diff --git a/Sources/Common/Execution.swift b/Sources/Common/Execution.swift
index e6d1541..bbf75d7 100644
--- a/Sources/Common/Execution.swift
+++ b/Sources/Common/Execution.swift
@@ -87,6 +87,12 @@ open class ProgramExecution: CustomStringConvertible {
     return new_pe
   }
 
+  public func replaceScopes(_ new_scopes: VarValueScopes) -> ProgramExecution {
+    let new_pe = ProgramExecution(copy: self)
+    new_pe.scopes = new_scopes
+    return new_pe
+  }
+
   public func declare(identifier: Identifier, withValue value: P4Value) -> ProgramExecution {
     let new_pe = ProgramExecution(copy: self)
     let new_scopes = new_pe.scopes.declare(identifier: identifier, withValue: value)
diff --git a/Sources/Common/P4Types.swift b/Sources/Common/P4Types.swift
index e3a7ea0..fcf3afa 100644
--- a/Sources/Common/P4Types.swift
+++ b/Sources/Common/P4Types.swift
@@ -15,6 +15,13 @@
 // You should have received a copy of the GNU General Public License
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
+public enum TypeCheckResults: Equatable {
+  case Ok
+  case ReadOnly
+  case WrongDirection
+  case IncompatibleTypes
+}
+
 public enum Direction: Equatable, CustomStringConvertible {
   case In
   case Out
@@ -115,11 +122,11 @@ public struct P4Type: CustomStringConvertible {
   }
 
   public func update(removeAttribute attribute: P4TypeAttribute) -> P4Type {
-    return P4Type(self._data_type, self._attributes.update(addAttribute: attribute))
+    return P4Type(self._data_type, self._attributes.update(removeAttribute: attribute))
   }
 
   public func update(addAttribute attribute: P4TypeAttribute) -> P4Type {
-    return P4Type(self._data_type, self._attributes.update(removeAttribute: attribute))
+    return P4Type(self._data_type, self._attributes.update(addAttribute: attribute))
   }
 
   public func direction() -> Direction? {
@@ -143,6 +150,37 @@ public struct P4Type: CustomStringConvertible {
       && self.dataType().eq(rhs: rhs.dataType())
   }
 
+  public func assignable() -> TypeCheckResults {
+    if self.readOnly() {
+      return TypeCheckResults.ReadOnly
+    }
+
+    if let direction = direction(),
+      direction == Direction.In
+    {
+      return TypeCheckResults.WrongDirection
+    }
+    return TypeCheckResults.Ok
+  }
+
+  public func assignableFromType(_ rhs: P4Type) -> TypeCheckResults {
+    if !self.dataType().eq(rhs: rhs.dataType()) {
+      return TypeCheckResults.IncompatibleTypes
+    }
+
+    if self.readOnly() {
+      return TypeCheckResults.ReadOnly
+    }
+
+    if let direction = direction(),
+      direction == Direction.In
+    {
+      return TypeCheckResults.WrongDirection
+    }
+
+    return TypeCheckResults.Ok
+  }
+
   public static func ReadOnly(_ type: P4DataType) -> P4Type {
     return P4Type(type, P4TypeAttributes.ReadOnly())
   }
diff --git a/Sources/Common/Protocols.swift b/Sources/Common/Protocols.swift
index bce3be1..03a5f2a 100644
--- a/Sources/Common/Protocols.swift
+++ b/Sources/Common/Protocols.swift
@@ -20,7 +20,7 @@ public protocol EvaluatableExpression {
   /// - Parameters
   ///  - execution: The execution context in which to evaluate the expression
   /// - Returns: The value of expression
-  func evaluate(execution: ProgramExecution) -> Result<P4Value>
+  func evaluate(execution: ProgramExecution) -> (Result<P4Value>, ProgramExecution)
   func type() -> P4Type
 }
 
diff --git a/Sources/P4Compiler/Common.swift b/Sources/P4Compiler/Common.swift
index 2b7d92f..a292182 100644
--- a/Sources/P4Compiler/Common.swift
+++ b/Sources/P4Compiler/Common.swift
@@ -83,8 +83,8 @@ func parameter_list_compiler(
 
