gp4/Sources/P4Compiler/Expression.swift

// 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 TreeSitterP4

protocol CompilableExpression {
  static func compile(
    node: Node, withContext context: CompilerContext
  ) -> Result<EvaluatableExpression?>
}

protocol CompilableLValueExpression {
  static func compile_as_lvalue(
    node: Node, withContext context: CompilerContext
  ) -> Result<EvaluatableLValueExpression?>
}

extension TypedIdentifier: CompilableExpression {
  static func compile(
    node: SwiftTreeSitter.Node, withContext context: CompilerContext
  ) -> Result<EvaluatableExpression?> {

    let node = node.child(at: 0)!
    #SkipUnlessNodeType<SwiftTreeSitter.Node, EvaluatableExpression?>(
      node: node, type: "identifier")

    guard
      case Result.Ok(let attributed_type) = context.instances.lookup(
        identifier: Common.Identifier(name: node.text!))
    else {
      return .Error(ErrorOnNode(node: node, withError: "Cannot find \(node.text!) in scope"))
    }

    return .Ok(TypedIdentifier(name: node.text!, withType: attributed_type.type))
  }
}

extension TypedIdentifier: CompilableLValueExpression {
  static func compile_as_lvalue(
    node: SwiftTreeSitter.Node, withContext context: CompilerContext
  ) -> Result<EvaluatableLValueExpression?> {

    let expression = node.child(at: 0)!
    #SkipUnlessNodeType<SwiftTreeSitter.Node, EvaluatableExpression?>(
      node: expression, type: "identifier")

    let maybe_parsed_expression = TypedIdentifier.compile(node: node, withContext: context)
    guard case .Ok(let maybe_typed_identifier) = maybe_parsed_expression else {
      return .Error(maybe_parsed_expression.error()!)
    }

    let typed_identifier_expression = maybe_typed_identifier as! TypedIdentifier

    return Result.Ok(typed_identifier_expression)
  }
}

extension P4BooleanValue: CompilableExpression {
  static func compile(
    node: SwiftTreeSitter.Node, withContext context: CompilerContext
  ) -> Result<EvaluatableExpression?> {
    let node = node.child(at: 0)!
    #SkipUnlessNodeType<SwiftTreeSitter.Node, EvaluatableExpression?>(
      node: node, type: "booleanLiteralExpression")

    if node.text == "false" {
      return .Ok(P4BooleanValue(withValue: false))
    } else if node.text == "true" {
      return .Ok(P4BooleanValue(withValue: true))
    }

    return .Error(
      ErrorOnNode(node: node, withError: "Failed to parse boolean literal: \(node.text!)"))
  }
}

extension P4IntValue: CompilableExpression {
  static func compile(
    node: SwiftTreeSitter.Node, withContext context: CompilerContext
  ) -> Result<EvaluatableExpression?> {
    let node = node.child(at: 0)!
    #SkipUnlessNodeType<SwiftTreeSitter.Node, EvaluatableExpression?>(node: node, type: "integer")
    if let parsed_int = Int(node.text!) {
      return .Ok(P4IntValue(withValue: parsed_int))
    } else {
      return .Error(ErrorOnNode(node: node, withError: "Failed to parse integer: \(node.text!)"))
    }
  }
}

extension P4StringValue: CompilableExpression {
  static func compile(
    node: SwiftTreeSitter.Node, withContext scopes: CompilerContext
  ) -> Result<EvaluatableExpression?> {
    let node = node.child(at: 0)!
    #SkipUnlessNodeType<SwiftTreeSitter.Node, EvaluatableExpression?>(
      node: node, type: "string_literal")
    return .Ok(P4StringValue(withValue: node.text!))
  }
}

extension KeysetExpression: CompilableExpression {
  static func compile(
    node: SwiftTreeSitter.Node, withContext context: CompilerContext
  ) -> Common.Result<(any Common.EvaluatableExpression)?> {
    let keyset_expression_node = node.child(at: 0)!

