Support Function Calls

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

Sources/Common/Execution.swift

@@ -21,6 +21,13 @@ open class ProgramExecution: CustomStringConvertible {
   var error: Error?
   var debug: DebugLevel = DebugLevel.Error
 
+  init(copy: ProgramExecution) {
+    self.scopes = copy.scopes
+    self.initialValues = copy.initialValues
+    self.error = copy.error
+    self.debug = copy.debug
+  }
+
   public init() {
     initialValues = .none
   }
@@ -42,7 +49,7 @@ open class ProgramExecution: CustomStringConvertible {
   }
 
   public func setError(error: Error) -> ProgramExecution {
-    let npe = self
+    let npe = ProgramExecution(copy: self)
     npe.error = error
     return npe
   }
@@ -52,7 +59,7 @@ open class ProgramExecution: CustomStringConvertible {
   }
 
   public func setDebugLevel(_ dl: DebugLevel) -> ProgramExecution {
-    let pe = self
+    let pe = ProgramExecution(copy: self)
     pe.debug = dl
     return pe
   }
@@ -67,22 +74,22 @@ open class ProgramExecution: CustomStringConvertible {
   }
 
   public func enter_scope() -> ProgramExecution {
-    let new_pe = self
-    new_pe.scopes = self.scopes.enter()
+    let new_pe = ProgramExecution(copy: self)
+    new_pe.scopes = new_pe.scopes.enter()
 
     return new_pe
   }
 
   public func exit_scope() -> ProgramExecution {
-    let new_pe = self
-    new_pe.scopes = self.scopes.exit()
+    let new_pe = ProgramExecution(copy: self)
+    new_pe.scopes = new_pe.scopes.exit()
 
     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)
+    let new_pe = ProgramExecution(copy: self)
+    let new_scopes = new_pe.scopes.declare(identifier: identifier, withValue: value)
 
     new_pe.scopes = new_scopes
     return new_pe
@@ -98,3 +105,12 @@ public typealias VarValueScope = Scope<P4Value>
 
 /// Scopes that resolves variable identifiers to their values.
 public typealias VarValueScopes = Scopes<P4Value>
+
+/// Indicate the control flow result of a particular statement.
+public enum ControlFlow {
+    case Next
+    case Continue
+    case Break
+    case Return(P4Value?)
+    case Error
+}

Sources/Common/Protocols.swift

@@ -28,8 +28,10 @@ public protocol EvaluatableStatement {
   /// Evaluate a statement for a given execution
   /// - Parameters
   ///  - execution: The execution context in which to evaluate the parser statement
-  /// - Returns: An updated execution after evaluating the parser statement
-  func evaluate(execution: ProgramExecution) -> ProgramExecution
+  /// - Returns: A tuple of
+  /// 1. Whether this statement affects control flow.
+  /// 2. An updated execution after evaluating the parser statement
+  func evaluate(execution: ProgramExecution) -> (ControlFlow, ProgramExecution)
 }
 
