     }
     return self.value == int_rhs.value
   }
-  public override var description: String {
+  public var description: String {
     "\(self.value) of \(self.type()) type"
   }
 }
@@ -229,23 +226,28 @@ public struct P4String: P4Type {
   }
 }
 /// An instance of a P4 string
-public class P4StringValue: P4ValueBase<P4String> {
+public class P4StringValue: P4Value {
+  public func type() -> any P4Type {
+    return P4String()
+  }
+
   let value: String
   public init(withValue value: String) {
     self.value = value
   }
-  public override func eq(rhs: P4Value) -> Bool {
+  public func eq(rhs: P4Value) -> Bool {
     guard let string_rhs = rhs as? P4StringValue else {
       return false
     }
     return self.value == string_rhs.value
   }
 
-  public override var description: String {
+  public var description: String {
     "\(self.value) of \(self.type()) type"
   }
 }
 
+
 public class Packet {
   public init() {}
 }
diff --git a/Sources/Common/Protocols.swift b/Sources/Common/Protocols.swift
index a6edc9a..9d93d4d 100644
--- a/Sources/Common/Protocols.swift
+++ b/Sources/Common/Protocols.swift
@@ -33,7 +33,6 @@ public protocol EvaluatableStatement {
 }
 
 public protocol P4Type: CustomStringConvertible {
-  static func create() -> any P4Type
   func eq(rhs: any P4Type) -> Bool
 }
 
diff --git a/Sources/Common/Support.swift b/Sources/Common/Support.swift
index 6c70117..40a0d7d 100644
--- a/Sources/Common/Support.swift
+++ b/Sources/Common/Support.swift
@@ -141,10 +141,10 @@ extension Result: CustomStringConvertible {
   #externalMacro(module: "Macros", type: "UseOkResult")
 @freestanding(expression) public macro UseErrorResult<T>(_: Result<T>) -> Error =
   #externalMacro(module: "Macros", type: "UseErrorResult")
-@freestanding(codeItem) public macro RequireNodeType<N, T>(node: N, type: String, msg: String) =
+@freestanding(codeItem) public macro RequireNodeType<N, T>(node: N, type: String, nice_type_name: String) =
   #externalMacro(module: "Macros", type: "RequireNodeType")
 @freestanding(codeItem) public macro RequireNodesType<N, T>(
-  nodes: N, type: [String], msg: [String]
+  nodes: N, type: [String], nice_type_names: [String]
 ) =
   #externalMacro(module: "Macros", type: "RequireNodesType")
 @freestanding(codeItem) public macro SkipUnlessNodeType<N, T>(node: N, type: String) =
diff --git a/Sources/P4Compiler/Expression.swift b/Sources/P4Compiler/Expression.swift
index f03191e..9047e31 100644
--- a/Sources/P4Compiler/Expression.swift
+++ b/Sources/P4Compiler/Expression.swift
@@ -101,7 +101,7 @@ struct Expression {
 
     #RequireNodesType<Node, EvaluatableExpression>(
       nodes: node, type: ["expression", "keysetExpression"],
-      msg: ["expression", "keyset expression"])
+      nice_type_names: ["expression", "keyset expression"])
 
     // If the node is a keyset expression, then dig out the expression:
     let node =
@@ -152,3 +152,97 @@ struct Identifier {
     }
   }
 }
+
+extension SelectExpression: CompilableExpression {
+  static func compile(
+    node: Node, inTree tree: MutableTree, withScopes scopes: LexicalScopes
+  ) -> Result<EvaluatableExpression?> {
+    #RequireNodeType<Node, (SelectExpression, LexicalScopes)>(
+      node: node, type: "selectExpression", nice_type_name: "parser select expression")
+
+    guard let selector_node = node.child(at: 2),
+      selector_node.nodeType == "expression"
+    else {
+      return .Error(ErrorOnNode(node: node, withError: "Could not find selector expression"))
+    }
+
+    guard let select_body_node = node.child(at: 5),
+      select_body_node.nodeType == "selectBody"
+    else {
+      return .Error(ErrorOnNode(node: node, withError: "Could not find select expression body"))
+    }
+
+    let maybe_selector = Expression.Compile(node: selector_node, inTree: tree, withScopes: scopes)
+    guard case .Ok(let selector) = maybe_selector else {
+      return .Error(
+        Error(
+          withMessage:
+            "Could not parse transition select expression selector expression: \(maybe_selector.error()!)"
+        ))
+    }
+
+    var kses: [KeysetExpression] = Array()
+    var kses_errors: [Error] = Array()
+
+    select_body_node.enumerateNamedChildren() { current_node in
+      let maybe_parsed_kse = KeysetExpression.compile(node: current_node, inTree: tree, withScopes: scopes)
+      if case .Ok(let parsed_kse) = maybe_parsed_kse {
+        kses.append(parsed_kse as! KeysetExpression)
+      } else {
+        kses_errors.append(Error(withMessage: "\(maybe_parsed_kse.error()!)"))
+      }
+    }
+
+    if !kses_errors.isEmpty {
+      return .Error(
+        Error(
+          withMessage: "Error(s) parsing select cases: "
+            + (kses_errors.map { error in
+              return "\(error.msg)"
+            }.joined(separator: ";\n"))))
+    }
+    return .Ok(
+        SelectExpression(withSelector: selector, withKeysetExpressions: kses),
+      )
+  }
+}
+
+extension KeysetExpression: CompilableExpression {
+  static func compile(
+    node: Node, inTree tree: MutableTree, withScopes scopes: LexicalScopes
+  ) -> Result<EvaluatableExpression?> {
+      if node.nodeType != "selectCase" {
+        return Result.Error(Error(withMessage: "Expected select case not found"))
+      }
+
+      guard let keysetexpression_node = node.child(at: 0),
+        keysetexpression_node.nodeType == "keysetExpression"
+      else {
+        return Result.Error(Error(withMessage: "Missing keyset expression in select case"))
+      }
+
+      guard let targetstate_node = node.child(at: 2),
+        targetstate_node.nodeType == "identifier"
+      else {
+        return Result.Error(Error(withMessage: "Missing target state in select case"))
+      }
+
+      let maybe_parsed_keysetexpression = Expression.Compile(
+        node: keysetexpression_node, inTree: tree, withScopes: scopes)
+      guard case Result.Ok(let keysetexpression) = maybe_parsed_keysetexpression else {
+        return Result.Error(maybe_parsed_keysetexpression.error()!)
+      }
+
+      let maybe_parsed_targetstate = Identifier.Compile(
+        node: targetstate_node, inTree: tree, withScopes: scopes)
+      guard case .Ok(let targetstate) = maybe_parsed_targetstate else {
+        return Result.Error(maybe_parsed_targetstate.error()!)
+      }
+
+      return .Ok(
+          KeysetExpression(
+            withKey: keysetexpression, withNextState: targetstate) 
+        )
+    }
+  }
+
diff --git a/Sources/P4Compiler/Parser.swift b/Sources/P4Compiler/Parser.swift
index 34d5e7e..c1177ff 100644
--- a/Sources/P4Compiler/Parser.swift
+++ b/Sources/P4Compiler/Parser.swift
@@ -28,7 +28,7 @@ extension ParserAssignmentStatement: CompilableStatement {
   ) -> Result<(EvaluatableStatement, LexicalScopes)> {
 