    #RequireNodesType<Node, EvaluatableExpression>(
      nodes: keyset_expression_node, type: ["expression", "default_keyset"],
      nice_type_names: ["expression", "default keyset"])

    // If there is a default keyset, that's easy!
    if keyset_expression_node.nodeType == "default_keyset" {
      return .Ok(KeysetExpression(P4SetDefaultValue(withType: context.expected_type!)))
    }

    // Compile the expression:
    let maybe_compiled_set_expression = Expression.Compile(
      node: keyset_expression_node, withContext: context)
    guard case .Ok(let compiled_expression) = maybe_compiled_set_expression else {
      return .Error(maybe_compiled_set_expression.error()!)
    }

    return .Ok(KeysetExpression(compiled_expression))
  }
}

struct Expression {
  public static func Compile(
    node: Node, withContext context: CompilerContext
  ) -> Result<EvaluatableExpression> {
    #RequireNodeType<Node, EvaluatableExpression>(
      node: node, type: "expression", nice_type_name: "expression")

    let expression_node = node.child(at: 0)!
    #RequireNodesType<Node, EvaluatableExpression>(
      nodes: expression_node, type: ["grouped_expression", "simple_expression"],
      nice_type_names: ["grouped expression", "simple expression"])

    // If this is a grouped expression, recurse!
    if expression_node.nodeType == "grouped_expression" {
      return Expression.Compile(node: expression_node.child(at: 1)!, withContext: context)
    }

    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 {
      switch candidate_expression_parser.compile(
        node: expression_node, withContext: context)
      {
      case .Ok(.some(let parsed)): return .Ok(parsed)
      case .Error(let e): return .Error(e)
      default: continue
      }
    }

    return Result.Error(Error(withMessage: "\(node.range): Could not parse into expression"))
  }
}

struct LValue {
  public static func Compile(
    node: Node, withContext context: CompilerContext
  ) -> Result<EvaluatableLValueExpression> {
    #RequireNodeType<Node, EvaluatableExpression>(
      node: node, type: "expression", nice_type_name: "expression")

    let expression_node = node.child(at: 0)!
    #RequireNodesType<Node, EvaluatableExpression>(
      nodes: expression_node, type: ["grouped_expression", "simple_expression"],
      nice_type_names: ["grouped expression", "simple expression"])

    // If this is a grouped expression, recurse!
    if expression_node.nodeType == "grouped_expression" {
      return LValue.Compile(node: expression_node.child(at: 1)!, withContext: context)
    }

    let lvalue_parsers: [CompilableLValueExpression.Type] = [
      TypedIdentifier.self, FieldAccessExpression.self, ArrayAccessExpression.self,
    ]

    for candidate_lvalue_parser in lvalue_parsers {
      switch candidate_lvalue_parser.compile_as_lvalue(
        node: expression_node, withContext: context)
      {
      case .Ok(.some(let parsed)): return .Ok(parsed)
      case .Error(let e): return .Error(e)
      default: continue
      }
    }

    return Result.Error(Error(withMessage: "\(node.range): Could not parse into expression"))
  }
}

struct Identifier {
  public static func Compile(
    node: Node, withContext context: CompilerContext
  ) -> Result<Common.Identifier> {
    return if let node_text_value = node.text {
      .Ok(Common.Identifier(name: node_text_value))
    } else {
      .Error(Error(withMessage: "Could not parse an identifier from \(node)"))
    }
  }
}

extension SelectExpression: CompilableExpression {
  static func compile(
    node: Node, withContext context: CompilerContext
  ) -> Result<EvaluatableExpression?> {
    #RequireNodeType<Node, (SelectExpression, CompilerContext)>(
      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, withContext: context)
    guard case .Ok(let selector) = maybe_selector else {
      return .Error(
        ErrorOnNode(
          node: selector_node,
          withError:
            "Could not parse transition select expression selector expression: \(maybe_selector.error()!)"
        ))
    }