 public protocol P4Type: CustomStringConvertible {

Sources/P4Compiler/Common.swift

@@ -0,0 +1,318 @@
+// 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
+import P4Runtime
+import SwiftTreeSitter
+import TreeSitterExtensions
+import TreeSitterP4
+
+func parameter_list_compiler(
+  node: SwiftTreeSitter.Node, withContext context: CompilerContext
+) -> Common.Result<(ParameterList, CompilerContext)> {
+
+  var currentChildIdx = 0
+  var currentChildIdxSafe = 1
+  var currentChild: Node? = .none
+
+  if node.text == ")" {
+    // There are no parameters!
+    return Result.Ok((ParameterList([]), context))
+  }
+
+  #RequireNodeType<Node, (ParameterList, CompilerContext)>(
+    node: node, type: "parameter_list", nice_type_name: "Parameter List")
+
+  var parameters: ParameterList = ParameterList([])
+
+  if node.childCount < currentChildIdxSafe {
+    return Result.Error(
+      ErrorOnNode(node: node, withError: "Missing parameter list component"))
+  }
+
+  currentChild = node.child(at: currentChildIdx)
+  if currentChild?.nodeType == "parameter_list" {
+    switch parameter_list_compiler(node: currentChild!, withContext: context) {
+    case .Ok(let (ps, _)):
+      parameters = ps
+    case .Error(let e): return Result.Error(e)
+    }
+
+    currentChildIdx += 1
+    currentChildIdxSafe += 1
+  }
+
+  // We may have moved nodes, check/reset currentChild.
+  if node.childCount < currentChildIdxSafe {
+    return Result.Error(
+      ErrorOnNode(node: node, withError: "Missing parameter list component"))
+  }
+  currentChild = node.child(at: currentChildIdx)
+
+  // If this is a ')', we are done.
+  if currentChild?.text == ")" {
+    return Result.Ok((parameters, context))
+  }
+
+  // If this is a comma, we skip it!
+  if currentChild?.text == "," {
+    currentChildIdx += 1
+    currentChildIdxSafe += 1
+  }
+
+  if node.childCount < currentChildIdxSafe {
+    return Result.Error(
+      ErrorOnNode(node: node, withError: "Missing parameter list component"))
+  }
+  currentChild = node.child(at: currentChildIdx)
+
+  // 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 .Error(let e): return Result.Error(e)
+  }
+}
+
+extension ParameterList: Compilable {
+  public typealias T = ParameterList
+  public static func Compile(
+    node: SwiftTreeSitter.Node, withContext context: CompilerContext
+  ) -> Common.Result<(ParameterList, CompilerContext)> {
+
+    let parameter_node = node
+    #RequireNodeType<Node, (ParameterList, CompilerContext)>(
+      node: parameter_node, type: "parameters", nice_type_name: "Parameters")
+
+    var currentChildIdx = 0
+    var currentChildIdxSafe = 1
+
+    // Let's eat the '(' before we start ...
+    if parameter_node.childCount < currentChildIdxSafe {
+      return .Error(
+        ErrorOnNode(node: parameter_node, withError: "Missing '(' in parameter list component"))
+    }
+
+    currentChildIdx += 1
+    currentChildIdxSafe += 1
+    if parameter_node.childCount < currentChildIdxSafe {
+      return .Error(
+        ErrorOnNode(node: parameter_node, withError: "Missing parameter list component"))
+    }
+    let currentChild = parameter_node.child(at: currentChildIdx)
+
+    return parameter_list_compiler(node: currentChild!, withContext: context)
+  }
+}
+
+extension Parameter: Compilable {
+  public typealias T = Parameter
+  public static func Compile(
+    node: Node, withContext context: CompilerContext
+  ) -> Result<(Parameter, CompilerContext)> {
+
+    #RequireNodeType<Node, (EvaluatableStatement, CompilerContext)>(
+      node: node, type: "parameter", nice_type_name: "parameter")
+
+    var currentChildIdx = 0
+    var currentChildIdxSafe = 1
+    var currentChild: Node? = .none
+
+    if node.childCount < currentChildIdxSafe {
+      return .Error(
+        ErrorOnNode(node: node, withError: "Missing parameter declaration component"))
+    }
+
+    currentChild = node.child(at: currentChildIdx)
+
+    // Annotation?
+    if currentChild!.nodeType == "annotations" {
+      return .Error(
+        ErrorOnNode(
+          node: currentChild!,
+          withError: "Annotations in parameter declarations are not yet handled"))
+      // Will increment indexes here.
+    }
+
+    // Direction?
+    if currentChild!.nodeType == "direction" {
+      return .Error(
+        ErrorOnNode(
+          node: currentChild!, withError: "Direction in parameter declarations are not yet handled"
+        ))
+      // Will increment indexes here.
+    }
+
+    if currentChild!.nodeType != "typeRef" {
+      return Result.Error(
+        ErrorOnNode(
+          node: node, withError: "Did not find type name for parameter declaration"))
+    }
+
+    guard
+      case .Ok(let parameter_type) = Types.CompileType(type: currentChild!, withContext: context)
+    else {
+      return Result.Error(