   // Otherwise, there should be one parameter left!
   switch Parameter.Compile(node: currentChild!, withContext: context) {
-  case .Ok(let (vds, updated_context)):
-    return Result.Ok((parameters.addParameter(vds), updated_context))
+  case .Ok(let (parsed_parameter, updated_context)):
+    return Result.Ok((parameters.addParameter(parsed_parameter), updated_context))
   case .Error(let e): return Result.Error(e)
   }
 }
diff --git a/Sources/P4Compiler/Statement.swift b/Sources/P4Compiler/Statement.swift
index 54cf3ce..4ffdebf 100644
--- a/Sources/P4Compiler/Statement.swift
+++ b/Sources/P4Compiler/Statement.swift
@@ -312,7 +312,7 @@ extension ReturnStatement: CompilableStatement {
 
     return switch Expression.Compile(node: expression_node, withContext: context) {
     case .Ok(let result):
-      if result.type().eq(context.expected_type!) {
+      if result.type().dataType().eq(rhs: context.expected_type!.dataType()) {
         .Ok((ReturnStatement(result), context))
       } else {
         .Error(
diff --git a/Sources/P4Lang/Common.swift b/Sources/P4Lang/Common.swift
index adf0690..7500006 100644
--- a/Sources/P4Lang/Common.swift
+++ b/Sources/P4Lang/Common.swift
@@ -39,7 +39,16 @@ public struct Parameter: CustomStringConvertible, Equatable {
   /// Calculate whether the `argument` is compatible with this parameter.
   public func compatible(_ argument: Argument) -> Bool {
     let arg_type = argument.argument.type()
-    return arg_type.eq(self.type)
+
+    // If the parameter is (in)out, then the argument must be an lvalue.
+    if let param_direction = self.type.direction(),
+      param_direction == Direction.In || param_direction == Direction.InOut
+    {
+      if !(argument.argument is EvaluatableLValueExpression) {
+        return false
+      }
+    }
+    return arg_type.dataType().eq(rhs: self.type.dataType())
   }
 
 }
diff --git a/Sources/P4Runtime/Common.swift b/Sources/P4Runtime/Common.swift
index 7419abc..d486da3 100644
--- a/Sources/P4Runtime/Common.swift
+++ b/Sources/P4Runtime/Common.swift
@@ -17,3 +17,57 @@
 
 import Common
 import P4Lang
+
+public func Call<T>(
+  body: (ProgramExecution) -> (Result<T>, ProgramExecution), withArguments args: ArgumentList,
+  withParameters params: ParameterList, inExecution execution: ProgramExecution
+) -> (Result<T>, ProgramExecution) {
+    
+  if case .Error(let e) = args.compatible(params) {
+    return (.Error(e), execution)
+  }
+
+  var called_execution = execution.enter_scope()
+
+  for (parameter, argument) in zip(params.parameters, args.arguments) {
+    let arg_idx = argument.index
+    let arg_value = argument.argument
+    let maybe_argument_value = arg_value.evaluate(execution: called_execution)
+    guard case (.Ok(let argument_value), let updated_execution) = maybe_argument_value else {
+      return (
+        .Error(Error(withMessage: "Cannot evaluate argument \(arg_idx): \(argument)")),
+        called_execution.exit_scope()
+      )
+    }
+    called_execution = updated_execution.declare(
+      identifier: parameter.name, withValue: argument_value)
+  }
+
+  let (maybe_call_result, updated_execution) = body(called_execution)
+  guard case .Ok(let call_result) = maybe_call_result else {
+    return (.Error(maybe_call_result.error()!), updated_execution.exit_scope())
+  }
+
+  // Before returning, update the (in)out parameters!
+  var inout_scopes = updated_execution.exit_scope().scopes
+
+  for (parameter, argument) in zip(params.parameters, args.arguments) {
+    if let param_direction = parameter.type.direction(),
+    param_direction == Direction.InOut || param_direction == Direction.Out {
+      // Let's make sure that it is an evaluatable l value!
+      guard let arg_lvalue = argument.argument as? EvaluatableLValueExpression else {
+        return (.Error(Error(withMessage: "(in)out parameter argument is not lvalue")), updated_execution.exit_scope())
+      }
+
+      guard case .Ok(let arg_new_value) = updated_execution.scopes.lookup(identifier: parameter.name) else {
+        return (.Error(Error(withMessage: "Could not get (in)out parameter value from scope")), updated_execution.exit_scope())
+      }
+
+      switch arg_lvalue.set(to: arg_new_value, inScopes: inout_scopes, duringExecution: updated_execution) {
+        case .Ok((let updated_scopes, _)): inout_scopes = updated_scopes
+        case .Error(let e): return (.Error(e), updated_execution.exit_scope())
+      }
+    } 
+  }
+  return (.Ok(call_result), updated_execution.replaceScopes(inout_scopes))
+}
\ No newline at end of file
diff --git a/Sources/P4Runtime/Expressions.swift b/Sources/P4Runtime/Expressions.swift
index b09931c..9565eb3 100644
--- a/Sources/P4Runtime/Expressions.swift
+++ b/Sources/P4Runtime/Expressions.swift
@@ -19,8 +19,8 @@ import Common
 import P4Lang
 
 extension SelectCaseExpression: EvaluatableExpression {
-  public func evaluate(execution: Common.ProgramExecution) -> Common.Result<P4Value> {
-    return execution.scopes.lookup(identifier: next_state_identifier)
+  public func evaluate(execution: ProgramExecution) -> (Result<P4Value>, ProgramExecution) {
+    return (execution.scopes.lookup(identifier: next_state_identifier), execution)
   }
 