    var sces: [SelectCaseExpression] = Array()
    var sces_errors: [Error] = Array()

    select_body_node.enumerateNamedChildren { current_node in
      let maybe_parsed_cse = SelectCaseExpression.compile(
        node: current_node, withContext: context.update(newExpectation: selector.type()))
      if case .Ok(let parsed_cse) = maybe_parsed_cse {
        sces.append(parsed_cse as! SelectCaseExpression)
      } else {
        sces_errors.append(Error(withMessage: "\(maybe_parsed_cse.error()!)"))
      }
    }

    if !sces_errors.isEmpty {
      return .Error(
        Error(
          withMessage: "Error(s) parsing select cases: "
            + (sces_errors.map { error in
              return "\(error.msg)"
            }.joined(separator: ";"))))
    }
    return .Ok(
      SelectExpression(withSelector: selector, withSelectCaseExpressions: sces),
    )
  }
}

extension SelectCaseExpression: CompilableExpression {
  static func compile(
    node: Node, withContext context: CompilerContext
  ) -> 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 = KeysetExpression.compile(
      node: keysetexpression_node, withContext: context)
    guard case Result.Ok(let maybe_keysetexpression) = maybe_parsed_keysetexpression else {
      return Result.Error(maybe_parsed_keysetexpression.error()!)
    }

    guard let maybe_keysetexpression = maybe_keysetexpression else {
      return Result.Error(
        ErrorOnNode(node: keysetexpression_node, withError: "Missing expected keyset expression"))
    }

    let keysetexpression = maybe_keysetexpression as! KeysetExpression

    if case .Error(let e) = keysetexpression.compatible(type: context.expected_type!) {
      return .Error(ErrorOnNode(node: keysetexpression_node, withError: e.msg))
    }

    let maybe_parsed_targetstate = Identifier.Compile(
      node: targetstate_node, withContext: context)
    guard case .Ok(let targetstate) = maybe_parsed_targetstate else {
      return Result.Error(maybe_parsed_targetstate.error()!)
    }

    return .Ok(
      SelectCaseExpression(
        withKey: keysetexpression, withNextState: targetstate)
    )
  }
}

extension BinaryOperatorExpression: CompilableExpression {
  static func compile(
    node: SwiftTreeSitter.Node, withContext context: CompilerContext
  ) -> Result<(EvaluatableExpression)?> {
    let expression = node.child(at: 0)!

    #SkipUnlessNodeType<Node, EvaluatableExpression?>(
      node: expression, type: "binaryOperatorExpression")

    var currentChildIdx = 0
    var currentChildIdxSafe = 1
    var currentChild: Node? = .none

    if expression.childCount < currentChildIdxSafe {
      return Result.Error(
        ErrorOnNode(node: node, withError: "Malformed binary operator expression"))
    }
    currentChild = expression.child(at: currentChildIdx)

    let binary_operator_expression_node = currentChild!

    // TODO: This macro cannot handle new lines in the arrays
    // swift-format-ignore
    #RequireNodesType<Node, EvaluatableExpression?>(
      nodes: binary_operator_expression_node,
      type: ["binaryEqualOperatorExpression", "binaryLessThanOperatorExpression", "binaryLessThanEqualOperatorExpression", "binaryGreaterThanOperatorExpression", "binaryGreaterThanEqualOperatorExpression", "binaryAndOperatorExpression", "binaryOrOperatorExpression", "binaryAddOperatorExpression", "binarySubtractOperatorExpression", "binaryMultiplyOperatorExpression", "binaryDivideOperatorExpression"],
      nice_type_names: [ "binary equal operator", "binary less than operator", "binary less than or equal to operator", "binary greater than operator", "binary greater than or equal to operator", "binary and operator", "binary or operator", "binary add operator", "binary subtract operator", "binary multiply operator", "binary divide operator"])

    if binary_operator_expression_node.childCount < currentChildIdxSafe {
      return Result.Error(
        ErrorOnNode(node: node, withError: "Missing LHS for binary operator expression"))
    }
    currentChild = binary_operator_expression_node.child(at: currentChildIdx)
    let left_hand_side_raw = currentChild!