     #RequireNodeType<Node, (EvaluatableStatement, LexicalScopes)>(
-      node: node, type: "assignmentStatement", msg: "assignment statement")
+      node: node, type: "assignmentStatement", nice_type_name: "assignment statement")
 
     guard let lvalue_node = node.child(at: 0),
       lvalue_node.nodeType == "expression"
@@ -142,137 +142,26 @@ public struct Parser {
     }
   }
 
-  public struct TransitionSelectExpressionCaseStatement {
-    static func Compile(
-      node: Node, inTree tree: MutableTree, withLexicalScopes scopes: LexicalScopes
-    ) -> Result<(KeysetExpression, LexicalScopes)> {
-      if node.nodeType != "selectCase" {
-        return Result.Error(Error(withMessage: "Expected select case not found"))
-      }
-
-      guard let keysetexpression_node = node.child(at: 0),
-        keysetexpression_node.nodeType == "keysetExpression"
-      else {
-        return Result.Error(Error(withMessage: "Missing keyset expression in select case"))
-      }
-
-      guard let targetstate_node = node.child(at: 2),
-        targetstate_node.nodeType == "identifier"
-      else {
-        return Result.Error(Error(withMessage: "Missing target state in select case"))
-      }
-
-      let maybe_parsed_keysetexpression = Expression.Compile(
-        node: keysetexpression_node, inTree: tree, withScopes: scopes)
-      guard case Result.Ok(let keysetexpression) = maybe_parsed_keysetexpression else {
-        return Result.Error(maybe_parsed_keysetexpression.error()!)
-      }
-
-      let maybe_parsed_targetstate = Identifier.Compile(
-        node: targetstate_node, inTree: tree, withScopes: scopes)
-      guard case .Ok(let targetstate) = maybe_parsed_targetstate else {
-        return Result.Error(maybe_parsed_targetstate.error()!)
-      }
-
-      return .Ok(
-        (
-          KeysetExpression(
-            withKey: keysetexpression, withNextState: targetstate), scopes
-        ))
-    }
-  }
-
-  public struct TransitionSelectExpression {
-    static func Compile(
-      node: Node, inTree tree: MutableTree, withScope scopes: LexicalScopes
-    ) -> Result<(ParserTransitionSelectExpression, LexicalScopes)> {
-      guard
-        let transition_selection_expression_query = try? SwiftTreeSitter.Query(
-          language: p4lang,
-          data: String(
-            "(parserTransitionStatement (transition) (transitionSelectionExpression (selectExpression (select) (expression) @selector (selectBody) @body)))"
-          ).data(using: String.Encoding.utf8)!)
-      else {
-        return Result.Error(Error(withMessage: "Could not compile the tree sitter query"))
-      }
-
-      let qr = transition_selection_expression_query.execute(node: node, in: tree)
-
-      guard let query_result = qr.next() else {
-        return .Error(Error(withMessage: "Could not find transition select expression"))
-      }
-
-      let selector = query_result.captures(named: "selector")
-      let body = query_result.captures(named: "body")
-
-      if selector.isEmpty {
-        return .Error(
-          Error(withMessage: "Could not find transition select expression selector expression"))
-      }
-      let selector_node = selector[0].node
-      let maybe_selector = Expression.Compile(node: selector_node, inTree: tree, withScopes: scopes)
-      guard case .Ok(let selector) = maybe_selector else {
-        return .Error(
-          Error(
-            withMessage:
-              "Could not parse transition select expression selector expression: \(maybe_selector.error()!)"
-          ))
-      }
-
-      if body.isEmpty {
-        return .Error(Error(withMessage: "Could not find transition select expression body"))
-      }
-      let body_node = body[0].node
-      var kses: [KeysetExpression] = Array()
-      var kses_errors: [Error] = Array()
-
-      body_node.enumerateNamedChildren { current_node in
-        let maybe_parsed_kse = TransitionSelectExpressionCaseStatement.Compile(
-          node: current_node, inTree: tree, withLexicalScopes: scopes)
-        if case .Ok((let parsed_kse, _)) = maybe_parsed_kse {
-          kses.append(parsed_kse)
-        } else {
-          kses_errors.append(Error(withMessage: "\(maybe_parsed_kse.error()!)"))
-        }
-      }
-
-      if !kses_errors.isEmpty {
-        return .Error(
-          Error(
-            withMessage: "Error(s) parsing select cases: "
-              + (kses_errors.map { error in
-                return "\(error.msg)"
-              }.joined(separator: ";\n"))))
-      }
-
-      return .Ok(
-        (
-          ParserTransitionSelectExpression(withSelector: selector, withKeysetExpressions: kses),
-          scopes
-        ))
-    }
-  }
-
   public struct TransitionStatement {
     static func Compile(
       node: Node, inTree tree: MutableTree, withScope scopes: LexicalScopes
     ) -> Result<(ParserTransitionStatement, LexicalScopes)> {
-      guard
-        let next_state_query = try? SwiftTreeSitter.Query(
-          language: p4lang,
-          data: String(
-            "(parserTransitionStatement (transition) (transitionSelectionExpression (identifier) @next-state))"
-          ).data(using: String.Encoding.utf8)!)
-      else {
-        return Result.Error(Error(withMessage: "Could not compile the tree sitter query"))
+
+      #RequireNodeType<Node, (EvaluatableStatement, LexicalScopes)>(node: node, type: "parserTransitionStatement", nice_type_name: "parser transition statement")
+
+      guard let tse_node = node.child(at: 1),
+      tse_node.nodeType! == "transitionSelectionExpression" else {
+        return .Error(ErrorOnNode(node: node, withError: "Could not find transition select expression"))
       }
 