   public func type() -> P4Type {
@@ -29,19 +29,19 @@ extension SelectCaseExpression: EvaluatableExpression {
 }
 
 extension SelectExpression: EvaluatableExpression {
-  public func evaluate(execution: Common.ProgramExecution) -> Common.Result<P4Value> {
+  public func evaluate(execution: ProgramExecution) -> (Result<P4Value>, ProgramExecution) {
     switch self.selector.evaluate(execution: execution) {
-    case .Ok(let selector_value):
+    case (.Ok(let selector_value), let updated_execution):
       for sce in self.case_expressions {
-        if case .Ok(let kse) = sce.key.evaluate(execution: execution),
+        if case (.Ok(let kse), let updated_execution) = sce.key.evaluate(execution: updated_execution),
           kse.eq(selector_value)
         {
-          let result = sce.evaluate(execution: execution)
+          let result = sce.evaluate(execution: updated_execution)
           return result
         }
       }
-      return .Error(Error(withMessage: "No key matched the selector"))
-    case .Error(let e): return .Error(e)
+      return (.Error(Error(withMessage: "No key matched the selector")), updated_execution)
+    case (.Error(let e), let updated_execution): return (.Error(e), updated_execution)
     }
   }
 
@@ -56,8 +56,8 @@ extension TypedIdentifier: EvaluatableExpression {
     return self.type
   }
 
-  public func evaluate(execution: Common.ProgramExecution) -> Result<P4Value> {
-    return execution.scopes.lookup(identifier: self)
+  public func evaluate(execution: ProgramExecution) -> (Result<P4Value>, ProgramExecution) {
+    return (execution.scopes.lookup(identifier: self), execution)
   }
 }
 
@@ -81,14 +81,12 @@ extension TypedIdentifier: EvaluatableLValueExpression {
       return .Error(Error(withMessage: "Cannot assign to identifier not in scope"))
     }
 
-    if !type.eq(to.type()) {
-      return .Error(
-        Error(
-          withMessage:
-            "Cannot assign value with type \(to.type()) to identifier \(self) with type \(type)"
-        ))
+    return switch type.assignableFromType(to.type()) {
+      case TypeCheckResults.IncompatibleTypes: .Error( Error( withMessage: "Cannot assign value with type \(to.type()) to identifier \(self) with type \(type)"))
+      case TypeCheckResults.ReadOnly: .Error( Error( withMessage: "Cannot assign value with type \(to.type()) to identifier \(self) that is read only"))
+      case TypeCheckResults.WrongDirection: .Error( Error( withMessage: "Cannot assign value with type \(to.type()) to identifier \(self) that is in parameter"))
+      case TypeCheckResults.Ok: .Ok(())
     }
-    return .Ok(())
   }
 }
 
@@ -196,18 +194,19 @@ public func binary_int_math_operator_checker(
 }
 
 extension BinaryOperatorExpression: EvaluatableExpression {
-  public func evaluate(execution: Common.ProgramExecution) -> Common.Result<P4Value> {
-    let maybe_evaluated_left = self.left.evaluate(execution: execution)
-    guard case Result.Ok(let evaluated_left) = maybe_evaluated_left else {
+  public func evaluate(execution: ProgramExecution) -> (Result<P4Value>, ProgramExecution) {
+    let updated_execution = execution
+    let maybe_evaluated_left = self.left.evaluate(execution: updated_execution)
+    guard case (.Ok(let evaluated_left), let updated_execution) = maybe_evaluated_left else {
       return maybe_evaluated_left
     }
 
-    let maybe_evaluated_right = self.right.evaluate(execution: execution)
-    guard case Result.Ok(let evaluated_right) = maybe_evaluated_right else {
+    let maybe_evaluated_right = self.right.evaluate(execution: updated_execution)
+    guard case (.Ok(let evaluated_right), let updated_execution) = maybe_evaluated_right else {
       return maybe_evaluated_right
     }
 
-    return Result.Ok(P4Value(self.evaluator.2(evaluated_left, evaluated_right)))
+    return (.Ok(P4Value(self.evaluator.2(evaluated_left, evaluated_right))), updated_execution)
   }
 
   public func type() -> P4Type {
@@ -216,27 +215,28 @@ extension BinaryOperatorExpression: EvaluatableExpression {
 }
 
 extension ArrayAccessExpression: EvaluatableExpression {
-  public func evaluate(execution: Common.ProgramExecution) -> Common.Result<P4Value> {
-    let maybe_name = self.name.evaluate(execution: execution)
-    guard case Result.Ok(let name) = maybe_name else {
+  public func evaluate(execution: ProgramExecution) -> (Result<P4Value>, ProgramExecution) {
+    let updated_execution = execution
+    let maybe_name = self.name.evaluate(execution: updated_execution)
+    guard case (.Ok(let name), let updated_execution) = maybe_name else {
       return maybe_name
     }
 
