ILoggable

LINQ: Immutability vs. Deferred execution

In the meantime, I wanted to write up some non-obvious observation about deferred execution with Linq. Considering the gotchas with lambdas, it's easy to extend the lessons learned to linq, since it is after all deferred execution. But what's different with Linq is that, while execution is deferred, the expression tree built via a query is also immutable. I came across this trying to do some simple query re-use.

Let's start with a simple DTO:

public class Order
{
  public Order(int id, int val, bool buyOrder)
  {
    Id = id;
    Value = val;
    IsBuyOrder = buyOrder;
  }
  public int Id { get; set; }
  public int Value { get; set; }
  public bool IsBuyOrder { get; set; }
}

Order[] orders = new Order[]
{
  new Order(1,2,true),
  new Order(2,2,false),
  new Order(3,4,true),
  new Order(4,4,false),
  new Order(5,6,true),
  new Order(6,6,false),
};

Let's split those into buy and sell orders:

var buyOrders = from order in orders
          where order.IsBuyOrder
          select order;

var sellOrders = from order in orders
                 where !order.IsBuyOrder
                 select order;

If we want to find the buy and the sell order with a value of 2, you'd think we could write one query and re-use it for both of those queries. Since both queries results in IEnumerable<Order>, how about we define a query source and assign the value of either above query.

IEnumerable<Order> orders2 = null;

var orderAtTwo = from order in orders2
                 where order.Value == 2
                 select order;

orders2 = buyOrders;
int buyOrderId = orderAtTwo.First().Id;

orders2 = sellOrders;
int sellOrderId = orderAtTwo.First().Id;

Console.WriteLine("buy Id: {0}, sell Id: {1}", buyOrderId, sellOrderId);

A way around this is to replace the actual contents of orders2. However, for us to do that, we have to turn it into the query source into a collection first.

orders2.Clear();
orders2.AddRange(buyOrders);
int buyOrderId = orderAtTwo.First().Id;

orders2.Clear();
orders2.AddRange(sellOrders);
int sellOrderId = orderAtTwo.First().Id;

Console.WriteLine("buy Id: {0}, sell Id: {1}", buyOrderId, sellOrderId);

buy Id: 1, sell Id: 2

To achieve this, we need to move the shared query into a separate method such as:

private IEnumerable<Order> GetTwo(IEnumerable<Order> source)
{
  return from order in source
         where order.Value == 2
         select order;
}

int buyOrderId = GetTwo(buyOrders).First().Id;
int sellOrderId = GetTwo(sellOrders).First().Id;

Console.WriteLine("buy Id: {0}, sell Id: {1}", buyOrderId, sellOrderId);

Labels: c#, lambda, linq

posted by ether @ 10:05 AM 0 comments

More Deferred Execution Fun: foreach and delegation scope

public void SpawnActions()
{
  foreach (ActionContext context in contexts)
  {
    int id = context.Id;
    Action<int> callback =  
      (workerNumber) =>
      {
        Console.WriteLine("{0} Id: {1}/{2}", workerNumber, id, context.Id);
      };
    ThreadPool.QueueUserWorkItem(new WaitCallback(FutureExecute), callback);
  }
}

public void FutureExecute(object state)
{
  int id = worker++;
  Action<int> callback = state as Action<int>;
  Thread.Sleep(500);
  callback(id);
}

The output looks like this:

0 Id: 0/9
1 Id: 1/9
2 Id: 2/9
3 Id: 3/9
4 Id: 4/9
5 Id: 5/9
6 Id: 6/9
7 Id: 7/9
8 Id: 8/9
9 Id: 9/9

  foreach (ActionContext context in contexts)
  {
    int id = context.Id;
    // locally scoped variable
    ActionContext c2 = context;
    Action<int> callback =  
      (workerNumber) =>
      {
        Console.WriteLine("{0} Id: {1}/{2}", workerNumber, id, c2.Id);
      };
    ThreadPool.QueueUserWorkItem(new WaitCallback(FutureExecute), callback);
  }

0 Id: 0/0
1 Id: 1/1
2 Id: 2/2
3 Id: 3/3
4 Id: 4/4
5 Id: 5/5
6 Id: 6/6
7 Id: 7/7
8 Id: 8/8
9 Id: 9/9

The dangers of deferred execution

Utility.ActionDownloader.Download(
  Configuration.GetAssetUri(dto.Url),
  (Downloader d) =>
  {
    FloatContainer c = (FloatContainer)XamlReader.Load(d.ResponseText);
    c.Initialize(dto);
  });

What happened was that I needed my main routine to execute some code after FloatContainer was initialized, and by habit i created an Initialized event on FloatContainer. Of course this was superfluous, since my lambda expression called the synchronous Initialize, i.e my action could be placed inline after that call to c.Initialize(dto) and be guaranteed to be called after initialization had completed.