+        Error(withMessage: "Could not parse a P4 type from \(currentChild!.text!)"))
+    }
+
+    currentChildIdx += 1
+    currentChildIdxSafe += 1
+    if node.childCount < currentChildIdxSafe {
+      return .Error(
+        ErrorOnNode(node: node, withError: "Missing parameter declaration component"))
+    }
+
+    currentChild = node.child(at: currentChildIdx)
+    if currentChild!.nodeType != "identifier" {
+      return Result.Error(
+        ErrorOnNode(
+          node: node, withError: "Did not find identifier for parameter statement"))
+    }
+
+    guard
+      case .Ok(let parameter_name) = Identifier.Compile(node: currentChild!, withContext: context)
+    else {
+      return Result.Error(
+        Error(withMessage: "Could not parse a parameter name from \(currentChild!.text!)"))
+    }
+
+    return Result.Ok(
+      (
+        Parameter(
+          identifier: parameter_name, withType: parameter_type),
+        context
+      ))
+  }
+}
+
+func argument_list_compiler(
+  node: SwiftTreeSitter.Node, withContext context: CompilerContext
+) -> Common.Result<(ArgumentList, CompilerContext)> {
+
+  var currentChildIdx = 0
+  var currentChildIdxSafe = 1
+  var currentChild: Node? = .none
+
+  if node.text == ")" {
+    // There are no arguments!
+    return Result.Ok((ArgumentList([]), context))
+  }
+
+  #RequireNodeType<Node, (ArgumentList, CompilerContext)>(
+    node: node, type: "argument_list", nice_type_name: "argument List")
+
+  var arguments: ArgumentList = ArgumentList([])
+
+  if node.childCount < currentChildIdxSafe {
+    return Result.Error(
+      ErrorOnNode(node: node, withError: "Missing argument list component"))
+  }
+
+  currentChild = node.child(at: currentChildIdx)
+  if currentChild?.nodeType == "argument_list" {
+    switch argument_list_compiler(node: currentChild!, withContext: context) {
+    case .Ok(let (ps, _)):
+      arguments = ps
+    case .Error(let e): return Result.Error(e)
+    }
+
+    currentChildIdx += 1
+    currentChildIdxSafe += 1
+  }
+
+  // We may have moved nodes, check/reset currentChild.
+  if node.childCount < currentChildIdxSafe {
+    return Result.Error(
+      ErrorOnNode(node: node, withError: "Missing argument list component"))
+  }
+  currentChild = node.child(at: currentChildIdx)
+
+  // If this is a ')', we are done.
+  if currentChild?.text == ")" {
+    return Result.Ok((arguments, context))
+  }
+
+  // If this is a comma, we skip it!
+  if currentChild?.text == "," {
+    currentChildIdx += 1
+    currentChildIdxSafe += 1
+  }
+
+  if node.childCount < currentChildIdxSafe {
+    return Result.Error(
+      ErrorOnNode(node: node, withError: "Missing argument list component"))
+  }
+  currentChild = node.child(at: currentChildIdx)
+
+  // Otherwise, there should be one argument left!
+  switch Argument.Compile(node: currentChild!, withContext: context) {
+  case .Ok(let (ce, updated_context)):
+    return Result.Ok((arguments.addArgument(Argument(ce, atIndex: arguments.count() + 1)), updated_context))
+  case .Error(let e): return Result.Error(e)
+  }
+}
+
+extension ArgumentList: Compilable {
+  public typealias T = ArgumentList
+  public static func Compile(
+    node: SwiftTreeSitter.Node, withContext context: CompilerContext
+  ) -> Common.Result<(ArgumentList, CompilerContext)> {
+
+    let argument_node = node
+    #RequireNodeType<Node, (ArgumentList, CompilerContext)>(
+      node: argument_node, type: "arguments", nice_type_name: "arguments")
+
+    var currentChildIdx = 0
+    var currentChildIdxSafe = 1
+
+    // Let's eat the '(' before we start ...
+    if argument_node.childCount < currentChildIdxSafe {
+      return .Error(
+        ErrorOnNode(node: argument_node, withError: "Missing '(' in argument list component"))
+    }
+
+    currentChildIdx += 1
+    currentChildIdxSafe += 1
+    if argument_node.childCount < currentChildIdxSafe {
+      return .Error(
+        ErrorOnNode(node: argument_node, withError: "Missing argument list component"))
+    }
+    let currentChild = argument_node.child(at: currentChildIdx)
+
+    return argument_list_compiler(node: currentChild!, withContext: context)
+  }
+}
+
+extension Argument: Compilable {
+  public typealias T = EvaluatableExpression
+  public static func Compile(
+    node: SwiftTreeSitter.Node, withContext context: CompilerContext
+  ) -> Common.Result<(EvaluatableExpression, CompilerContext)> {
+    let argument_node = node
+    #RequireNodeType<Node, (EvaluatableExpression, CompilerContext)>(
+      node: argument_node, type: "argument", nice_type_name: "argument")
+
+    let expression_node = node.child(at: 0)!
+
+    return switch Expression.Compile(node: expression_node, withContext: context) {
+        case .Ok(let compiled_expression): .Ok((compiled_expression, context))
+        case .Error(let e): .Error(e)
+    }
+  }
+}