-    let maybe_indexor = self.indexor.evaluate(execution: execution)
-    guard case Result.Ok(let indexor) = maybe_indexor else {
+    let maybe_indexor = self.indexor.evaluate(execution: updated_execution)
+    guard case (.Ok(let indexor), let updated_execution) = maybe_indexor else {
       return maybe_indexor
     }
 
     guard let indexor_int = indexor.dataValue() as? P4IntValue else {
-      return Result.Error(Error(withMessage: "\(indexor) cannot index an array"))
+      return (.Error(Error(withMessage: "\(indexor) cannot index an array")), updated_execution)
     }
 
     guard let array = name.dataValue() as? P4ArrayValue else {
-      return Result.Error(Error(withMessage: "\(name) does not name an array"))
+      return (.Error(Error(withMessage: "\(name) does not name an array")), updated_execution)
     }
     let accessed = array.access(indexor_int.access())
 
-    return .Ok(accessed)
+    return (.Ok(accessed), updated_execution)
   }
 
   public func type() -> P4Type {
@@ -250,20 +250,20 @@ extension ArrayAccessExpression: EvaluatableLValueExpression {
     duringExecution execution: ProgramExecution
   ) -> Common.Result<(Common.VarValueScopes, P4Value)> {
 
-    let maybe_value = self.name.evaluate(execution: execution)
-    guard case .Ok(let value) = maybe_value else {
-      return Result.Error(
-        Error(withMessage: "\(self.name) cannot be evaluated: \(maybe_value.error()!)"))
+    let updated_execution = execution
+    let maybe_value = self.name.evaluate(execution: updated_execution)
+    guard case (.Ok(let value), let updated_execution) = maybe_value else {
+      return .Error(Error(withMessage: "\(self.name) cannot be evaluated: \(maybe_value.0.error()!)"))
     }
     guard let array_value = value.dataValue() as? P4ArrayValue else {
       return Result.Error(Error(withMessage: "\(self.name) does not identify an array"))
     }
 
     // Now, get the indexor!
-    let maybe_indexor_value = self.indexor.evaluate(execution: execution)
-    guard case .Ok(let indexor_value) = maybe_indexor_value else {
+    let maybe_indexor_value = self.indexor.evaluate(execution: updated_execution)
+    guard case (.Ok(let indexor_value), let updated_execution) = maybe_indexor_value else {
       return Result.Error(
-        Error(withMessage: "\(self.indexor) cannot be evaluated: \(maybe_indexor_value.error()!)"))
+        Error(withMessage: "\(self.indexor) cannot be evaluated: \(maybe_indexor_value.0.error()!)"))
     }
     guard let indexor_int = indexor_value.dataValue() as? P4IntValue else {
       return Result.Error(Error(withMessage: "\(self.indexor) cannot be used to index an array"))
@@ -282,42 +282,63 @@ extension ArrayAccessExpression: EvaluatableLValueExpression {
     }
 
     let array_lvalue = self.name as! EvaluatableLValueExpression
-    return array_lvalue.set(to: updated_array_value, inScopes: scopes, duringExecution: execution)
+    return array_lvalue.set(to: updated_array_value, inScopes: scopes, duringExecution: updated_execution)
   }
 
   public func check(
     to: any Common.EvaluatableExpression, inScopes scopes: Common.VarTypeScopes
   ) -> Common.Result<()> {
 
-    if !self.type.value_type().eq(to.type()) {
-      return .Error(
+    return switch self.type.value_type().assignableFromType(to.type()) {
+    case TypeCheckResults.IncompatibleTypes:
+      .Error(
         Error(
           withMessage:
             "Cannot assign value of type \(to.type()) to array with values of type \(self.name.type())"
         ))
+    case TypeCheckResults.ReadOnly:
+      .Error(
+        Error(
+          withMessage: "Cannot assign value of type \(to.type()) to array \(self) that is read only"
+        ))
+    case TypeCheckResults.WrongDirection:
+      .Error(
+        Error(
+          withMessage:
+            "Cannot assign value of type \(to.type()) to array \(self) that is in parameter"))
+    case TypeCheckResults.Ok:
+      // Now, check the type of the array itself.
+      switch self.name.type().assignable() {
+      case TypeCheckResults.ReadOnly:
+        .Error(Error(withMessage: "Cannot assign to array \(self) that is read only"))
+      case TypeCheckResults.WrongDirection:
+        .Error(Error(withMessage: "Cannot assign to array \(self) that is in parameter"))
+      case TypeCheckResults.Ok: .Ok(())
+      default: .Error(Error(withMessage: "Cannot assign to array \(self)"))
+      }
     }
-    return .Ok(())
   }
 }
 
 extension FieldAccessExpression: EvaluatableExpression {
-  public func evaluate(execution: Common.ProgramExecution) -> Common.Result<P4Value> {
+  public func evaluate(execution: ProgramExecution) -> (Result<P4Value>, ProgramExecution) {
 