    currentChildIdx = currentChildIdx + 1
    currentChildIdxSafe = currentChildIdxSafe + 1
    if binary_operator_expression_node.childCount < currentChildIdxSafe {
      return Result.Error(
        ErrorOnNode(node: node, withError: "Missing binary operator for binary operator expression")
      )
    }
    currentChild = binary_operator_expression_node.child(at: currentChildIdx)

    currentChildIdx = currentChildIdx + 1
    currentChildIdxSafe = currentChildIdxSafe + 1
    if binary_operator_expression_node.childCount < currentChildIdxSafe {
      return Result.Error(
        ErrorOnNode(node: node, withError: "Missing binary operator for binary operator expression")
      )
    }
    currentChild = binary_operator_expression_node.child(at: currentChildIdx)
    let right_hand_side_raw = currentChild!

    let maybe_left_hand_side = Expression.Compile(node: left_hand_side_raw, withContext: context)
    guard case Result.Ok(let left_hand_side) = maybe_left_hand_side else {
      return Result.Error(maybe_left_hand_side.error()!)
    }

    let maybe_right_hand_side = Expression.Compile(node: right_hand_side_raw, withContext: context)
    guard case Result.Ok(let right_hand_side) = maybe_right_hand_side else {
      return Result.Error(maybe_right_hand_side.error()!)
    }

    let evaluators: [String: (String, P4Type, BinaryOperatorChecker?, BinaryOperatorEvaluator)] = [
      "binaryEqualOperatorExpression": (
        "Binary Equal", P4Boolean(), Optional<BinaryOperatorChecker>.none,
        binary_equal_operator_evaluator
      ),
      "binaryLessThanOperatorExpression": (
        "Binary Less Than", P4Boolean(), Optional<BinaryOperatorChecker>.none,
        binary_lt_operator_evaluator
      ),
      "binaryLessThanEqualOperatorExpression": (
        "Binary Less Than Or Equal", P4Boolean(), Optional<BinaryOperatorChecker>.none,
        binary_lte_operator_evaluator
      ),
      "binaryGreaterThanOperatorExpression": (
        "Binary Greater Than", P4Boolean(), Optional<BinaryOperatorChecker>.none,
        binary_gt_operator_evaluator
      ),
      "binaryGreaterThanEqualOperatorExpression": (
        "Binary Greater Than Or Equal", P4Boolean(), Optional<BinaryOperatorChecker>.none,
        binary_gte_operator_evaluator
      ),
      "binaryAndOperatorExpression": (
        "Binary Or", P4Boolean(), binary_and_or_operator_checker, binary_and_operator_evaluator
      ),
      "binaryOrOperatorExpression": (
        "Binary And", P4Boolean(), binary_and_or_operator_checker, binary_or_operator_evaluator
      ),
      "binaryAddOperatorExpression": (
        "Binary Add", P4Int(), binary_int_math_operator_checker, binary_add_operator_evaluator
      ),
      "binarySubtractOperatorExpression": (
        "Binary Subtract", P4Int(), binary_int_math_operator_checker,
        binary_subtract_operator_evaluator
      ),
      "binaryMultiplyOperatorExpression": (
        "Binary Multiply", P4Int(), binary_int_math_operator_checker,
        binary_multiply_operator_evaluator
      ),
      "binaryDivideOperatorExpression": (
        "Binary Divide", P4Int(), binary_int_math_operator_checker, binary_divide_operator_evaluator
      ),
    ]

    guard let selected_evaluator = evaluators[binary_operator_expression_node.nodeType!] else {
      return Result.Error(
        Error(withMessage: "No evaluator for \(binary_operator_expression_node.nodeType!)"))
    }