Sources/P4Compiler/Declarations.swift

@@ -379,185 +379,6 @@ extension P4Lang.Parser: CompilableDeclaration {
   }
 }
 
-func parameter_list_compiler(
-  node: SwiftTreeSitter.Node, withContext context: CompilerContext
-) -> Common.Result<(ParameterList, CompilerContext)> {
-
-  var currentChildIdx = 0
-  var currentChildIdxSafe = 1
-  var currentChild: Node? = .none
-
-  if node.text == ")" {
-    // There are no parameters!
-    return Result.Ok((ParameterList([]), context))
-  }
-
-  #RequireNodeType<Node, (ParameterList, CompilerContext)>(
-    node: node, type: "parameter_list", nice_type_name: "Parameter List")
-
-  var parameters: ParameterList = ParameterList([])
-
-  if node.childCount < currentChildIdxSafe {
-    return Result.Error(
-      ErrorOnNode(node: node, withError: "Missing parameter list component"))
-  }
-
-  currentChild = node.child(at: currentChildIdx)
-  if currentChild?.nodeType == "parameter_list" {
-    switch parameter_list_compiler(node: currentChild!, withContext: context) {
-    case .Ok(let (ps, _)):
-      parameters = ps
-    case .Error(let e): return Result.Error(e)
-    }
-
-    currentChildIdx += 1
-    currentChildIdxSafe += 1
-  }
-
-  // We may have moved nodes, check/reset currentChild.
-  if node.childCount < currentChildIdxSafe {
-    return Result.Error(
-      ErrorOnNode(node: node, withError: "Missing parameter list component"))
-  }
-  currentChild = node.child(at: currentChildIdx)
-
-  // If this is a ')', we are done.
-  if currentChild?.text == ")" {
-    return Result.Ok((parameters, context))
-  }
-
-  // If this is a comma, we skip it!
-  if currentChild?.text == "," {
-    currentChildIdx += 1
-    currentChildIdxSafe += 1
-  }
-
-  if node.childCount < currentChildIdxSafe {
-    return Result.Error(
-      ErrorOnNode(node: node, withError: "Missing parameter list component"))
-  }
-  currentChild = node.child(at: currentChildIdx)
-
-  // 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 .Error(let e): return Result.Error(e)
-  }
-}
-
-extension ParameterList: Compilable {
-  public typealias T = ParameterList
-  public static func Compile(
-    node: SwiftTreeSitter.Node, withContext context: CompilerContext
-  ) -> Common.Result<(ParameterList, CompilerContext)> {
-
-    let parameter_node = node
-    #RequireNodeType<Node, (ParameterList, CompilerContext)>(
-      node: parameter_node, type: "parameters", nice_type_name: "Parameters")
-
-    var currentChildIdx = 0
-    var currentChildIdxSafe = 1
-
-    // Let's eat the '(' before we start ...
-    if parameter_node.childCount < currentChildIdxSafe {
-      return .Error(
-        ErrorOnNode(node: parameter_node, withError: "Missing '(' in parameter list component"))
-    }
-
-    currentChildIdx += 1
-    currentChildIdxSafe += 1
-    if parameter_node.childCount < currentChildIdxSafe {
-      return .Error(
-        ErrorOnNode(node: parameter_node, withError: "Missing parameter list component"))
-    }
-    let currentChild = parameter_node.child(at: currentChildIdx)
-
-    return parameter_list_compiler(node: currentChild!, withContext: context)
-  }
-}
-
-extension Parameter: Compilable {
-  public typealias T = Parameter
-  public static func Compile(
-    node: Node, withContext context: CompilerContext
-  ) -> Result<(Parameter, CompilerContext)> {
-
-    #RequireNodeType<Node, (EvaluatableStatement, CompilerContext)>(
-      node: node, type: "parameter", nice_type_name: "parameter")
-
-    var currentChildIdx = 0
-    var currentChildIdxSafe = 1
-    var currentChild: Node? = .none
-
-    if node.childCount < currentChildIdxSafe {
-      return .Error(
-        ErrorOnNode(node: node, withError: "Missing parameter declaration component"))
-    }
-
-    currentChild = node.child(at: currentChildIdx)
-
-    // Annotation?
-    if currentChild!.nodeType == "annotations" {
-      return .Error(
-        ErrorOnNode(
-          node: currentChild!,
-          withError: "Annotations in parameter declarations are not yet handled"))
-      // Will increment indexes here.
-    }
-
-    // Direction?
-    if currentChild!.nodeType == "direction" {
-      return .Error(
-        ErrorOnNode(
-          node: currentChild!, withError: "Direction in parameter declarations are not yet handled"
-        ))
-      // Will increment indexes here.
-    }
-
-    if currentChild!.nodeType != "typeRef" {
-      return Result.Error(
-        ErrorOnNode(
-          node: node, withError: "Did not find type name for parameter declaration"))
-    }
-
-    guard
-      case .Ok(let parameter_type) = Types.CompileType(type: currentChild!, withContext: context)
-    else {
-      return Result.Error(
-        Error(withMessage: "Could not parse a P4 type from \(currentChild!.text!)"))
-    }
-
-    currentChildIdx += 1
-    currentChildIdxSafe += 1
-    if node.childCount < currentChildIdxSafe {
-      return .Error(
-        ErrorOnNode(node: node, withError: "Missing parameter declaration component"))
-    }
-
-    currentChild = node.child(at: currentChildIdx)
-    if currentChild!.nodeType != "identifier" {
-      return Result.Error(
-        ErrorOnNode(
-          node: node, withError: "Did not find identifier for parameter statement"))
-    }
-
-    guard
-      case .Ok(let parameter_name) = Identifier.Compile(node: currentChild!, withContext: context)
-    else {
-      return Result.Error(
-        Error(withMessage: "Could not parse a parameter name from \(currentChild!.text!)"))
-    }
-
-    return Result.Ok(
-      (
-        Parameter(
-          identifier: parameter_name, withType: parameter_type),
-        context
-      ))
-  }
-}
-
 extension Control: CompilableDeclaration {
   public static func Compile(
     node: SwiftTreeSitter.Node, withContext context: CompilerContext