-    let maybe_struct = self.strct.evaluate(execution: execution)
-    guard case Result.Ok(let strct) = maybe_struct else {
+    let updated_execution = execution
+    let maybe_struct = self.strct.evaluate(execution: updated_execution)
+    guard case (.Ok(let strct), let updated_execution) = maybe_struct else {
       return maybe_struct
     }
 
     guard let struct_strct = strct.dataValue() as? P4StructValue else {
-      return Result.Error(Error(withMessage: "\(strct) does not identify a struct"))
+      return (.Error(Error(withMessage: "\(strct) does not identify a struct")), updated_execution)
     }
 
     // TODO: Create a default value?
     guard let value = struct_strct.get(field: self.field) else {
-      return .Error(Error(withMessage: "Missing value"))
+      return (.Error(Error(withMessage: "Missing value")), updated_execution)
     }
 
-    return .Ok(value)
+    return (.Ok(value), updated_execution)
   }
 
   public func type() -> P4Type {
@@ -335,15 +356,16 @@ extension FieldAccessExpression: EvaluatableLValueExpression {
     // where strct_id expands to
     // (identifier.field_id1.field_id2...).field_id = new_field_value
 
+    let updated_execution = execution
     // First, evaluate strct_id and make sure that it names a struct.
-    let maybe_value = self.strct.evaluate(execution: execution)
-    guard case .Ok(let value) = maybe_value else {
-      return Result.Error(
-        Error(withMessage: "\(self.strct) cannot be evaluated: \(maybe_value.error()!)"))
+    let maybe_value = self.strct.evaluate(execution: updated_execution)
+    guard case (.Ok(let value), let updated_execution) = maybe_value else {
+      return .Error(
+        Error(withMessage: "\(self.strct) cannot be evaluated: \(maybe_value.0.error()!)"))
     }
 
     guard let struct_value = value.dataValue() as? P4StructValue else {
-      return Result.Error(Error(withMessage: "\(self.strct) does not identify a struct"))
+      return .Error(Error(withMessage: "\(self.strct) does not identify a struct"))
     }
 
     // Now we know that struct_id identifies a structure value.
@@ -363,25 +385,47 @@ extension FieldAccessExpression: EvaluatableLValueExpression {
     // We use recursion here -- ultimately finding our way to a TypedIdentifier that
     // will update the scope. Pretty cool!
     let struct_lvalue = self.strct as! EvaluatableLValueExpression
-    return struct_lvalue.set(to: new_struct_value, inScopes: scopes, duringExecution: execution)
+    return struct_lvalue.set(to: new_struct_value, inScopes: scopes, duringExecution: updated_execution)
   }
 
   public func check(
     to: any Common.EvaluatableExpression, inScopes scopes: Common.VarTypeScopes
   ) -> Common.Result<()> {
-
-    if !self.field.type.eq(to.type()) {
-      return .Error(
+    return switch self.field.type().assignableFromType(to.type()) {
+    case TypeCheckResults.IncompatibleTypes:
+      .Error(
         Error(
           withMessage:
-            "Cannot assign value of type \(to.type()) to field with type \(self.field.type)"))
+            "Cannot assign value of type \(to.type()) to field \(self.field) of type \(self.type())"
+        ))
+    case TypeCheckResults.ReadOnly:
+      .Error(
+        Error(
+          withMessage:
+            "Cannot assign value of type \(to.type()) to field \(self.field) that is read only"
+        ))
+    case TypeCheckResults.Ok:
+      // Now, check the type of the struct itself.
+      switch self.strct.type().assignable() {
+      case TypeCheckResults.ReadOnly:
+        .Error(
+          Error(
+            withMessage: "Cannot assign to field \(self.field) of \(self.strct) that is read only"))
+      case TypeCheckResults.WrongDirection:
+        .Error(
+          Error(
+            withMessage:
+              "Cannot assign to field \(self.field) of \(self.strct) that is in parameter"))
+      case TypeCheckResults.Ok: .Ok(())
+      default: .Error(Error(withMessage: "Cannot assign to field \(self.field) of \(self.strct)"))
+      }
+    default: .Error(Error(withMessage: "Cannot assign to field \(self.field) of \(self.strct)"))
     }
-    return .Ok(())
   }
 }
 
 extension KeysetExpression: EvaluatableExpression {
-  public func evaluate(execution: Common.ProgramExecution) -> Result<P4Value> {
+  public func evaluate(execution: ProgramExecution) -> (Result<P4Value>, ProgramExecution) {
     return self.key.evaluate(execution: execution)
   }
 