-      let qr = next_state_query.execute(node: node, in: tree)
+      guard let next_node = tse_node.child(at: 0) else {
+        return .Error(ErrorOnNode(node: node, withError: "Could not find the next token in a transition selection expression"))
+      }
 
-      if let next_state_result = qr.next() {
-        let transition_capture = next_state_result.captures(named: "next-state")
+      // If the next node is an identifier, we have the simple form ...
+      if next_node.nodeType == "identifier" {
         let maybe_parsed_next_state = Identifier.Compile(
-          node: transition_capture[0].node, inTree: tree, withScopes: scopes)
+          node: next_node, inTree: tree, withScopes: scopes)
         if case .Ok(let next_state) = maybe_parsed_next_state {
           return .Ok(
             (ParserTransitionStatement(withNextState: next_state), scopes))
@@ -285,11 +174,12 @@ public struct Parser {
         }
       }
 
+      // We know that the next node is a select expression.
       return
-        switch TransitionSelectExpression.Compile(node: node, inTree: tree, withScope: scopes)
+        switch SelectExpression.compile(node: next_node, inTree: tree, withScopes: scopes)
       {
-      case .Ok((let tse, _)):
-        .Ok((ParserTransitionStatement(withTransitionExpression: tse), scopes))
+      case .Ok(let tse):
+        .Ok((ParserTransitionStatement(withTransitionExpression: tse! as! SelectExpression), scopes))
       case .Error(let e): .Error(e)
       }
     }
diff --git a/Sources/P4Compiler/Statement.swift b/Sources/P4Compiler/Statement.swift
index ffc2914..eae6c63 100644
--- a/Sources/P4Compiler/Statement.swift
+++ b/Sources/P4Compiler/Statement.swift
@@ -26,10 +26,7 @@ extension BlockStatement: CompilableStatement {
   public static func Compile(
     node: Node, inTree tree: MutableTree, withScopes scopes: LexicalScopes
   ) -> Result<(EvaluatableStatement, LexicalScopes)> {
-    if node.nodeType != "blockStatement" {
-      return Result.Error(
-        ErrorOnNode(node: node, withError: "Did not find expected block statement"))
-    }
+    #RequireNodeType<Node, (EvaluatableStatement, LexicalScopes)>(node: node, type: "blockStatement", nice_type_name: "block statement")
 
     var currentChildIdx = 0
     var currentChildIdxSafe = 1
@@ -94,12 +91,7 @@ extension ConditionalStatement: CompilableStatement {
     node: Node, inTree tree: MutableTree, withScopes scopes: LexicalScopes
   ) -> Result<(EvaluatableStatement, LexicalScopes)> {
 
-    guard let node_type = node.nodeType,
-      node_type == "conditionalStatement"
-    else {
-      return Result.Error(
-        ErrorOnNode(node: node, withError: "Did not find expected conditional statement"))
-    }
+    #RequireNodeType<Node, (EvaluatableStatement, LexicalScopes)>(node: node, type: "conditionalStatement", nice_type_name: "conditional statement")
 
     let maybe_condition_expression = node.child(at: 2)
     guard let condition_expression = maybe_condition_expression,
@@ -166,12 +158,7 @@ extension VariableDeclarationStatement: CompilableStatement {
     node: Node, inTree tree: MutableTree, withScopes scopes: LexicalScopes
   ) -> Result<(EvaluatableStatement, LexicalScopes)> {
 