This scenario just meant I created some superfluous code. However, I'm sure as I use lambda expression more, there will be more pitfalls of writing code that doesn't consider that its execution time is unknown, as is the state of the objects tied to the scope of the expression.

This last bit about objects tied to the expression scope is especially tricky and I think we will see some help in terms of Immutable concepts weaving their way into C# 3.x or 4.0, as the whole functional aspect of lambda expressions really work best when dealing with objects that cannot change state. Eric Lippert's been laying the groundwork in a number of posts on the subject and while he constantly disclaims that his ponderings are not a roadmap for C#, I am still going to assume that his interest and recognition of the subject of Immutables will have some impact in a future revision of the language. Well, I at least hope it does.

Labels: .net, c#, delegate, lambda

posted by ether @ 1:39 PM 0 comments

Action & Func: Never write another delegate

delegate int IntProducerDelegate();

public int x = 0;
public int IntProducer()
{
  return x++;
}

IntProducerDelegate p1 = new IntProducerDelegate(IntProducer);

IntProducerDelegate p2 = delegate { return IntProducer(); };

// or with IntProducer's action inlined...
IntProducerDelegate p3 = delegate { return x++; };

// create a new Func delegate just like the IntProducerDelegate
IntProducerDelegate p3 = new Func<int>(IntProducer);

// which means that we don't need IntProducerDelegate at all anymore
Func<int> p4 = delegate { return x++; };

// and the anonymous delegate can also be shorthanded with a lambda expression
Func<int> p5 = () => { return x++; };
// which says, given that we take no argument "()", execute and return the following "return x++;"

delegate void IntConsumerDelegate(int i);

public void IntConsumer(int i)
{
  Console.WriteLine("The number is {0}", i);
}

IntConsumerDelegate c1 = new IntConsumerDelegate(IntConsumer);

IntConsumerDelegate c2 = new Action<int>(IntConsumer);

Action<int> c3 = delegate(int i) { Console.WriteLine("The number is {0}", i); };

Action<int> c4 = (i) => { Console.WriteLine("The number is {0}", i); };

Searching a Tree of Objects with Linq

The scenario

I have a tree of objects that have both a type and a name. The name is unique, the Type is not. The interface is this:

public interface INode
{
  int Id { get; }
  string Name { get; set; }
  string Type { get; set; }
  List<INode> Children { get; }
}

I want to be able to find a single named Node in the tree and I want to be able to retrieve a collection of all nodes for a particular type. The searchable interface could be expressed as this:

public interface ISearchableNode : INode
{
  INode FindByName(string name);
  IEnumerable<INode> FindByType(string name);
}

The signature for the evaluation delegate looks like this:

delegate bool FindDelegate(INode node);

// Instead of this:
private void Find(INode node, List<INode> resultSet, FindDelegate findDelegate);
// called like this for a Name search:
Find(this, resultSet, delegate(INode node) { return node.Name == name; });

// I can use this:
private void Find(INode node, List<INode> resultSet, Func<INode, bool> f)
// called like this:
Find(this, resultSet, node => node.Name == name);

Thus giving me the following implementation for ISearchableNode:

public INode FindByName(string name)
{
  List<INode> resultSet = new List<INode>();
  Find(this, resultSet, x => x.Name == name);
  return resultSet.FirstOrDefault();
}

public IEnumerable<INode> FindByType(string type)
{
  List<INode> resultSet = new List<INode>();
  Find(this, resultSet, x => x.Type == type);
  return (IEnumerable<INode>)resultSet;
}

private void Find(INode node, List<INode> resultSet, Func<INode, bool> f)
{
  if (f(node))
  {
    resultSet.Add(node);
  }
  foreach (INode child in node.Children)
  {
    Find(child, resultSet, f);
  }
}

public interface IQueryableNode : IEnumerable<INode>
{
}

#region IEnumerable<Node> Members
public IEnumerator<INode> GetEnumerator()
{
  yield return this;
  foreach (Node child in Children)
  {
    foreach (Node subchild in child)
    {
      yield return subchild;
    }
  }
}
#endregion

#region IEnumerable Members
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
  return this.GetEnumerator();
}
#endregion

var allBar = from n in x
                where n.Type == "bar"
                select n;
foreach (Node n in allBar)
{
  // do something with that node
}

INode node = (from n in x
              where n.Name == "i"
              select n).FirstOrDefault();