@@ -391,38 +435,24 @@ extension KeysetExpression: EvaluatableExpression {
 }
 
 extension FunctionCall: EvaluatableExpression {
-  public func evaluate(execution: Common.ProgramExecution) -> Common.Result<P4Value> {
+  public func evaluate(execution: Common.ProgramExecution) -> (Common.Result<P4Value>, ProgramExecution) {
 
     guard let body = self.callee.body else {
-      return .Error(Error(withMessage: "No body for called function (\(self.callee.name))"))
+      return (.Error(Error(withMessage: "No body for called function (\(self.callee.name))")), execution)
     }
 
-    // Put the arguments into scope
-
-    var called_execution = execution.enter_scope()
-    for (parameter, argument) in zip(self.callee.params.parameters, arguments.arguments) {
-      let arg_idx = argument.index
-      let arg_value = argument.argument
-      let maybe_argument_value = arg_value.evaluate(execution: called_execution)
-      guard case .Ok(let argument_value) = maybe_argument_value else {
-        return .Error(Error(withMessage: "Cannot evaluate argument \(arg_idx): \(argument)"))
+    let call_body: (ProgramExecution) -> (Result<P4Value>, ProgramExecution) = { (execution: ProgramExecution) in 
+      let (control_flow, updated_execution) = body.evaluate(execution: execution)
+      return switch control_flow {
+      case ControlFlow.Return(.some(let value)): (.Ok(value), updated_execution)
+      default:
+          (.Error(Error(withMessage: "No value returned from called function (\(self.callee.name))")), execution)
       }
-      called_execution = called_execution.declare(
-        identifier: parameter.name, withValue: argument_value)
     }
 
-    let (control_flow, _) = body.evaluate(execution: called_execution)
-
-    return switch control_flow {
-    case ControlFlow.Return(let value):
-      if let value = value {
-        .Ok(value)
-      } else {
-        .Error(Error(withMessage: "No value returned from called function (\(self.callee.name))"))
-      }
-    default:
-      .Error(Error(withMessage: "No value returned from called function (\(self.callee.name))"))
-    }
+    return Call(
+        body: call_body, withArguments: self.arguments, withParameters: self.callee.params,
+        inExecution: execution)
   }
 
   public func type() -> P4Type {
@@ -431,7 +461,7 @@ extension FunctionCall: EvaluatableExpression {
 }
 
 extension P4Value: EvaluatableExpression {
-  public func evaluate(execution: ProgramExecution) -> Result<P4Value> {
-    return .Ok(self)
+  public func evaluate(execution: ProgramExecution) -> (Result<P4Value>, ProgramExecution) {
+    return (.Ok(self), execution)
   }
 }
diff --git a/Sources/P4Runtime/Parser.swift b/Sources/P4Runtime/Parser.swift
index 3d5f4ce..f705c1e 100644
--- a/Sources/P4Runtime/Parser.swift
+++ b/Sources/P4Runtime/Parser.swift
@@ -20,19 +20,20 @@ import P4Lang
 
 extension ParserAssignmentStatement: EvaluatableStatement {
   public func evaluate(execution: ProgramExecution) -> (ControlFlow, ProgramExecution) {
-    let result = self.value.evaluate(execution: execution)
-    guard case Result.Ok(let value) = result else {
-      return (ControlFlow.Error, execution.setError(error: result.error()!))
+    let updated_execution = execution
+    let result = self.value.evaluate(execution: updated_execution)
+    guard case (.Ok(let value), let updated_execution) = result else {
+      return (ControlFlow.Error, execution.setError(error: result.0.error()!))
     }
 
     let maybe_updated_scopes = self.lvalue.set(
-      to: value, inScopes: execution.scopes, duringExecution: execution)
+      to: value, inScopes: execution.scopes, duringExecution: updated_execution)
     guard case Result.Ok(let updated_scopes) = maybe_updated_scopes else {
       return (ControlFlow.Error, execution.setError(error: maybe_updated_scopes.error()!))
     }
     execution.scopes = updated_scopes.0
 
-    return (ControlFlow.Next, execution)
+    return (ControlFlow.Next, updated_execution)
   }
 }
 
@@ -116,9 +117,8 @@ extension ParserStateSelectTransition: EvaluatableParserState {
       }
     }
 