-    guard let node_type = node.nodeType,
-      node_type == "variableDeclaration"
-    else {
-      return Result.Error(
-        ErrorOnNode(node: node, withError: "Did not find expected variable declaration statement"))
-    }
+    #RequireNodeType<Node, (EvaluatableStatement, LexicalScopes)>(node: node, type: "variableDeclaration", nice_type_name: "variable declaration statement")
 
     let maybe_typeref = node.child(at: 0)
     guard let typeref = maybe_typeref,
@@ -248,10 +235,7 @@ extension ExpressionStatement: CompilableStatement {
   public static func Compile(
     node: Node, inTree tree: MutableTree, withScopes scopes: LexicalScopes
   ) -> Result<(EvaluatableStatement, LexicalScopes)> {
-    if node.nodeType != "expressionStatement" {
-      return Result.Error(
-        ErrorOnNode(node: node, withError: "Did not find expected expression statement"))
-    }
+    #RequireNodeType<Node, (EvaluatableStatement, LexicalScopes)>(node: node, type: "expressionStatement", nice_type_name: "expression statement")
 
     let _ = node.child(at: 0)
 
diff --git a/Sources/P4Lang/Parser.swift b/Sources/P4Lang/Parser.swift
index bab02df..f1f0b3d 100644
--- a/Sources/P4Lang/Parser.swift
+++ b/Sources/P4Lang/Parser.swift
@@ -56,7 +56,7 @@ public struct KeysetExpression {
 
 }
 
-public struct ParserTransitionSelectExpression {
+public struct SelectExpression {
   public let selector: EvaluatableExpression
   public let keyset_expressions: [KeysetExpression]
 
@@ -67,24 +67,24 @@ public struct ParserTransitionSelectExpression {
     self.keyset_expressions = kses
   }
 
-  public func append_checked_kse(kse: KeysetExpression) -> ParserTransitionSelectExpression {
+  public func append_checked_kse(kse: KeysetExpression) -> SelectExpression {
     var new_kse = self.keyset_expressions
     new_kse.append(kse)
-    return ParserTransitionSelectExpression(
+    return SelectExpression(
       withSelector: self.selector, withKeysetExpressions: new_kse)
   }
 }
 
 public struct ParserTransitionStatement {
   public let next_state: Identifier?
-  public let transition_expression: ParserTransitionSelectExpression?
+  public let transition_expression: SelectExpression?
 
   public init() {
     self.next_state = .none
     self.transition_expression = .none
   }
 
-  public init(withTransitionExpression transition_expression: ParserTransitionSelectExpression) {
+  public init(withTransitionExpression transition_expression: SelectExpression) {
     self.next_state = .none
     self.transition_expression = transition_expression
   }
@@ -163,6 +163,22 @@ public class ParserState: Equatable, CustomStringConvertible, Comparable {
   }
 }
 
+/// A P4 parser state type
+public struct P4ParserState: P4Type {
+  public static func create() -> any P4Type {
+    return P4ParserState()
+  }
+  public var description: String {
+    return "Parser State"
+  }
+  public func eq(rhs: any P4Type) -> Bool {
+    return switch rhs {
+    case is P4ParserState: true
+    default: false
+    }
+  }
+}
+
 nonisolated(unsafe) public let accept: ParserState = ParserState(name: Identifier(name: "accept"))
 nonisolated(unsafe) public let reject: ParserState = ParserState(name: Identifier(name: "reject"))
 
diff --git a/Sources/P4Runtime/Expressions.swift b/Sources/P4Runtime/Expressions.swift
new file mode 100644
index 0000000..060d5e6
--- /dev/null
+++ b/Sources/P4Runtime/Expressions.swift
@@ -0,0 +1,86 @@
+// 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
+import P4Lang
+
+extension KeysetExpression: EvaluatableExpression {
+  public func evaluate(execution: Common.ProgramExecution) -> Common.Result<any Common.P4Value> {
+    return execution.scopes.lookup(identifier: next_state_identifier)
+  }
+
+  public func type() -> any Common.P4Type {
+    return P4ParserState.create()
+  }
+}
+
+extension SelectExpression: EvaluatableExpression {
+  public func evaluate(execution: Common.ProgramExecution) -> Common.Result<any Common.P4Value> {
+    switch self.selector.evaluate(execution: execution) {
+    case .Ok(let selector_value):
+      for kse in self.keyset_expressions {
+        if case .Ok(let kse_key) = kse.key.evaluate(execution: execution),
+          kse_key.eq(rhs: selector_value)
+        {
+          let result = kse.evaluate(execution: execution)
+          return result
+        }
+      }
+      return .Error(Error(withMessage: "No key matched the selector"))
+    case .Error(let e): return .Error(e)
+    }
+  }
+
+  public func type() -> any Common.P4Type {
+    return P4ParserState.create()
+  }
+}
+
+extension P4StringValue: EvaluatableExpression {
+    public func evaluate(execution: Common.ProgramExecution) -> Common.Result<any Common.P4Value> {
+      return .Ok(self)
+    }
+}
+
+extension P4BooleanValue: EvaluatableExpression {
+    public func evaluate(execution: Common.ProgramExecution) -> Common.Result<any Common.P4Value> {
+      return .Ok(self)
+    }
+}
+
+extension P4StructValue: EvaluatableExpression {
+    public func evaluate(execution: Common.ProgramExecution) -> Common.Result<any Common.P4Value> {
+      return .Ok(self)
+    }
+}
+
+extension P4IntValue: EvaluatableExpression {
+    public func evaluate(execution: Common.ProgramExecution) -> Common.Result<any Common.P4Value> {
+      return .Ok(self)
+    }
+}
+
+// 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)
+  }
+}
\ No newline at end of file
diff --git a/Sources/P4Runtime/Parser.swift b/Sources/P4Runtime/Parser.swift
index a6329a9..13e70fc 100644
--- a/Sources/P4Runtime/Parser.swift
+++ b/Sources/P4Runtime/Parser.swift
@@ -20,8 +20,9 @@ import P4Lang
 