Sources/P4Compiler/Expression.swift

@@ -163,6 +163,7 @@ struct Expression {
     let expression_parsers: [CompilableExpression.Type] = [
       P4BooleanValue.self, P4StringValue.self, P4IntValue.self, TypedIdentifier.self,
       BinaryOperatorExpression.self, ArrayAccessExpression.self, FieldAccessExpression.self,
+      FunctionCall.self
     ]
 
     for candidate_expression_parser in expression_parsers {
@@ -653,3 +654,67 @@ extension ArrayAccessExpression: CompilableLValueExpression {
     return Result.Ok(array_access_expression)
   }
 }
+
+extension FunctionCall: CompilableExpression {
+  static func compile(
+    node: Node, withContext context: CompilerContext
+  ) -> Result<EvaluatableExpression?> {
+    let expression = node.child(at: 0)!
+    #SkipUnlessNodeType<Node, EvaluatableExpression?>(
+      node: expression, type: "function_call")
+
+    var currentChildIdx = 0
+    var currentChildIdxSafe = 1
+    var currentChild: Node? = .none
+
+    if expression.childCount < currentChildIdxSafe {
+      return Result.Error(
+        ErrorOnNode(node: node, withError: "Missing function call component"))
+    }
+
+    currentChild = expression.child(at: currentChildIdx)
+
+    let maybe_callee_name = Identifier.Compile(
+      node: currentChild!, withContext: context)
+    guard case .Ok(let callee_name) = maybe_callee_name else {
+      return Result.Error(maybe_callee_name.error()!)
+    }
+
+    let maybe_callee = switch context.types.lookup(identifier: callee_name) {
+      case .Ok(let looked_up): switch looked_up {
+        case let callee as FunctionDeclaration: Result.Ok(callee) // What we found is actually a function declaration
+        default: Result<FunctionDeclaration>.Error(ErrorOnNode(node: currentChild!, withError: "\(callee_name) is not a function"))
+      }
+      case .Error(let e): Result<FunctionDeclaration>.Error(e)
+    }
+
+    guard case .Ok(let callee) = maybe_callee else {
+      return .Error(maybe_callee.error()!)
+    }
+
+
+    currentChildIdx += 1
+    currentChildIdxSafe += 1
+    if expression.childCount < currentChildIdxSafe {
+      return Result.Error(
+        ErrorOnNode(node: node, withError: "Missing function call component"))
+    }
+    currentChild = expression.child(at: currentChildIdx)
+
+    let maybe_argument_list = ArgumentList.Compile(node: currentChild!, withContext: context)
+    
+    guard case .Ok((let arguments, _)) = maybe_argument_list  else {
+      return .Error(maybe_argument_list.error()!)
+    }
+
+    // Now, compare the arguments with the parameters:
+
+    if case .Error(let e) = arguments.compatible(callee.params) {
+      return .Error(e)
+    }
+
+    // All good!
+
+    return .Ok(FunctionCall(callee, withArguments: arguments))
+  }
+}

Sources/P4Compiler/Parser.swift

@@ -62,6 +62,7 @@ public struct Parser {
         "expressionStatement": ExpressionStatement.self,
         "variableDeclaration": VariableDeclarationStatement.self,
         "conditionalStatement": ConditionalStatement.self, "blockStatement": BlockStatement.self,
+        "return_statement": ReturnStatement.self,
       ]
       guard let parser = statementParsers[statement.nodeType ?? ""] else {
         return Result.Error(

Sources/P4Compiler/Statement.swift

@@ -299,3 +299,19 @@ extension ParserAssignmentStatement: CompilableStatement {
       ))
   }
 }
+
+extension ReturnStatement: CompilableStatement {
+  public static func Compile(
+    node: SwiftTreeSitter.Node, withContext context: CompilerContext
+  ) -> Common.Result<(any Common.EvaluatableStatement, CompilerContext)> {
+    #RequireNodeType<Node, (EvaluatableStatement, CompilerContext)>(
+      node: node, type: "return_statement", nice_type_name: "return statement")
+
+    let expression_node = node.child(at: 1)!
+
+    return switch Expression.Compile(node: expression_node, withContext: context) {
+    case .Ok(let result): .Ok((ReturnStatement(result), context))
+    case .Error(let e): .Error(e)
+    }
+  }
+}

