问无法命名函数表达式。文档错误

EN

  Future<UserModel> getUserById(String id){
    return FirebaseFirestore.instance.collection("collectionPath")
        .doc(id).get().then((doc) => UserModel.fromSnapShot(doc.data()));
  }

import 'package:flutter/foundation.dart';

@immutable
class UserModel{
  final String id;
  final String name;
  final String email;
  final String stripeId;
  final List<CartItemModel> cart;
  final int totalCartPrice;

  const UserModel({required this.id, required this.name, required this.email, required this.stripeId, required this.cart,
      required this.totalCartPrice});

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'name': name,
      'email': email,
      'stripeId': stripeId,
      'cart': cart,
      'totalCartPrice': totalCartPrice,
    };
  }

  factory UserModel.fromMap(Map<String, dynamic> map) {
    return UserModel(
      id: map['id'] as String,
      name: map['name'] as String,
      email: map['email'] as String,
      stripeId: map['stripeId'] as String,
      cart:  (map['cart'] as List).map((cart) => CartItemModel.fromMap(cart)).toList() ,
      totalCartPrice: map['totalCartPrice'] as int,
    );
  }
}

@immutable
class CartItemModel{
  final String id;


  const CartItemModel({required this.id});

  Map<String, dynamic> toMap() {
    return {
      'id': id,
    };
  }

  factory CartItemModel.fromMap(Map<String, dynamic> map) {
    return CartItemModel(
      id: map['id'] as String,
    );
  }
}