 extension ParserAssignmentStatement: EvaluatableStatement {
   public func evaluate(execution: ProgramExecution) -> ProgramExecution {
-    guard case Result.Ok(let value) = self.value.evaluate(execution: execution) else {
-      return execution.setError(error: Error(withMessage: "AHHHHH"))
+    let result = self.value.evaluate(execution: execution)
+    guard case Result.Ok(let value) = result else {
+      return execution.setError(error: result.error()!)
     }
     let updated_scopes = execution.scopes.set(identifier: self.lvalue, withValue: value)
 
@@ -32,38 +33,82 @@ extension ParserAssignmentStatement: EvaluatableStatement {
 }
 
 public struct ParserStateDirectTransition: ParserStateInstance {
+
+  public func type() -> any Common.P4Type {
+    return P4ParserState.create()
+  }
+
+  public func eq(rhs: any Common.P4Value) -> Bool {
+    switch rhs {
+    case let state as ParserStateInstance: return currrent_state == state.state()
+    default: return false
+    }
+  }
+
+  public var description: String {
+    get {
+      return "Instance of \(currrent_state)"
+    }
+  }
+
   public let currrent_state: ParserState
-  public let next_state: ParserStateInstance
+  public let next_state_identifier: Identifier
 
   public func execute(
     program: Common.ProgramExecution
   ) -> (any ParserStateInstance, Common.ProgramExecution) {
-    var program = program.exit_scope()
-    program = program.enter_scope()
+    var program = program.enter_scope()
 
     for statement in currrent_state.statements {
       program = statement.evaluate(execution: program)
     }
+    let res = program.scopes.lookup(identifier: next_state_identifier)
 
-    return (self.next_state, program)
+    if case .Ok(let value) = res {
+      if value.type().eq(rhs: P4ParserState.create()) {
+        return (value as! ParserStateInstance, program.exit_scope())
+      }
+    }
+
+    program = program.setError(error: res.error()!).exit_scope()
+   
+    return (self, program.exit_scope())
   }
 
   public func done() -> Bool {
     return false
   }
 
-  public func current() -> P4Lang.ParserState {
+  public func state() -> P4Lang.ParserState {
     return currrent_state
   }
 
 }
 
 public struct ParserStateNoTransition: ParserStateInstance {
+
+  public func type() -> any Common.P4Type {
+    return P4ParserState.create()
+  }
+
+  public func eq(rhs: any Common.P4Value) -> Bool {
+    switch rhs {
+    case let state as ParserStateInstance: return currrent_state == state.state()
+    default: return false
+    }
+  }
+
+  public var description: String {
+    get {
+      return "Instance of \(currrent_state)"
+    }
+  }
+
   public let currrent_state: ParserState
+
   public func execute(
     program: Common.ProgramExecution
   ) -> (any ParserStateInstance, Common.ProgramExecution) {
-    let program = program.exit_scope()
     return (self, program)
   }
 
@@ -71,90 +116,83 @@ public struct ParserStateNoTransition: ParserStateInstance {
     return true
   }
 
-  public func current() -> P4Lang.ParserState {
+  public func state() -> P4Lang.ParserState {
     return currrent_state
   }
 }
 
 public struct ParserStateSelectTransition: ParserStateInstance {
+  public func type() -> any Common.P4Type {
+    return P4ParserState.create()
+  }
+
+  public func eq(rhs: any Common.P4Value) -> Bool {
+    switch rhs {
+    case let state as ParserStateInstance: return currrent_state == state.state()
+    default: return false
+    }
+  }
+
+  public var description: String {
+    get {
+      return "Instance of \(currrent_state)"
+    }
+  }
+
   public func execute(
     program: Common.ProgramExecution
   ) -> (any ParserStateInstance, Common.ProgramExecution) {
-    // Otherwise, we exit the scope from the previous state and enter a new one!
-    var program = program.exit_scope()
-    program = program.enter_scope()
+    var program = program.enter_scope()
 
     // First, evaluate the statements.
     for statement in currrent_state.statements {
       program = statement.evaluate(execution: program)
     }
 
-    switch self.selector.evaluate(execution: program) {
-    case .Ok(let selector_value):
-      for (key, target) in zip(self.keys, self.states) {
-        if case .Ok(let kse_key) = key.evaluate(execution: program),
-          kse_key.eq(rhs: selector_value)
-        {
-          return (target, program)
-        }
+    let res = self.selectExpression.evaluate(execution: program)
+
+    if case .Ok(let value) = res {
+      if value.type().eq(rhs: P4ParserState.create()) {
+        return (value as! ParserStateInstance, program.exit_scope())
       }
-      return (
-        self, program.setError(error: Error(withMessage: "No selector key matched")).setDone()
-      )
-    case .Error(let e): return (self, program.setError(error: e))
     }
+
+    program = program.setError(error: res.error()!).exit_scope()
+    return (self, program.exit_scope())
   }
 
   public func done() -> Bool {
     return false
   }
 
-  public func current() -> P4Lang.ParserState {
+  public func state() -> P4Lang.ParserState {
     return currrent_state
   }
 