Sources/P4Lang/Common.swift

@@ -0,0 +1,120 @@
+// 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 struct Parameter: CustomStringConvertible, Equatable {
+  public static func == (lhs: Parameter, rhs: Parameter) -> Bool {
+    return lhs.name == rhs.name && lhs.type.eq(rhs: rhs.type)
+  }
+
+  public var name: Identifier
+  public var type: P4Type
+
+  public init(
+    identifier: Identifier, withType type: P4Type
+  ) {
+    self.name = identifier
+    self.type = type
+  }
+
+  public var description: String {
+    return "Parameter: \(self.name) with type \(self.type)"
+  }
+}
+
+public struct ParameterList: CustomStringConvertible, Equatable {
+  public static func == (lhs: ParameterList, rhs: ParameterList) -> Bool {
+    if lhs.parameters.count != rhs.parameters.count {
+      return false
+    }
+
+    return 0
+      == zip(lhs.parameters, rhs.parameters).count { (lparam, rparam) in
+        return lparam != rparam
+      }
+  }
+
+  public var parameters: [Parameter]
+
+  public init() {
+    self.parameters = Array()
+  }
+
+  public init(_ parameters: [Parameter]) {
+    self.parameters = parameters
+  }
+
+  public func addParameter(_ parameter: Parameter) -> ParameterList {
+    return ParameterList(self.parameters + [parameter])
+  }
+
+  public var description: String {
+    let parameters = self.parameters.map { parameter in
+      parameter.description
+    }.joined(separator: ";")
+    return "Parameter list: \(parameters)"
+  }
+}
+
+public struct ArgumentList {
+  public let arguments: [Argument]
+
+  public init(_ arguments: [Argument]) {
+    self.arguments = arguments
+  }
+
+  public func compatible(_ parameters: ParameterList) -> Result<()> {
+    if self.arguments.count != parameters.parameters.count {
+      return .Error(
+        Error(
+          withMessage:
+            "\(self.arguments.count) arguments found but \(parameters.parameters.count) required"))
+    }
+
+    for (arg, param) in zip(self.arguments, parameters.parameters) {
+      let arg_index = arg.index
+      let arg_type = arg.argument.type()
+      if !arg_type.eq(rhs: param.type) {
+        return .Error(
+          Error(
+            withMessage:
+              "Argument \(arg_index)'s type (\(arg_type)) is incompatible with the parameter type (\(param.type))"
+          ))
+      }
+    }
+    return .Ok(())
+  }
+
+  public func addArgument(_ argument: Argument) -> ArgumentList {
+    return ArgumentList(self.arguments + [argument])
+  }
+
+  public func count() -> Int {
+    return self.arguments.count
+  }
+}
+
+public struct Argument {
+  public let index: Int
+  public let argument: EvaluatableExpression
+
+  public init(_ argument: EvaluatableExpression, atIndex index: Int) {
+    self.argument = argument
+    self.index = index
+  }
+}

Sources/P4Lang/Declarations.swift

@@ -19,60 +19,6 @@ import Common
 
 public struct Declaration {}
 
-public struct Parameter: CustomStringConvertible, Equatable {
-  public static func == (lhs: Parameter, rhs: Parameter) -> Bool {
-    return lhs.name == rhs.name && lhs.type.eq(rhs: rhs.type)
-  }
-
-  public var name: Identifier
-  public var type: P4Type
-
-  public init(
-    identifier: Identifier, withType type: P4Type
-  ) {
-    self.name = identifier
-    self.type = type
-  }
-
-  public var description: String {
-    return "Parameter: \(self.name) with type \(self.type)"
-  }
-}
-
-public struct ParameterList: CustomStringConvertible, Equatable {
-  public static func == (lhs: ParameterList, rhs: ParameterList) -> Bool {
-    if lhs.parameters.count != rhs.parameters.count {
-      return false
-    }
-
-    return 0
-      == zip(lhs.parameters, rhs.parameters).count { (lparam, rparam) in
-        return lparam != rparam
-      }
-  }
-
-  public var parameters: [Parameter]
-
-  public init() {
-    self.parameters = Array()
-  }
-
-  public init(_ parameters: [Parameter]) {
-    self.parameters = parameters
-  }
-
-  public func addParameter(_ parameter: Parameter) -> ParameterList {
-    return ParameterList(self.parameters + [parameter])
-  }
-
-  public var description: String {
-    let parameters = self.parameters.map { parameter in
-      parameter.description
-    }.joined(separator: ";")
-    return "Parameter list: \(parameters)"
-  }
-}
-
 public struct FunctionDeclaration: P4Type, P4Value {
   public func type() -> any Common.P4Type {
     return self