    if let checker = selected_evaluator.2,
      case .Error(let e) = checker(left_hand_side, right_hand_side)
    {
      return Result.Error(e)
    }

    return .Ok(
      BinaryOperatorExpression(
        withEvaluator: (selected_evaluator.0, selected_evaluator.1, selected_evaluator.3),
        withLhs: left_hand_side, withRhs: right_hand_side))
  }
}

extension ArrayAccessExpression: CompilableExpression {
  static func compile(
    node: SwiftTreeSitter.Node, withContext context: CompilerContext
  ) -> Common.Result<(any Common.EvaluatableExpression)?> {
    let expression = node.child(at: 0)!

    #SkipUnlessNodeType<Node, EvaluatableExpression?>(
      node: expression, type: "arrayAccessExpression")

    let array_access_expression_node = expression

    var currentChildIdx = 0
    var currentChildIdxSafe = 1
    var currentChild: Node? = .none

    // What is the "name" of the array?
    if array_access_expression_node.childCount < currentChildIdxSafe {
      return Result.Error(
        ErrorOnNode(node: node, withError: "Malformed array access expression"))
    }
    currentChild = expression.child(at: currentChildIdx)

    #RequireNodeType<Node, EvaluatableExpression?>(
      node: currentChild!, type: "expression",
      nice_type_name: "array identifier expression")
    let array_access_identifier_node = currentChild!

    // Check for the [
    currentChildIdx = currentChildIdx + 1
    currentChildIdxSafe = currentChildIdxSafe + 1
    if array_access_expression_node.childCount < currentChildIdxSafe {
      return Result.Error(
        ErrorOnNode(node: node, withError: "Missing [ for array access expression")
      )
    }

    // What is the indexor of the array?
    currentChildIdx = currentChildIdx + 1
    currentChildIdxSafe = currentChildIdxSafe + 1
    if array_access_expression_node.childCount < currentChildIdxSafe {
      return Result.Error(
        ErrorOnNode(node: node, withError: "Missing indexor expression for array access expression")
      )
    }
    currentChild = array_access_expression_node.child(at: currentChildIdx)

    #RequireNodeType<Node, EvaluatableExpression?>(
      node: currentChild!, type: "expression",
      nice_type_name: "array indexor expression")

    let array_access_indexor_node = currentChild!

    let maybe_array_identifier = Expression.Compile(
      node: array_access_identifier_node, withContext: context)
    guard case Result.Ok(let array_identifier) = maybe_array_identifier else {
      return Result.Error(maybe_array_identifier.error()!)
    }

    let maybe_array_type = array_identifier.type()
    guard let array_type = maybe_array_type as? P4Array else {
      return Result.Error(
        ErrorOnNode(
          node: array_access_identifier_node,
          withError: "\(array_identifier) does not name an array type")
      )
    }

    let maybe_array_indexor = Expression.Compile(
      node: array_access_indexor_node, withContext: context)
    guard case Result.Ok(let array_indexor) = maybe_array_indexor else {
      return Result.Error(maybe_array_indexor.error()!)
    }

    return .Ok(
      ArrayAccessExpression(
        withName: array_identifier, withType: array_type, withIndexor: array_indexor))
  }
}

extension FieldAccessExpression: CompilableExpression {
  static func compile(
    node: SwiftTreeSitter.Node, withContext context: CompilerContext
  ) -> Common.Result<(any Common.EvaluatableExpression)?> {
    let expression = node.child(at: 0)!

    #SkipUnlessNodeType<Node, EvaluatableExpression?>(
      node: expression, type: "fieldAccessExpression")

    let field_access_expression_node = expression

    var currentChildIdx = 0
    var currentChildIdxSafe = 1
    var currentChild: Node? = .none

    // What is the "name" of the struct?
    if field_access_expression_node.childCount < currentChildIdxSafe {
      return Result.Error(
        ErrorOnNode(node: node, withError: "Malformed field access expression"))
    }
    currentChild = expression.child(at: currentChildIdx)

    #RequireNodeType<Node, EvaluatableExpression?>(
      node: currentChild!, type: "expression",
      nice_type_name: "struct identifier expression")
    let struct_identifier_node = currentChild!