-  public let keys: [any EvaluatableExpression]
-  public let states: [ParserStateInstance]
-  public let selector: any EvaluatableExpression
+  public let selectExpression: SelectExpression
   public let currrent_state: ParserState
 }
 
 extension ParserState: Compilable {
-  public typealias ToCompile = (ParserState, [Identifier: ParserStateInstance])
+  public typealias ToCompile = ParserState
   public typealias Compiled = ParserStateInstance
-  public static func compile(_ parser: ToCompile) -> Result<Compiled> {
-    let (state, current) = parser
-    if state == accept || state == reject {
-      return .Ok(ParserStateNoTransition(currrent_state: state))
-    }
+  public static func compile(_ state: ToCompile) -> Result<Compiled> {
 
     if state.direct_transition(),
       let transition_statement = state.transition
     {
       return .Ok(
         ParserStateDirectTransition(
-          currrent_state: state, next_state: current[transition_statement.next_state!]!))
+          currrent_state: state, next_state_identifier: transition_statement.next_state!))
     }
 
     if let transition_select_statement = state.transition,
       let transition_select_expression = transition_select_statement.transition_expression
     {
 
-      var keys: [any EvaluatableExpression] = Array()
-      var states: [any ParserStateInstance] = Array()
-
-      for kse in transition_select_expression.keyset_expressions {
-        guard let next_state = current[kse.next_state_identifier] else {
-          return .Error(
-            Error(withMessage: "Cannot find \(kse.next_state_identifier) as transition target"))
-        }
-        keys.append(kse.key)
-        states.append(next_state)
-      }
       return .Ok(
         ParserStateSelectTransition(
-          keys: keys, states: states, selector: transition_select_expression.selector,
-          currrent_state: state))
+          selectExpression: transition_select_expression, currrent_state: state))
     }
 
     return .Error(Error(withMessage: "Invalid parser state: No meaningful transition"))
@@ -163,24 +201,30 @@ extension ParserState: Compilable {
 
 extension ParserStates: Compilable {
   public typealias ToCompile = ParserStates
-  public typealias Compiled = ParserStateInstance
+  public typealias Compiled = (ParserStateInstance, [ParserStateInstance])
   public static func compile(_ parser: ToCompile) -> Result<Compiled> {
-    var compiled_states = [Identifier: ParserStateInstance]()
+    var compiled_states:[ParserStateInstance] = Array()
 
-    compiled_states[Identifier(name: "accept")] = ParserStateNoTransition(currrent_state: accept)
-    compiled_states[Identifier(name: "reject")] = ParserStateNoTransition(currrent_state: reject)
+    compiled_states.append(ParserStateNoTransition(currrent_state: accept))
+    compiled_states.append(ParserStateNoTransition(currrent_state: reject))
+
+    var start_state: ParserStateInstance? = .none
 
     // TODO: We assume that states are in transition-order!
     for state in parser.states {
-      switch ParserState.compile((state, compiled_states)) {
-      case .Ok(let compiled): compiled_states[state.state] = compiled
+      switch ParserState.compile(state) {
+      case .Ok(let compiled): 
+        if compiled.state().state == Identifier(name: "start") {
+          start_state = compiled
+        }
+        compiled_states.append(compiled)
       case .Error(let e): return .Error(e)
       }
     }
 
     // Now, find the start state:
-    if let start_state = compiled_states[Identifier(name: "start")] {
-      return .Ok(start_state)
+    if let start_state = start_state {
+      return .Ok((start_state, compiled_states))
     } else {
       return .Error(Error(withMessage: "No start state defined"))
     }
@@ -189,14 +233,16 @@ extension ParserStates: Compilable {
 
 public class ParserInstance: ProgramExecution {
 
+  let states: [ParserStateInstance]
   let start_state: ParserStateInstance
 
-  public init(_ _start_state: ParserStateInstance) {
-    self.start_state = _start_state
+  public init(start: ParserStateInstance, states: [ParserStateInstance]) {
+    start_state = start
+    self.states = states
   }
 
   public override var description: String {
-    return "Execution: \(super.description)\nStart State: \(start_state)"
+    return "Execution: \(super.description)\nStates: \(states)"
   }
 }
 
@@ -206,7 +252,7 @@ extension ParserInstance: Compilable {
 
   public static func compile(_ parser: ToCompile) -> Result<Compiled> {
     return switch ParserStates.compile(parser.states) {
-    case .Ok(let start_state): Result.Ok(ParserInstance(start_state))
+    case .Ok(let (start_state, states)): Result.Ok(ParserInstance(start: start_state, states: states))
     case .Error(let e): Result.Error(e)
     }
   }
diff --git a/Sources/P4Runtime/Program.swift b/Sources/P4Runtime/Program.swift
index aa7e78d..af0d28a 100644
--- a/Sources/P4Runtime/Program.swift
+++ b/Sources/P4Runtime/Program.swift
@@ -66,15 +66,4 @@ 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)
-  }
-}
+}
\ No newline at end of file
diff --git a/Sources/P4Runtime/Protocols.swift b/Sources/P4Runtime/Protocols.swift
index 742d4e9..5a0866d 100644
--- a/Sources/P4Runtime/Protocols.swift
+++ b/Sources/P4Runtime/Protocols.swift
@@ -18,14 +18,6 @@
 import Common
 import P4Lang
 