-    let res = self.selectExpression.evaluate(execution: program)
-
-    if case .Ok(let value) = res {
+    switch self.selectExpression.evaluate(execution: program) {
+    case (.Ok(let value), let program):
       if value.type().dataType().eq(rhs: self) {
         return (value.dataValue() as! EvaluatableParserState, program.exit_scope())
       } else {
@@ -128,10 +128,8 @@ extension ParserStateSelectTransition: EvaluatableParserState {
             error: Error(withMessage: "Select transition transitioned to a none state"))
         )
       }
+    case (.Error(let e), let program): return (self, program.setError(error: e).exit_scope())
     }
-
-    program = program.setError(error: res.error()!).exit_scope()
-    return (self, program.exit_scope())
   }
 
   public func done() -> Bool {
@@ -143,7 +141,7 @@ extension ParserStateSelectTransition: EvaluatableParserState {
   }
 }
 
-extension Parser: CallableExecution {
+extension Parser: LibraryCallable {
   public typealias T = InstantiatedParserState
   public func call(
     execution: Common.ProgramExecution, arguments: P4Lang.ArgumentList
@@ -178,34 +176,24 @@ extension Parser: CallableExecution {
       )
     }
 
-    // Now that we are assured that there is a start state,
-    // let's set the arguments.
-
-    if case .Error(let e) = arguments.compatible(self.parameters) {
-      return (
-        reject, execution.setError(error: Error(withMessage: "Cannot call parser: \(e)"))
-      )
-    }
-
-    for (parameter, argument) in zip(self.parameters.parameters, arguments.arguments) {
-      let arg_idx = argument.index
-      let arg_value = argument.argument
-      let maybe_argument_value = arg_value.evaluate(execution: execution)
-      guard case .Ok(let argument_value) = maybe_argument_value else {
-        return (
-          reject,
-          execution.setError(
-            error: Error(withMessage: "Cannot evaluate argument \(arg_idx): \(argument)"))
-        )
+    let call_body: (ProgramExecution) -> (Result<P4Value>, ProgramExecution) = {
+      (execution: ProgramExecution) in
+      var current_execution = execution
+      // Evaluate until the state is either accept or reject.
+      while !current_state.done() && !current_execution.hasError() {
+        (current_state, current_execution) = current_state.execute(program: current_execution)
       }
-      execution = execution.declare(identifier: parameter.name, withValue: argument_value)
+      return (.Ok(P4Value(AsInstantiatedParserState(current_state.state()))), current_execution)
     }
 
-    // Evaluate until the state is either accept or reject.
-    while !current_state.done() && !execution.hasError() {
-      (current_state, execution) = current_state.execute(program: execution)
+    return
+      switch Call(
+        body: call_body, withArguments: arguments, withParameters: parameters, inExecution: execution)
+    {
+    case (.Ok(let value), let updated_execution):
+      (value.dataValue() as! InstantiatedParserState, updated_execution)
+    case (.Error(let e), let updated_execution):
+      (reject, updated_execution.setError(error: Error(withMessage: "Cannot call parser: \(e)")))
     }
-
-    return (AsInstantiatedParserState(current_state.state()), execution.exit_scope())
   }
 }
diff --git a/Sources/P4Runtime/Protocols.swift b/Sources/P4Runtime/Protocols.swift
index c8088fb..c131ea0 100644
--- a/Sources/P4Runtime/Protocols.swift
+++ b/Sources/P4Runtime/Protocols.swift
@@ -28,7 +28,8 @@ public protocol EvaluatableParserState: P4DataValue {
   func state() -> ParserState
 }
 
-public protocol CallableExecution<T> {
+/// Defines an interface for P4 components that can be invoked directly by the p4rse library user
+public protocol LibraryCallable<T> {
   associatedtype T
   func call(execution: ProgramExecution, arguments: ArgumentList) -> (T, ProgramExecution)
 }
diff --git a/Sources/P4Runtime/Statements.swift b/Sources/P4Runtime/Statements.swift
index 0d47ad9..977ba53 100644
--- a/Sources/P4Runtime/Statements.swift
+++ b/Sources/P4Runtime/Statements.swift
@@ -41,7 +41,7 @@ extension BlockStatement: EvaluatableStatement {
 
 extension VariableDeclarationStatement: EvaluatableStatement {
   public func evaluate(execution: ProgramExecution) -> (ControlFlow, ProgramExecution) {
-    guard case .Ok(let initial_value) = self.initializer.evaluate(execution: execution) else {
+    guard case (.Ok(let initial_value), let execution) = self.initializer.evaluate(execution: execution) else {
       return (
         ControlFlow.Error,
         execution.setError(error: Error(withMessage: "Could not evaluate \(self.initializer)"))
@@ -55,7 +55,7 @@ extension VariableDeclarationStatement: EvaluatableStatement {
 