    // Check for the .
    currentChildIdx = currentChildIdx + 1
    currentChildIdxSafe = currentChildIdxSafe + 1
    if field_access_expression_node.childCount < currentChildIdxSafe {
      return Result.Error(
        ErrorOnNode(node: node, withError: "Missing . for field access expression")
      )
    }

    // What is the field of the struct?
    currentChildIdx = currentChildIdx + 1
    currentChildIdxSafe = currentChildIdxSafe + 1
    if field_access_expression_node.childCount < currentChildIdxSafe {
      return Result.Error(
        ErrorOnNode(node: node, withError: "Missing field name for field access expression")
      )
    }
    currentChild = field_access_expression_node.child(at: currentChildIdx)

    #RequireNodeType<Node, EvaluatableExpression?>(
      node: currentChild!, type: "identifier",
      nice_type_name: "field name")

    let field_name_node = currentChild!

    // Make sure that the identifier really identifies a struct.
    let maybe_struct_identifier = Expression.Compile(
      node: struct_identifier_node, withContext: context)
    guard case Result.Ok(let struct_identifier) = maybe_struct_identifier else {
      return Result.Error(maybe_struct_identifier.error()!)
    }
    guard let struct_type = struct_identifier.type() as? P4Struct else {
      return .Error(
        ErrorOnNode(
          node: struct_identifier_node,
          withError: "\(struct_identifier_node.text!) does not have struct type"))
    }

    let maybe_field_name = Identifier.Compile(
      node: field_name_node, withContext: context)
    guard case Result.Ok(let field_name) = maybe_field_name else {
      return Result.Error(maybe_field_name.error()!)
    }

    // Make sure that the field is valid for the struct type.
    let maybe_field_type = struct_type.fields.get_field_type(field_name)
    guard let field_type = maybe_field_type else {
      return .Error(
        ErrorOnNode(
          node: field_name_node,
          withError: "\(field_name) is not a valid field for struct with type \(struct_type)"))
    }

    return .Ok(
      FieldAccessExpression(
        withStruct: struct_identifier,
        withField: P4StructFieldIdentifier(id: field_name, withType: field_type)))
  }
}

extension FieldAccessExpression: CompilableLValueExpression {
  static func compile_as_lvalue(
    node: SwiftTreeSitter.Node, withContext context: CompilerContext
  ) -> Result<EvaluatableLValueExpression?> {
    let expression = node.child(at: 0)!
    #SkipUnlessNodeType<Node, EvaluatableExpression?>(
      node: expression, type: "fieldAccessExpression")

    let maybe_parsed_expression = FieldAccessExpression.compile(node: node, withContext: context)
    guard case .Ok(let maybe_field_access_expression) = maybe_parsed_expression else {
      return .Error(maybe_parsed_expression.error()!)
    }

    let field_access_expression = maybe_field_access_expression as! FieldAccessExpression

    return Result.Ok(field_access_expression)
  }
}

extension ArrayAccessExpression: CompilableLValueExpression {
  static func compile_as_lvalue(
    node: SwiftTreeSitter.Node, withContext context: CompilerContext
  ) -> Result<EvaluatableLValueExpression?> {
    let expression = node.child(at: 0)!
    #SkipUnlessNodeType<Node, EvaluatableExpression?>(
      node: expression, type: "arrayAccessExpression")

    let maybe_parsed_expression = ArrayAccessExpression.compile(node: node, withContext: context)
    guard case .Ok(let maybe_array_access_expression) = maybe_parsed_expression else {
      return .Error(maybe_parsed_expression.error()!)
    }

    let array_access_expression = maybe_array_access_expression as! ArrayAccessExpression

    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))
  }
}