-protocol EvaluatableParserTransition {
-  func program(execution: ProgramExecution) -> (ParserState, ProgramExecution)
-}
-
-protocol EvaluatableParserTransitionStatement {
-  func transition(execution: ProgramExecution) -> (ParserState, ProgramExecution)
-}
-
 public protocol Execution {
   func execute() -> (ParserState, ProgramExecution)
 }
@@ -36,10 +28,10 @@ public protocol Compilable {
   static func compile(_: ToCompile) -> Result<Compiled>
 }
 
-public protocol ParserStateInstance {
+public protocol ParserStateInstance: P4Value {
   func execute(program: ProgramExecution) -> (ParserStateInstance, ProgramExecution)
   func done() -> Bool
-  func current() -> ParserState
+  func state() -> ParserState
 }
 
 public protocol ParserExecution {
diff --git a/Sources/P4Runtime/Runtime.swift b/Sources/P4Runtime/Runtime.swift
index d4837dc..0429507 100644
--- a/Sources/P4Runtime/Runtime.swift
+++ b/Sources/P4Runtime/Runtime.swift
@@ -41,7 +41,11 @@ public class ParserRuntime: CustomStringConvertible {
 
   /// Run the P4 parser on a given packet
   public func run() -> Result<(ParserState, ProgramExecution)> {
-    return .Ok(parser.execute())
+    let (end_state, execution) = parser.execute()
+    if let error = execution.getError() {
+      return .Error(error)
+    }
+    return .Ok((end_state, execution))
   }
 
   public var description: String {
@@ -53,12 +57,19 @@ public class ParserRuntime: CustomStringConvertible {
 extension ParserInstance: ParserExecution {
   public func execute() -> (ParserState, ProgramExecution) {
     var execution = self as ProgramExecution
+
+    execution = execution.enter_scope()
+    // First, add every state to the scope!
+    for state in self.states {
+      execution = execution.declare(identifier: state.state().state, withValue: state)
+    }
+
     var c = self.start_state
 
     // Evaluate until the state is either accept or reject.
     while !c.done() && !execution.hasError() {
       (c, execution) = c.execute(program: execution)
     }
-    return (c.current(), execution)
+    return (c.state(), execution)
   }
 }
diff --git a/Tests/p4rseTests/RuntimeTests.swift b/Tests/p4rseTests/RuntimeTests.swift
index 4e65cd7..3c5ff68 100644
--- a/Tests/p4rseTests/RuntimeTests.swift
+++ b/Tests/p4rseTests/RuntimeTests.swift
@@ -142,17 +142,10 @@ import TreeSitterP4
     """
 
   let program = try #UseOkResult(Program.Compile(simple_parser_declaration))
-  let parser = try #UseOkResult(program.find_parser(withName: Identifier(name: "main_parser")))
   let runtime = try #UseOkResult(P4Runtime.ParserRuntime.create(program: program))
-  let (state_result, exec_result) = try! #UseOkResult(runtime.run())
+  let (state_result, _) = try! #UseOkResult(runtime.run())
 
-  #expect(parser.states.count() == 1)
   #expect(state_result == P4Lang.reject)
-
-  let x = try #UseOkResult(exec_result.scopes.lookup(identifier: Identifier(name: "x")))
-  #expect(x.eq(rhs: P4BooleanValue(withValue: false)))
-  let check = try #UseOkResult(exec_result.scopes.lookup(identifier: Identifier(name: "check")))
-  #expect(check.eq(rhs: P4StringValue(withValue: "\"valid\"")))
 }
 
 @Test func test_simple_parser_with_conditional_statement_and_else() async throws {
@@ -177,15 +170,29 @@ import TreeSitterP4
     """
 
   let program = try #UseOkResult(Program.Compile(simple_parser_declaration))
-  let parser = try #UseOkResult(program.find_parser(withName: Identifier(name: "main_parser")))
   let runtime = try #UseOkResult(P4Runtime.ParserRuntime.create(program: program))
-  let (state_result, exec_result) = try! #UseOkResult(runtime.run())
+  let (state_result, _) = try! #UseOkResult(runtime.run())
 
-  #expect(parser.states.count() == 1)
   #expect(state_result == P4Lang.accept)
 
-  let x = try #UseOkResult(exec_result.scopes.lookup(identifier: Identifier(name: "x")))
-  #expect(x.eq(rhs: P4BooleanValue(withValue: true)))
-  let check = try #UseOkResult(exec_result.scopes.lookup(identifier: Identifier(name: "check")))
-  #expect(check.eq(rhs: P4StringValue(withValue: "\"b\"")))
+}
+
+@Test func test_no_matching_key_transition() async throws {
+  let simple_parser_declaration = """
+      parser main_parser() {
+        state start {
+          transition select (false) {
+            true: accept;
+          };
+        }
+      };
+    """
+
+  let program = try #UseOkResult(Program.Compile(simple_parser_declaration))
+  let runtime = try #UseOkResult(P4Runtime.ParserRuntime.create(program: program))
+
+  #expect(
+    #RequireErrorResult<(ParserState, ProgramExecution)>(
+      Error(withMessage: "No key matched the selector"),
+      runtime.run()))
 }
diff --git a/Tests/p4rseTests/ScopeRuntimeTests.swift b/Tests/p4rseTests/ScopeRuntimeTests.swift
index 9d48143..61c2a95 100644
--- a/Tests/p4rseTests/ScopeRuntimeTests.swift
+++ b/Tests/p4rseTests/ScopeRuntimeTests.swift
@@ -44,25 +44,9 @@ import TreeSitterP4
 
   let program = try #UseOkResult(Program.Compile(simple_parser_declaration))
   let runtime = try #UseOkResult(P4Runtime.ParserRuntime.create(program: program))
-  let (state_result, execution_result) = try! #UseOkResult(runtime.run())
+  let (state_result, _) = try! #UseOkResult(runtime.run())
 