 extension ConditionalStatement: EvaluatableStatement {
   public func evaluate(execution: ProgramExecution) -> (ControlFlow, ProgramExecution) {
-    guard case .Ok(let evaluated_condition) = self.condition.evaluate(execution: execution) else {
+    guard case (.Ok(let evaluated_condition), let execution) = self.condition.evaluate(execution: execution) else {
       return (
         ControlFlow.Error,
         execution.setError(error: Error(withMessage: "Could not evaluate \(self.condition)"))
@@ -108,8 +108,8 @@ extension ExpressionStatement: EvaluatableStatement {
 extension ReturnStatement: EvaluatableStatement {
   public func evaluate(execution: ProgramExecution) -> (ControlFlow, ProgramExecution) {
     return switch self.value.evaluate(execution: execution) {
-    case .Ok(let v): (ControlFlow.Return(v), execution)
-    case .Error(let e): (ControlFlow.Error, execution.setError(error: e))
+    case (.Ok(let v), let execution): (ControlFlow.Return(v), execution)
+    case (.Error(let e), let execution): (ControlFlow.Error, execution.setError(error: e))
     }
   }
 }
diff --git a/Tests/p4rseTests/Declarations.swift b/Tests/p4rseTests/Declarations.swift
index 5a99f20..f99936e 100644
--- a/Tests/p4rseTests/Declarations.swift
+++ b/Tests/p4rseTests/Declarations.swift
@@ -172,6 +172,34 @@ import TreeSitterP4
         x = true;
       };
    """
-  #expect(#RequireOkResult(Program.Compile(simple_parser_declaration)))
+  #expect(
+    #RequireErrorResult(
+      Error(
+        withMessage:
+        "{56, 21}: Failed to parse a statement element: {63, 9}: Failed to parse a statement element: {63, 1}: Cannot assign value with type Boolean to identifier x that is in parameter"
+      ),
+      Program.Compile(simple_parser_declaration))
+  )
+}
+
+@Test func test_function_declaration_with_parameters_and_direction_struct() async throws {
+  let simple_parser_declaration = """
+     struct Testing {
+       bool yesno;
+       int count;
+     };
+     bool functionb(in Testing x, out string y, inout int z) {
+        x.yesno = true;
+      };
+   """
+  #expect(
+    #RequireErrorResult(
+      Error(
+        withMessage:
+        "{113, 27}: Failed to parse a statement element: {120, 15}: Failed to parse a statement element: {120, 7}: Cannot assign to field yesno of x that is in parameter"
+      ),
+      Program.Compile(simple_parser_declaration))
+  )
+
 }
 
diff --git a/Tests/p4rseTests/ExpressionTests/FunctionCall.swift b/Tests/p4rseTests/ExpressionTests/FunctionCall.swift
index 0a675ed..c8a6e37 100644
--- a/Tests/p4rseTests/ExpressionTests/FunctionCall.swift
+++ b/Tests/p4rseTests/ExpressionTests/FunctionCall.swift
@@ -83,6 +83,35 @@ import TreeSitterP4
   #expect(AsInstantiatedParserState(state_result) == P4Lang.reject)
 }
 
+@Test func test_function_call_scoped_name_collision_inout() async throws {
+  let simple_parser_declaration = """
+      bool functionb(inout bool c) {
+        c = true;
+        return c;
+      };
+      parser main_parser() {
+        state start {
+          bool c = false;
+          bool b = functionb(c);
+          transition select (c) {
+            false: reject;
+            true: accept;
+          };
+        }
+      };
+    """
+
+  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, _) = try! #UseOkResult(runtime.run())
+
+  #expect(parser.states.count() == 1)
+
+  #expect(AsInstantiatedParserState(state_result) == P4Lang.accept)
+}
+
+
 @Test func test_function_call_integer_return_value() async throws {
   let simple_parser_declaration = """
       int functionb(int c) {
diff --git a/Tests/p4rseTests/StructTests.swift b/Tests/p4rseTests/StructTests.swift
index 1840d5e..5da57be 100644
--- a/Tests/p4rseTests/StructTests.swift
+++ b/Tests/p4rseTests/StructTests.swift
@@ -327,7 +327,7 @@ import TreeSitterP4
   #expect(
     #RequireErrorResult(
       Error(
-        withMessage: "{49, 13}: Failed to parse a statement element: {49, 8}: Cannot assign value of type Int to field with type Boolean"
+        withMessage: "{49, 13}: Failed to parse a statement element: {49, 8}: Cannot assign value of type Int to field yesno of type Boolean"
       ),
       Program.Compile(simple_parser_declaration, withGlobalInstances: test_declarations))
   )
@@ -457,7 +457,7 @@ import TreeSitterP4
   #expect(
     #RequireErrorResult(
       Error(
-        withMessage: "{49, 20}: Failed to parse a statement element: {49, 11}: Cannot assign value of type Boolean to field with type Int"
+        withMessage: "{49, 20}: Failed to parse a statement element: {49, 11}: Cannot assign value of type Boolean to field count of type Int"
       ),
       Program.Compile(simple_parser_declaration, withGlobalInstances: test_declarations))
   )