Sources/P4Lang/Expressions.swift

@@ -201,33 +201,12 @@ public struct FieldAccessExpression {
   }
 }
 
-public struct ArgumentList {
-  public let arguments: [(Int, EvaluatableExpression)]
-  public init(_ arguments: [EvaluatableExpression]) {
-    self.arguments = zip(1..., arguments).map { (idx, argument) in
-      (idx, argument)
-    }
-  }
+public struct FunctionCall {
+  public let callee: FunctionDeclaration
+  public let arguments: ArgumentList
 
-  public func compatible(_ parameters: ParameterList) -> Result<()> {
-    if self.arguments.count != parameters.parameters.count {
-      return .Error(
-        Error(
-          withMessage:
-            "\(self.arguments.count) arguments found but \(parameters.parameters.count) required"))
-    }
-
-    for (arg, param) in zip(self.arguments, parameters.parameters) {
-      let arg_index = arg.0
-      let arg_type = arg.1.type()
-      if !arg_type.eq(rhs: param.type) {
-        return .Error(
-          Error(
-            withMessage:
-              "Argument \(arg_index)'s type (\(arg_type)) is incompatible with the parameter type (\(param.type))"
-          ))
-      }
-    }
-    return .Ok(())
+  public init(_ callee: FunctionDeclaration, withArguments arguments: ArgumentList) {
+    self.callee = callee
+    self.arguments = arguments
   }
-}
+}

Sources/P4Lang/Statement.swift

@@ -55,3 +55,11 @@ public struct BlockStatement {
   }
 
 }
+
+public struct ReturnStatement {
+  public let value: EvaluatableExpression
+
+  public init(_ value: EvaluatableExpression) {
+    self.value = value
+  }
+}

Sources/P4Runtime/Common.swift

@@ -0,0 +1,19 @@
+// 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

Sources/P4Runtime/Expressions.swift

@@ -382,3 +382,40 @@ extension KeysetExpression: EvaluatableExpression {
     return self.kse_type()
   }
 }
+
+extension FunctionCall: EvaluatableExpression {
+  public func evaluate(execution: Common.ProgramExecution) -> Common.Result<any Common.P4Value> {
+
+    guard let body = self.callee.body else {
+      return .Error(Error(withMessage: "No body for called function (\(self.callee.name))"))
+    }
+
+    // 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)"))
+      }
+      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))"))
+    }
+  }
+
+  public func type() -> any Common.P4Type {
+    return self.callee.tipe
+  }
+}

Sources/P4Runtime/Parser.swift

@@ -19,20 +19,20 @@ import Common
 import P4Lang
 
 extension ParserAssignmentStatement: EvaluatableStatement {
-  public func evaluate(execution: ProgramExecution) -> ProgramExecution {
+  public func evaluate(execution: ProgramExecution) -> (ControlFlow, ProgramExecution) {
     let result = self.value.evaluate(execution: execution)
     guard case Result.Ok(let value) = result else {
-      return execution.setError(error: result.error()!)
+      return (ControlFlow.Error, execution.setError(error: result.error()!))
     }
 
     let maybe_updated_scopes = self.lvalue.set(
       to: value, inScopes: execution.scopes, duringExecution: execution)
     guard case Result.Ok(let updated_scopes) = maybe_updated_scopes else {
-      return execution.setError(error: maybe_updated_scopes.error()!)
+      return (ControlFlow.Error, execution.setError(error: maybe_updated_scopes.error()!))
     }
     execution.scopes = updated_scopes.0
 
-    return execution
+    return (ControlFlow.Next, execution)
   }
 }
 
@@ -43,7 +43,12 @@ extension ParserStateDirectTransition: EvaluatableParserState {
     var program = program.enter_scope()
 
     for statement in statements {
-      program = statement.evaluate(execution: program)
+      let (control_flow, next_program) = statement.evaluate(execution: program)
+      switch control_flow {
+        case .Next: program = next_program // Ok!
+        case .Error: return (reject, next_program)
+        default: return (reject, next_program.setError(error: Error(withMessage: "Invalid control flow (\(control_flow) in parser)")))
+      }
     }
     let res = program.scopes.lookup(identifier: get_next_state())
 
@@ -93,7 +98,12 @@ extension ParserStateSelectTransition: EvaluatableParserState {
 
     // First, evaluate the statements.
     for statement in statements {
-      program = statement.evaluate(execution: program)
+      let (control_flow, next_program) = statement.evaluate(execution: program)
+      switch control_flow {
+        case .Next: program = next_program // Ok!
+        case .Error: return (reject, next_program)
+        default: return (reject, next_program.setError(error: Error(withMessage: "Invalid control flow (\(control_flow) in parser)")))
+      }
     }
 
     let res = self.selectExpression.evaluate(execution: program)
@@ -159,8 +169,8 @@ extension Parser: CallableExecution {
     }
 
     for (parameter, argument) in zip(self.parameters.parameters, arguments.arguments) {
-      let arg_idx = argument.0
-      let arg_value = argument.1
+      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 (