-  #expect(execution_result.scopes.count == 1)
-
-  guard let scope = execution_result.scopes.current else {
-    assert(false)
-  }
-
-  // We should be in the accept state.
   #expect(state_result == P4Lang.reject)
-
-  // There are two variables declared.
-  #expect(scope.count == 2)
-
-  // Check the names/values of the variables in scope.
-  let b = try #require(scope.lookup(identifier: Identifier(name: "b")))
-  let s = try #require(scope.lookup(identifier: Identifier(name: "s")))
-  #expect(b.eq(rhs: P4BooleanValue(withValue: false)))
-  #expect(s.eq(rhs: P4StringValue(withValue: "\"testing\"")))
 }
 
 @Test func test_simple_scope() async throws {
@@ -86,17 +70,10 @@ import TreeSitterP4
 
   let program = try #UseOkResult(Program.Compile(simple_parser_declaration))
   let runtime = try #UseOkResult(P4Runtime.ParserRuntime.create(program: program))
-  let (state_result, execution_result) = try! #UseOkResult(runtime.run())
+  let (state_result, _) = try! #UseOkResult(runtime.run())
 
   #expect(state_result == P4Lang.accept)
 
-  #expect(execution_result.scopes.count == 1)
-  let scope = try! #require(execution_result.scopes.current)
-  #expect(scope.count == 2)
-  let va = try #require(scope.lookup(identifier: Identifier(name: "va")))
-  let where_to = try #require(scope.lookup(identifier: Identifier(name: "where_to")))
-  #expect(where_to.eq(rhs: P4BooleanValue(withValue: false)))
-  #expect(va.eq(rhs: P4IntValue(withValue: 5)))
 }
 
 @Test func test_simple_scope2() async throws {
@@ -120,20 +97,10 @@ import TreeSitterP4
 
   
   let program = try #UseOkResult(Program.Compile(simple_parser_declaration))
-  let parser = try #UseOkResult(program.find_parser(withName: Identifier(name: "main_parser")))
   let runtime = try #UseOkResult(P4Runtime.ParserRuntime.create(program: program))
-  let (state_result, execution_result) = try! #UseOkResult(runtime.run())
-
-  #expect(parser.states.count() == 2)
+  let (state_result, _) = try! #UseOkResult(runtime.run())
 
   #expect(state_result == P4Lang.reject)
-
-  #expect(execution_result.scopes.count == 1)
-  let scope = try! #require(execution_result.scopes.current)
-
-  #expect(scope.count == 1)
-  let where_to = try #require(scope.lookup(identifier: Identifier(name: "where_to")))
-  #expect(where_to.eq(rhs: P4BooleanValue(withValue: false)))
 }
 
 @Test func test_simple_assignment() async throws {
@@ -144,7 +111,7 @@ import TreeSitterP4
           string where_from = "here";
           where_to = false;
           where_from = "there";
-          transition select (true) {
+          transition select (where_to) {
             false: reject;
             true: accept;
           };
@@ -154,22 +121,11 @@ import TreeSitterP4
 
   
   let program = try #UseOkResult(Program.Compile(simple_parser_declaration))
-  let parser = try #UseOkResult(program.find_parser(withName: Identifier(name: "main_parser")))
   let runtime = try #UseOkResult(P4Runtime.ParserRuntime.create(program: program))
-  let (state_result, execution_result) = try! #UseOkResult(runtime.run())
+  let (state_result, _) = try! #UseOkResult(runtime.run())
 
-  #expect(parser.states.count() == 1)
+  #expect(state_result == P4Lang.reject)
 
-  #expect(state_result == P4Lang.accept)
-
-  #expect(execution_result.scopes.count == 1)
-  let scope = try! #require(execution_result.scopes.current)
-
-  #expect(scope.count == 2)
-  let where_to = try #require(scope.lookup(identifier: Identifier(name: "where_to")))
-  #expect(where_to.eq(rhs: P4BooleanValue(withValue: false)))
-  let where_from = try #require(scope.lookup(identifier: Identifier(name: "where_from")))
-  #expect(where_from.eq(rhs: P4StringValue(withValue: "\"there\"")))
 }
 
 @Test func test_nested_declaration_assignment() async throws {
@@ -196,20 +152,8 @@ import TreeSitterP4
 
   
   let program = try #UseOkResult(Program.Compile(simple_parser_declaration))
-  let parser = try #UseOkResult(program.find_parser(withName: Identifier(name: "main_parser")))
   let runtime = try #UseOkResult(P4Runtime.ParserRuntime.create(program: program))
-  let (state_result, execution_result) = try! #UseOkResult(runtime.run())
-
-  #expect(parser.states.count() == 1)
+  let (state_result, _) = try! #UseOkResult(runtime.run())
 
   #expect(state_result == P4Lang.reject)
-
-  #expect(execution_result.scopes.count == 1)
-  let scope = try! #require(execution_result.scopes.current)
-
-  #expect(scope.count == 3)
-  let where_to = try #require(scope.lookup(identifier: Identifier(name: "where_to")))
-  #expect(where_to.eq(rhs: P4BooleanValue(withValue: false)))
-  let where_from = try #require(scope.lookup(identifier: Identifier(name: "where_from")))
-  #expect(where_from.eq(rhs: P4StringValue(withValue: "\"there\"")))
 }
\ No newline at end of file