Sources/P4Runtime/Protocols.swift

@@ -22,12 +22,6 @@ public protocol Execution {
   func execute(execution: ProgramExecution) -> ProgramExecution
 }
 
-public protocol Compilable {
-  associatedtype ToCompile
-  associatedtype Compiled
-  static func compile(_: ToCompile) -> Result<Compiled>
-}
-
 public protocol EvaluatableParserState: P4Value {
   func execute(program: ProgramExecution) -> (EvaluatableParserState, ProgramExecution)
   func done() -> Bool

Sources/P4Runtime/Statements.swift

@@ -19,51 +19,99 @@ import Common
 import P4Lang
 
 extension BlockStatement: EvaluatableStatement {
-  public func evaluate(execution: ProgramExecution) -> ProgramExecution {
+  public func evaluate(execution: ProgramExecution) -> (ControlFlow, ProgramExecution) {
     var execution = execution
     for s in self.statements {
-      execution = s.evaluate(execution: execution)
+      let (control_flow, next_execution) = s.evaluate(execution: execution)
+      switch control_flow {
+      case ControlFlow.Return(let value): return (ControlFlow.Return(value), next_execution)
+      case ControlFlow.Next: execution = next_execution
+      case ControlFlow.Error: return (ControlFlow.Error, next_execution)
+      default:
+        return (
+          ControlFlow.Next,
+          next_execution.setError(
+            error: Error(withMessage: "Invalid control flow \(control_flow) in block statement"))
+        )
+      }
     }
-    return execution
+    return (ControlFlow.Next, execution)
   }
 }
 
 extension VariableDeclarationStatement: EvaluatableStatement {
-  public func evaluate(execution: ProgramExecution) -> ProgramExecution {
+  public func evaluate(execution: ProgramExecution) -> (ControlFlow, ProgramExecution) {
     guard case .Ok(let initial_value) = self.initializer.evaluate(execution: execution) else {
-      return execution.setError(error: Error(withMessage: "Could not evaluate \(self.initializer)"))
+      return (
+        ControlFlow.Error,
+        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
+    return (ControlFlow.Next, execution)
   }
 }
 
 extension ConditionalStatement: EvaluatableStatement {
-  public func evaluate(execution: ProgramExecution) -> ProgramExecution {
+  public func evaluate(execution: ProgramExecution) -> (ControlFlow, ProgramExecution) {
     guard case .Ok(let evaluated_condition) = self.condition.evaluate(execution: execution) else {
-      return execution.setError(error: Error(withMessage: "Could not evaluate \(self.condition)"))
+      return (
+        ControlFlow.Error,
+        execution.setError(error: Error(withMessage: "Could not evaluate \(self.condition)"))
+      )
     }
+
     if !evaluated_condition.type().eq(rhs: P4Boolean()) {
-      return execution.setError(error: Error(withMessage: "Condition expression is not a Boolean"))
+      return (
+        ControlFlow.Error,
+        execution.setError(error: Error(withMessage: "Condition expression is not a Boolean"))
+      )
     }
+
     if evaluated_condition.eq(rhs: P4BooleanValue.init(withValue: true)) {
       let execution = execution.enter_scope()
-      var result = self.thenn.evaluate(execution: execution)
-      result = result.exit_scope()
-      return result
+      switch self.thenn.evaluate(execution: execution) {
+      case (ControlFlow.Next, let result): return (ControlFlow.Next, result.exit_scope())
+      case (ControlFlow.Error, let result): return (ControlFlow.Error, result.exit_scope())
+      case (let cf, let result):
+        return (
+          ControlFlow.Next,
+          result.setError(
+            error: Error(withMessage: "Invalid control flow \(cf) in conditional statement"))
+        )
+      }
     } else if let elss = self.elss {
       let execution = execution.enter_scope()
-      var result = elss.evaluate(execution: execution)
-      result = result.exit_scope()
-      return result
+      switch elss.evaluate(execution: execution) {
+      case (ControlFlow.Next, let result): return (ControlFlow.Next, result.exit_scope())
+      case (ControlFlow.Error, let result): return (ControlFlow.Error, result.exit_scope())
+      case (let cf, let result):
+        return (
+          ControlFlow.Next,
+          result.setError(
+            error: Error(withMessage: "Invalid control flow \(cf) in conditional statement"))
+        )
+      }
     }
-    return execution
+    return (ControlFlow.Next, execution)
   }
 }
 
 extension ExpressionStatement: EvaluatableStatement {
-  public func evaluate(execution: ProgramExecution) -> ProgramExecution {
-    return execution
+  public func evaluate(execution: ProgramExecution) -> (ControlFlow, ProgramExecution) {
+    // TODO: Should this do something? Side effects?
+    return (ControlFlow.Next, execution)
   }
 }
+
+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))
+    }
+  }
+}
+
+